Fix some border drawing issues.
[sgt/puzzles] / loopy.c
diff --git a/loopy.c b/loopy.c
index 42e7856..f0344fe 100644 (file)
--- a/loopy.c
+++ b/loopy.c
@@ -1,6 +1,6 @@
 /*
  * loopy.c: An implementation of the Nikoli game 'Loop the loop'.
- * (c) Mike Pinna, 2005
+ * (c) Mike Pinna, 2005, 2006
  *
  * vim: set shiftwidth=4 :set textwidth=80:
  */ 
@@ -8,40 +8,24 @@
 /*
  * TODO:
  *
- *  - setting very high recursion depth seems to cause memory
- *    munching: are we recursing before checking completion, by any
- *    chance?
+ *  - Setting very high recursion depth seems to cause memory munching: are we
+ *    recursing before checking completion, by any chance?
  *
- *  - there's an interesting deductive technique which makes use of
- *    topology rather than just graph theory. Each _square_ in the
- *    grid is either inside or outside the loop; you can tell that
- *    two squares are on the same side of the loop if they're
- *    separated by an x (or, more generally, by a path crossing no
- *    LINE_UNKNOWNs and an even number of LINE_YESes), and on the
- *    opposite side of the loop if they're separated by a line (or
- *    an odd number of LINE_YESes and no LINE_UNKNOWNs). Oh, and
- *    any square separated from the outside of the grid by a
- *    LINE_YES or a LINE_NO is on the inside or outside
- *    respectively. So if you can track this for all squares, you
- *    can occasionally spot that two squares are separated by a
- *    LINE_UNKNOWN but their relative insideness is known, and
- *    therefore deduce the state of the edge between them.
- *     + An efficient way to track this would be by augmenting the
- *      disjoint set forest data structure. Each element, along
- *      with a pointer to a parent member of its equivalence
- *      class, would also carry a one-bit field indicating whether
- *      it was equal or opposite to its parent. Then you could
- *      keep flipping a bit as you ascended the tree during
- *      dsf_canonify(), and hence you'd be able to return the
- *      relationship of the input value to its ultimate parent
- *      (and also you could then get all those bits right when you
- *      went back up the tree rewriting). So you'd be able to
- *      query whether any two elements were known-equal,
- *      known-opposite, or not-known, and you could add new
- *      equalities or oppositenesses to increase your knowledge.
- *      (Of course the algorithm would have to fail an assertion
- *      if you tried to tell it two things it already knew to be
- *      opposite were equal, or vice versa!)
+ *  - There's an interesting deductive technique which makes use of topology
+ *    rather than just graph theory. Each _square_ in the grid is either inside
+ *    or outside the loop; you can tell that two squares are on the same side
+ *    of the loop if they're separated by an x (or, more generally, by a path
+ *    crossing no LINE_UNKNOWNs and an even number of LINE_YESes), and on the
+ *    opposite side of the loop if they're separated by a line (or an odd
+ *    number of LINE_YESes and no LINE_UNKNOWNs). Oh, and any square separated
+ *    from the outside of the grid by a LINE_YES or a LINE_NO is on the inside
+ *    or outside respectively. So if you can track this for all squares, you
+ *    figure out the state of the line between a pair once their relative
+ *    insideness is known.
+ *
+ *  - (Just a speed optimisation.)  Consider some todo list queue where every
+ *    time we modify something we mark it for consideration by other bits of
+ *    the solver, to save iteration over things that have already been done.
  */
 
 #include <stdio.h>
 #include "puzzles.h"
 #include "tree234.h"
 
+/* Debugging options */
+/*#define DEBUG_CACHES*/
+/*#define SHOW_WORKING*/
+
+/* ----------------------------------------------------------------------
+ * Struct, enum and function declarations
+ */
+
+enum {
+    COL_BACKGROUND,
+    COL_FOREGROUND,
+    COL_HIGHLIGHT,
+    COL_MISTAKE,
+    NCOLOURS
+};
+
+struct game_state {
+    int w, h;
+    
+    /* Put -1 in a square that doesn't get a clue */
+    char *clues;
+    
+    /* Arrays of line states, stored left-to-right, top-to-bottom */
+    char *hl, *vl;
+
+    int solved;
+    int cheated;
+
+    int recursion_depth;
+};
+
+enum solver_status {
+    SOLVER_SOLVED,    /* This is the only solution the solver could find */
+    SOLVER_MISTAKE,   /* This is definitely not a solution */
+    SOLVER_AMBIGUOUS, /* This _might_ be an ambiguous solution */
+    SOLVER_INCOMPLETE /* This may be a partial solution */
+};
+
+typedef struct normal {
+    char *dot_atleastone;
+    char *dot_atmostone;
+} normal_mode_state;
+
+typedef struct hard {
+    int *linedsf;
+} hard_mode_state;
+
+typedef struct solver_state {
+    game_state *state;
+    int recursion_remaining;
+    enum solver_status solver_status;
+    /* NB looplen is the number of dots that are joined together at a point, ie a
+     * looplen of 1 means there are no lines to a particular dot */
+    int *looplen;
+
+    /* caches */
+    char *dot_yescount;
+    char *dot_nocount;
+    char *square_yescount;
+    char *square_nocount;
+    char *dot_solved, *square_solved;
+    int *dotdsf;
+
+    normal_mode_state *normal;
+    hard_mode_state *hard;
+} solver_state;
+
+/*
+ * Difficulty levels. I do some macro ickery here to ensure that my
+ * enum and the various forms of my name list always match up.
+ */
+
+#define DIFFLIST(A) \
+    A(EASY,Easy,e,easy_mode_deductions) \
+    A(NORMAL,Normal,n,normal_mode_deductions) \
+    A(HARD,Hard,h,hard_mode_deductions)
+#define ENUM(upper,title,lower,fn) DIFF_ ## upper,
+#define TITLE(upper,title,lower,fn) #title,
+#define ENCODE(upper,title,lower,fn) #lower
+#define CONFIG(upper,title,lower,fn) ":" #title
+#define SOLVER_FN_DECL(upper,title,lower,fn) static int fn(solver_state *);
+#define SOLVER_FN(upper,title,lower,fn) &fn,
+enum { DIFFLIST(ENUM) DIFF_MAX };
+static char const *const diffnames[] = { DIFFLIST(TITLE) };
+static char const diffchars[] = DIFFLIST(ENCODE);
+#define DIFFCONFIG DIFFLIST(CONFIG)
+DIFFLIST(SOLVER_FN_DECL);
+static int (*(solver_fns[]))(solver_state *) = { DIFFLIST(SOLVER_FN) };
+
+struct game_params {
+    int w, h;
+    int diff;
+    int rec;
+};
+
+enum line_state { LINE_YES, LINE_UNKNOWN, LINE_NO };
+
+#define OPP(state) \
+    (2 - state)
+
+enum direction { UP, LEFT, RIGHT, DOWN };
+
+#define OPP_DIR(dir) \
+    (3 - dir) 
+
+struct game_drawstate {
+    int started;
+    int tilesize, linewidth;
+    int flashing;
+    char *hl, *vl;
+    char *clue_error;
+};
+
+static char *game_text_format(game_state *state);
+static char *state_to_text(const game_state *state);
+static char *validate_desc(game_params *params, char *desc);
+static int get_line_status_from_point(const game_state *state,
+                                      int x, int y, enum direction d);
+static int dot_order(const game_state* state, int i, int j, char line_type);
+static int square_order(const game_state* state, int i, int j, char line_type);
+static solver_state *solve_game_rec(const solver_state *sstate,
+                                    int diff);
+
+#ifdef DEBUG_CACHES
+static void check_caches(const solver_state* sstate);
+#else
+#define check_caches(s)
+#endif
+
+/* ----------------------------------------------------------------------
+ * Preprocessor magic 
+ */
+
+/* General constants */
 #define PREFERRED_TILE_SIZE 32
 #define TILE_SIZE (ds->tilesize)
-#define LINEWIDTH TILE_SIZE / 16
+#define LINEWIDTH (ds->linewidth)
 #define BORDER (TILE_SIZE / 2)
+#define FLASH_TIME 0.5F
 
-#define FLASH_TIME 0.4F
-
+/* Counts of various things that we're interested in */
 #define HL_COUNT(state) ((state)->w * ((state)->h + 1))
 #define VL_COUNT(state) (((state)->w + 1) * (state)->h)
+#define LINE_COUNT(state) (HL_COUNT(state) + VL_COUNT(state))
 #define DOT_COUNT(state) (((state)->w + 1) * ((state)->h + 1))
 #define SQUARE_COUNT(state) ((state)->w * (state)->h)
 
+/* For indexing into arrays */
+#define DOT_INDEX(state, x, y) ((x) + ((state)->w + 1) * (y))
+#define SQUARE_INDEX(state, x, y) ((x) + ((state)->w) * (y))
+#define HL_INDEX(state, x, y) SQUARE_INDEX(state, x, y)
+#define VL_INDEX(state, x, y) DOT_INDEX(state, x, y)
+
+/* Useful utility functions */
+#define LEGAL_DOT(state, i, j) ((i) >= 0 && (j) >= 0 && \
+                                (i) <= (state)->w && (j) <= (state)->h)
+#define LEGAL_SQUARE(state, i, j) ((i) >= 0 && (j) >= 0 && \
+                                   (i) < (state)->w && (j) < (state)->h)
+
+#define CLUE_AT(state, i, j) (LEGAL_SQUARE(state, i, j) ? \
+                              LV_CLUE_AT(state, i, j) : -1)
+                             
+#define LV_CLUE_AT(state, i, j) ((state)->clues[SQUARE_INDEX(state, i, j)])
+
+#define BIT_SET(field, bit) ((field) & (1<<(bit)))
+
+#define SET_BIT(field, bit)  (BIT_SET(field, bit) ? FALSE : \
+                              ((field) |= (1<<(bit)), TRUE))
+
+#define CLEAR_BIT(field, bit) (BIT_SET(field, bit) ? \
+                               ((field) &= ~(1<<(bit)), TRUE) : FALSE)
+
+#define DIR2STR(d) \
+    ((d == UP) ? "up" : \
+     (d == DOWN) ? "down" : \
+     (d == LEFT) ? "left" : \
+     (d == RIGHT) ? "right" : "oops")
+
+#define CLUE2CHAR(c) \
+    ((c < 0) ? ' ' : c + '0')
+
+/* Lines that have particular relationships with given dots or squares */
 #define ABOVE_SQUARE(state, i, j) ((state)->hl[(i) + (state)->w * (j)])
 #define BELOW_SQUARE(state, i, j) ABOVE_SQUARE(state, i, (j)+1)
-
 #define LEFTOF_SQUARE(state, i, j)  ((state)->vl[(i) + ((state)->w + 1) * (j)])
 #define RIGHTOF_SQUARE(state, i, j) LEFTOF_SQUARE(state, (i)+1, j)
 
-#define LEGAL_DOT(state, i, j) ((i) >= 0 && (j) >= 0 &&                 \
-                                (i) <= (state)->w && (j) <= (state)->h)
-
 /*
  * These macros return rvalues only, but can cope with being passed
  * out-of-range coordinates.
  */
-#define ABOVE_DOT(state, i, j) ((!LEGAL_DOT(state, i, j) || j <= 0) ?  \
+/* XXX replace these with functions so we can create an array of function
+ * pointers for nicer iteration over them.  This could probably be done with
+ * loads of other things for eliminating many nasty hacks. */
+#define ABOVE_DOT(state, i, j) ((!LEGAL_DOT(state, i, j) || j <= 0) ? \
                                 LINE_NO : LV_ABOVE_DOT(state, i, j))
 #define BELOW_DOT(state, i, j) ((!LEGAL_DOT(state, i, j) || j >= (state)->h) ? \
                                 LINE_NO : LV_BELOW_DOT(state, i, j))
 
 #define LEFTOF_DOT(state, i, j)  ((!LEGAL_DOT(state, i, j) || i <= 0) ? \
                                   LINE_NO : LV_LEFTOF_DOT(state, i, j))
-#define RIGHTOF_DOT(state, i, j) ((!LEGAL_DOT(state, i, j) || i >= (state)->w)?\
+#define RIGHTOF_DOT(state, i, j) ((!LEGAL_DOT(state, i, j) || i >= (state)->w)? \
                                   LINE_NO : LV_RIGHTOF_DOT(state, i, j))
 
 /*
  * These macros expect to be passed valid coordinates, and return
  * lvalues.
  */
-#define LV_BELOW_DOT(state, i, j) ((state)->vl[(i) + ((state)->w + 1) * (j)])
+#define LV_BELOW_DOT(state, i, j) ((state)->vl[VL_INDEX(state, i, j)])
 #define LV_ABOVE_DOT(state, i, j) LV_BELOW_DOT(state, i, (j)-1)
 
-#define LV_RIGHTOF_DOT(state, i, j) ((state)->hl[(i) + (state)->w * (j)])
+#define LV_RIGHTOF_DOT(state, i, j) ((state)->hl[HL_INDEX(state, i, j)])
 #define LV_LEFTOF_DOT(state, i, j)  LV_RIGHTOF_DOT(state, (i)-1, j)
 
-#define CLUE_AT(state, i, j) ((i < 0 || i >= (state)->w || \
-                               j < 0 || j >= (state)->h) ? \
-                             ' ' : LV_CLUE_AT(state, i, j))
-                             
-#define LV_CLUE_AT(state, i, j) ((state)->clues[(i) + (state)->w * (j)])
+/* Counts of interesting things */
+#define DOT_YES_COUNT(sstate, i, j) \
+    ((sstate)->dot_yescount[DOT_INDEX((sstate)->state, i, j)])
 
-#define OPP(dir) (dir == LINE_UNKNOWN ? LINE_UNKNOWN : \
-                  dir == LINE_YES ? LINE_NO : LINE_YES)
+#define DOT_NO_COUNT(sstate, i, j) \
+    ((sstate)->dot_nocount[DOT_INDEX((sstate)->state, i, j)])
 
-static char *game_text_format(game_state *state);
+#define SQUARE_YES_COUNT(sstate, i, j) \
+    ((sstate)->square_yescount[SQUARE_INDEX((sstate)->state, i, j)])
 
-enum {
-    COL_BACKGROUND,
-    COL_FOREGROUND,
-    COL_HIGHLIGHT,
-    NCOLOURS
-};
+#define SQUARE_NO_COUNT(sstate, i, j) \
+    ((sstate)->square_nocount[SQUARE_INDEX((sstate)->state, i, j)])
 
-enum line_state { LINE_UNKNOWN, LINE_YES, LINE_NO };
+/* Iterators.  NB these iterate over height more slowly than over width so that
+ * the elements come out in 'reading' order */
+/* XXX considering adding a 'current' element to each of these which gets the
+ * address of the current dot, say.  But expecting we'd need more than that
+ * most of the time.  */
+#define FORALL(i, j, w, h) \
+    for ((j) = 0; (j) < (h); ++(j)) \
+        for ((i) = 0; (i) < (w); ++(i))
 
-enum direction { UP, DOWN, LEFT, RIGHT };
+#define FORALL_DOTS(state, i, j) \
+    FORALL(i, j, (state)->w + 1, (state)->h + 1)
 
-struct game_params {
-    int w, h, rec;
-};
+#define FORALL_SQUARES(state, i, j) \
+    FORALL(i, j, (state)->w, (state)->h)
 
-struct game_state {
-    int w, h;
-    
-    /* Put ' ' in a square that doesn't get a clue */
-    char *clues;
-    
-    /* Arrays of line states, stored left-to-right, top-to-bottom */
-    char *hl, *vl;
+#define FORALL_HL(state, i, j) \
+    FORALL(i, j, (state)->w, (state)->h+1)
 
-    int solved;
-    int cheated;
+#define FORALL_VL(state, i, j) \
+    FORALL(i, j, (state)->w+1, (state)->h)
 
-    int recursion_depth;
-};
+/* ----------------------------------------------------------------------
+ * General struct manipulation and other straightforward code
+ */
 
 static game_state *dup_game(game_state *state)
 {
@@ -149,13 +299,13 @@ static game_state *dup_game(game_state *state)
     ret->solved = state->solved;
     ret->cheated = state->cheated;
 
-    ret->clues   = snewn(SQUARE_COUNT(state), char);
+    ret->clues = snewn(SQUARE_COUNT(state), char);
     memcpy(ret->clues, state->clues, SQUARE_COUNT(state));
 
-    ret->hl      = snewn(HL_COUNT(state), char);
+    ret->hl = snewn(HL_COUNT(state), char);
     memcpy(ret->hl, state->hl, HL_COUNT(state));
 
-    ret->vl      = snewn(VL_COUNT(state), char);
+    ret->vl = snewn(VL_COUNT(state), char);
     memcpy(ret->vl, state->vl, VL_COUNT(state));
 
     ret->recursion_depth = state->recursion_depth;
@@ -173,49 +323,62 @@ static void free_game(game_state *state)
     }
 }
 
-enum solver_status {
-    SOLVER_SOLVED,    /* This is the only solution the solver could find */
-    SOLVER_MISTAKE,   /* This is definitely not a solution */
-    SOLVER_AMBIGUOUS, /* This _might_ be an ambiguous solution */
-    SOLVER_INCOMPLETE /* This may be a partial solution */
-};
-
-typedef struct solver_state {
-  game_state *state;
-   /* XXX dot_atleastone[i,j, dline] is equivalent to */
-   /*     dot_atmostone[i,j,OPP_DLINE(dline)] */
-  char *dot_atleastone;
-  char *dot_atmostone;
-/*   char *dline_identical; */
-  int recursion_remaining;
-  enum solver_status solver_status;
-  int *dotdsf, *looplen;
-} solver_state;
-
-static solver_state *new_solver_state(game_state *state) {
+static solver_state *new_solver_state(const game_state *state, int diff) {
+    int i, j;
     solver_state *ret = snew(solver_state);
-    int i;
 
-    ret->state = dup_game(state);
+    ret->state = dup_game((game_state *)state);
     
-    ret->dot_atmostone = snewn(DOT_COUNT(state), char);
-    memset(ret->dot_atmostone, 0, DOT_COUNT(state));
-    ret->dot_atleastone = snewn(DOT_COUNT(state), char);
-    memset(ret->dot_atleastone, 0, DOT_COUNT(state));
-
-#if 0
-    dline_identical = snewn(DOT_COUNT(state), char);
-    memset(dline_identical, 0, DOT_COUNT(state));
-#endif
-
     ret->recursion_remaining = state->recursion_depth;
-    ret->solver_status = SOLVER_INCOMPLETE; /* XXX This may be a lie */
+    ret->solver_status = SOLVER_INCOMPLETE; 
 
-    ret->dotdsf = snewn(DOT_COUNT(state), int);
+    ret->dotdsf = snew_dsf(DOT_COUNT(state));
     ret->looplen = snewn(DOT_COUNT(state), int);
+
     for (i = 0; i < DOT_COUNT(state); i++) {
-       ret->dotdsf[i] = i;
-       ret->looplen[i] = 1;
+        ret->looplen[i] = 1;
+    }
+
+    ret->dot_solved = snewn(DOT_COUNT(state), char);
+    ret->square_solved = snewn(SQUARE_COUNT(state), char);
+    memset(ret->dot_solved, FALSE, DOT_COUNT(state));
+    memset(ret->square_solved, FALSE, SQUARE_COUNT(state));
+
+    ret->dot_yescount = snewn(DOT_COUNT(state), char);
+    memset(ret->dot_yescount, 0, DOT_COUNT(state));
+    ret->dot_nocount = snewn(DOT_COUNT(state), char);
+    memset(ret->dot_nocount, 0, DOT_COUNT(state));
+    ret->square_yescount = snewn(SQUARE_COUNT(state), char);
+    memset(ret->square_yescount, 0, SQUARE_COUNT(state));
+    ret->square_nocount = snewn(SQUARE_COUNT(state), char);
+    memset(ret->square_nocount, 0, SQUARE_COUNT(state));
+
+    /* dot_nocount needs special initialisation as we define lines coming off
+     * dots on edges as fixed at NO */
+
+    FORALL_DOTS(state, i, j) {
+        if (i == 0 || i == state->w)
+            ++ret->dot_nocount[DOT_INDEX(state, i, j)];
+        if (j == 0 || j == state->h)
+            ++ret->dot_nocount[DOT_INDEX(state, i, j)];
+    }
+
+    if (diff < DIFF_NORMAL) {
+        ret->normal = NULL;
+    } else {
+        ret->normal = snew(normal_mode_state);
+
+        ret->normal->dot_atmostone = snewn(DOT_COUNT(state), char);
+        memset(ret->normal->dot_atmostone, 0, DOT_COUNT(state));
+        ret->normal->dot_atleastone = snewn(DOT_COUNT(state), char);
+        memset(ret->normal->dot_atleastone, 0, DOT_COUNT(state));
+    }
+
+    if (diff < DIFF_HARD) {
+        ret->hard = NULL;
+    } else {
+        ret->hard = snew(hard_mode_state);
+        ret->hard->linedsf = snew_dsf(LINE_COUNT(state));
     }
 
     return ret;
@@ -224,149 +387,91 @@ static solver_state *new_solver_state(game_state *state) {
 static void free_solver_state(solver_state *sstate) {
     if (sstate) {
         free_game(sstate->state);
-        sfree(sstate->dot_atleastone);
-        sfree(sstate->dot_atmostone);
-        /*    sfree(sstate->dline_identical); */
         sfree(sstate->dotdsf);
         sfree(sstate->looplen);
+        sfree(sstate->dot_solved);
+        sfree(sstate->square_solved);
+        sfree(sstate->dot_yescount);
+        sfree(sstate->dot_nocount);
+        sfree(sstate->square_yescount);
+        sfree(sstate->square_nocount);
+
+        if (sstate->normal) {
+            sfree(sstate->normal->dot_atleastone);
+            sfree(sstate->normal->dot_atmostone);
+            sfree(sstate->normal);
+        }
+
+        if (sstate->hard) {
+            sfree(sstate->hard->linedsf);
+            sfree(sstate->hard);
+        }
+
         sfree(sstate);
     }
 }
 
-static solver_state *dup_solver_state(solver_state *sstate) {
+static solver_state *dup_solver_state(const solver_state *sstate) {
     game_state *state;
 
     solver_state *ret = snew(solver_state);
 
     ret->state = state = dup_game(sstate->state);
 
-    ret->dot_atmostone = snewn(DOT_COUNT(state), char);
-    memcpy(ret->dot_atmostone, sstate->dot_atmostone, DOT_COUNT(state));
-
-    ret->dot_atleastone = snewn(DOT_COUNT(state), char);
-    memcpy(ret->dot_atleastone, sstate->dot_atleastone, DOT_COUNT(state));
-
-#if 0
-    ret->dline_identical = snewn((state->w + 1) * (state->h + 1), char);
-    memcpy(ret->dline_identical, state->dot_atmostone, 
-           (state->w + 1) * (state->h + 1));
-#endif
-
     ret->recursion_remaining = sstate->recursion_remaining;
     ret->solver_status = sstate->solver_status;
 
     ret->dotdsf = snewn(DOT_COUNT(state), int);
     ret->looplen = snewn(DOT_COUNT(state), int);
-    memcpy(ret->dotdsf, sstate->dotdsf, DOT_COUNT(state) * sizeof(int));
-    memcpy(ret->looplen, sstate->looplen, DOT_COUNT(state) * sizeof(int));
-
-    return ret;
-}
-
-/*
- * Merge two dots due to the existence of an edge between them.
- * Updates the dsf tracking equivalence classes, and keeps track of
- * the length of path each dot is currently a part of.
- */
-static void merge_dots(solver_state *sstate, int x1, int y1, int x2, int y2)
-{
-    int i, j, len;
-
-    i = y1 * (sstate->state->w + 1) + x1;
-    j = y2 * (sstate->state->w + 1) + x2;
-
-    i = dsf_canonify(sstate->dotdsf, i);
-    j = dsf_canonify(sstate->dotdsf, j);
-
-    if (i != j) {
-       len = sstate->looplen[i] + sstate->looplen[j];
-       dsf_merge(sstate->dotdsf, i, j);
-       i = dsf_canonify(sstate->dotdsf, i);
-       sstate->looplen[i] = len;
-    }
-}
-
-/* Count the number of lines of a particular type currently going into the
- * given dot.  Lines going off the edge of the board are assumed fixed no. */
-static int dot_order(const game_state* state, int i, int j, char line_type)
-{
-    int n = 0;
-
-    if (i > 0) {
-        if (LEFTOF_DOT(state, i, j) == line_type)
-            ++n;
-    } else {
-        if (line_type == LINE_NO)
-            ++n;
-    }
-    if (i < state->w) {
-        if (RIGHTOF_DOT(state, i, j) == line_type)
-            ++n;
-    } else {
-        if (line_type == LINE_NO)
-            ++n;
-    }
-    if (j > 0) {
-        if (ABOVE_DOT(state, i, j) == line_type)
-            ++n;
+    memcpy(ret->dotdsf, sstate->dotdsf, 
+           DOT_COUNT(state) * sizeof(int));
+    memcpy(ret->looplen, sstate->looplen, 
+           DOT_COUNT(state) * sizeof(int));
+
+    ret->dot_solved = snewn(DOT_COUNT(state), char);
+    ret->square_solved = snewn(SQUARE_COUNT(state), char);
+    memcpy(ret->dot_solved, sstate->dot_solved, 
+           DOT_COUNT(state));
+    memcpy(ret->square_solved, sstate->square_solved, 
+           SQUARE_COUNT(state));
+
+    ret->dot_yescount = snewn(DOT_COUNT(state), char);
+    memcpy(ret->dot_yescount, sstate->dot_yescount,
+           DOT_COUNT(state));
+    ret->dot_nocount = snewn(DOT_COUNT(state), char);
+    memcpy(ret->dot_nocount, sstate->dot_nocount,
+           DOT_COUNT(state));
+
+    ret->square_yescount = snewn(SQUARE_COUNT(state), char);
+    memcpy(ret->square_yescount, sstate->square_yescount,
+           SQUARE_COUNT(state));
+    ret->square_nocount = snewn(SQUARE_COUNT(state), char);
+    memcpy(ret->square_nocount, sstate->square_nocount,
+           SQUARE_COUNT(state));
+
+    if (sstate->normal) {
+        ret->normal = snew(normal_mode_state);
+        ret->normal->dot_atmostone = snewn(DOT_COUNT(state), char);
+        memcpy(ret->normal->dot_atmostone, sstate->normal->dot_atmostone,
+               DOT_COUNT(state));
+
+        ret->normal->dot_atleastone = snewn(DOT_COUNT(state), char);
+        memcpy(ret->normal->dot_atleastone, sstate->normal->dot_atleastone,
+               DOT_COUNT(state));
     } else {
-        if (line_type == LINE_NO)
-            ++n;
+        ret->normal = NULL;
     }
-    if (j < state->h) {
-        if (BELOW_DOT(state, i, j) == line_type)
-            ++n;
+
+    if (sstate->hard) {
+        ret->hard = snew(hard_mode_state);
+        ret->hard->linedsf = snewn(LINE_COUNT(state), int);
+        memcpy(ret->hard->linedsf, sstate->hard->linedsf, 
+               LINE_COUNT(state) * sizeof(int));
     } else {
-        if (line_type == LINE_NO)
-            ++n;
+        ret->hard = NULL;
     }
 
-    return n;
-}
-/* Count the number of lines of a particular type currently surrounding the
- * given square */
-static int square_order(const game_state* state, int i, int j, char line_type)
-{
-    int n = 0;
-
-    if (ABOVE_SQUARE(state, i, j) == line_type)
-        ++n;
-    if (BELOW_SQUARE(state, i, j) == line_type)
-        ++n;
-    if (LEFTOF_SQUARE(state, i, j) == line_type)
-        ++n;
-    if (RIGHTOF_SQUARE(state, i, j) == line_type)
-        ++n;
-
-    return n;
-}
-
-/* Set all lines bordering a dot of type old_type to type new_type */
-static void dot_setall(game_state *state, int i, int j,
-                       char old_type, char new_type)
-{
-/*    printf("dot_setall([%d,%d], %d, %d)\n", i, j, old_type, new_type); */
-    if (i > 0        && LEFTOF_DOT(state, i, j) == old_type)
-        LV_LEFTOF_DOT(state, i, j) = new_type;
-    if (i < state->w && RIGHTOF_DOT(state, i, j) == old_type)
-        LV_RIGHTOF_DOT(state, i, j) = new_type;
-    if (j > 0        && ABOVE_DOT(state, i, j) == old_type)
-        LV_ABOVE_DOT(state, i, j) = new_type;
-    if (j < state->h && BELOW_DOT(state, i, j) == old_type)
-        LV_BELOW_DOT(state, i, j) = new_type;
-}
-/* Set all lines bordering a square of type old_type to type new_type */
-static void square_setall(game_state *state, int i, int j,
-                          char old_type, char new_type)
-{
-    if (ABOVE_SQUARE(state, i, j) == old_type)
-        ABOVE_SQUARE(state, i, j) = new_type;
-    if (BELOW_SQUARE(state, i, j) == old_type)
-        BELOW_SQUARE(state, i, j) = new_type;
-    if (LEFTOF_SQUARE(state, i, j) == old_type)
-        LEFTOF_SQUARE(state, i, j) = new_type;
-    if (RIGHTOF_SQUARE(state, i, j) == old_type)
-        RIGHTOF_SQUARE(state, i, j) = new_type;
+    return ret;
 }
 
 static game_params *default_params(void)
@@ -380,6 +485,7 @@ static game_params *default_params(void)
     ret->h = 10;
     ret->w = 10;
 #endif
+    ret->diff = DIFF_EASY;
     ret->rec = 0;
 
     return ret;
@@ -392,32 +498,41 @@ static game_params *dup_params(game_params *params)
     return ret;
 }
 
-static const struct {
-    char *desc;
-    game_params params;
-} loopy_presets[] = {
-    { "4x4 Easy",   {  4,  4, 0 } },
-    { "4x4 Hard",   {  4,  4, 2 } },
-    { "7x7 Easy",   {  7,  7, 0 } },
-    { "7x7 Hard",   {  7,  7, 2 } },
-    { "10x10 Easy", { 10, 10, 0 } },
+static const game_params presets[] = {
+    {  4,  4, DIFF_EASY, 0 },
+    {  4,  4, DIFF_NORMAL, 0 },
+    {  4,  4, DIFF_HARD, 0 },
+    {  7,  7, DIFF_EASY, 0 },
+    {  7,  7, DIFF_NORMAL, 0 },
+    {  7,  7, DIFF_HARD, 0 },
+    { 10, 10, DIFF_EASY, 0 },
+    { 10, 10, DIFF_NORMAL, 0 },
+    { 10, 10, DIFF_HARD, 0 },
 #ifndef SLOW_SYSTEM
-    { "10x10 Hard", { 10, 10, 2 } },
-    { "15x15 Easy", { 15, 15, 0 } },
-    { "30x20 Easy", { 30, 20, 0 } }
+    { 15, 15, DIFF_EASY, 0 },
+    { 15, 15, DIFF_NORMAL, 0 },
+    { 15, 15, DIFF_HARD, 0 },
+#ifndef SMALL_SCREEN
+    { 30, 20, DIFF_EASY, 0 },
+    { 30, 20, DIFF_NORMAL, 0 },
+    { 30, 20, DIFF_HARD, 0 }
+#endif
 #endif
 };
 
 static int game_fetch_preset(int i, char **name, game_params **params)
 {
-    game_params tmppar;
+    game_params *tmppar;
+    char buf[80];
 
-    if (i < 0 || i >= lenof(loopy_presets))
+    if (i < 0 || i >= lenof(presets))
         return FALSE;
 
-    tmppar = loopy_presets[i].params;
-    *params = dup_params(&tmppar);
-    *name = dupstr(loopy_presets[i].desc);
+    tmppar = snew(game_params);
+    *tmppar = presets[i];
+    *params = tmppar;
+    sprintf(buf, "%dx%d %s", tmppar->h, tmppar->w, diffnames[tmppar->diff]);
+    *name = dupstr(buf);
 
     return TRUE;
 }
@@ -431,16 +546,25 @@ static void decode_params(game_params *params, char const *string)
 {
     params->h = params->w = atoi(string);
     params->rec = 0;
+    params->diff = DIFF_EASY;
     while (*string && isdigit((unsigned char)*string)) string++;
     if (*string == 'x') {
         string++;
         params->h = atoi(string);
-       while (*string && isdigit((unsigned char)*string)) string++;
+        while (*string && isdigit((unsigned char)*string)) string++;
     }
     if (*string == 'r') {
         string++;
         params->rec = atoi(string);
-       while (*string && isdigit((unsigned char)*string)) string++;
+        while (*string && isdigit((unsigned char)*string)) string++;
+    }
+    if (*string == 'd') {
+        int i;
+        string++;
+        for (i = 0; i < DIFF_MAX; i++)
+            if (*string == diffchars[i])
+                params->diff = i;
+        if (*string) string++;
     }
 }
 
@@ -449,7 +573,7 @@ static char *encode_params(game_params *params, int full)
     char str[80];
     sprintf(str, "%dx%d", params->w, params->h);
     if (full)
-       sprintf(str + strlen(str), "r%d", params->rec);
+    sprintf(str + strlen(str), "r%dd%c", params->rec, diffchars[params->diff]);
     return dupstr(str);
 }
 
@@ -472,11 +596,10 @@ static config_item *game_configure(game_params *params)
     ret[1].sval = dupstr(buf);
     ret[1].ival = 0;
 
-    ret[2].name = "Recursion depth";
-    ret[2].type = C_STRING;
-    sprintf(buf, "%d", params->rec);
-    ret[2].sval = dupstr(buf);
-    ret[2].ival = 0;
+    ret[2].name = "Difficulty";
+    ret[2].type = C_CHOICES;
+    ret[2].sval = DIFFCONFIG;
+    ret[2].ival = params->diff;
 
     ret[3].name = NULL;
     ret[3].type = C_END;
@@ -492,7 +615,8 @@ static game_params *custom_params(config_item *cfg)
 
     ret->w = atoi(cfg[0].sval);
     ret->h = atoi(cfg[1].sval);
-    ret->rec = atoi(cfg[2].sval);
+    ret->rec = 0;
+    ret->diff = cfg[2].ival;
 
     return ret;
 }
@@ -503,1390 +627,2584 @@ static char *validate_params(game_params *params, int full)
         return "Width and height must both be at least 4";
     if (params->rec < 0)
         return "Recursion depth can't be negative";
+
+    /*
+     * This shouldn't be able to happen at all, since decode_params
+     * and custom_params will never generate anything that isn't
+     * within range.
+     */
+    assert(params->diff < DIFF_MAX);
+
     return NULL;
 }
 
-/* We're going to store a list of current candidate squares for lighting.
- * Each square gets a 'score', which tells us how adding that square right
- * now would affect the length of the solution loop.  We're trying to
- * maximise that quantity so will bias our random selection of squares to
- * light towards those with high scores */
-struct square { 
-    int score;
-    unsigned long random;
-    int x, y;
-};
-
-static int get_square_cmpfn(void *v1, void *v2) 
+/* Returns a newly allocated string describing the current puzzle */
+static char *state_to_text(const game_state *state)
 {
-    struct square *s1 = (struct square *)v1;
-    struct square *s2 = (struct square *)v2;
-    int r;
-    
-    r = s1->x - s2->x;
-    if (r)
-        return r;
+    char *retval;
+    char *description = snewn(SQUARE_COUNT(state) + 1, char);
+    char *dp = description;
+    int empty_count = 0;
+    int i, j;
 
-    r = s1->y - s2->y;
-    if (r)
-        return r;
+    FORALL_SQUARES(state, i, j) {
+        if (CLUE_AT(state, i, j) < 0) {
+            if (empty_count > 25) {
+                dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
+                empty_count = 0;
+            }
+            empty_count++;
+        } else {
+            if (empty_count) {
+                dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
+                empty_count = 0;
+            }
+            dp += sprintf(dp, "%c", (int)CLUE2CHAR(CLUE_AT(state, i, j)));
+        }
+    }
 
-    return 0;
-}
+    if (empty_count)
+        dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
 
-static int square_sort_cmpfn(void *v1, void *v2)
+    retval = dupstr(description);
+    sfree(description);
+
+    return retval;
+}
+
+/* We require that the params pass the test in validate_params and that the
+ * description fills the entire game area */
+static char *validate_desc(game_params *params, char *desc)
+{
+    int count = 0;
+
+    for (; *desc; ++desc) {
+        if (*desc >= '0' && *desc <= '9') {
+            count++;
+            continue;
+        }
+        if (*desc >= 'a') {
+            count += *desc - 'a' + 1;
+            continue;
+        }
+        return "Unknown character in description";
+    }
+
+    if (count < SQUARE_COUNT(params))
+        return "Description too short for board size";
+    if (count > SQUARE_COUNT(params))
+        return "Description too long for board size";
+
+    return NULL;
+}
+
+/* Sums the lengths of the numbers in range [0,n) */
+/* See equivalent function in solo.c for justification of this. */
+static int len_0_to_n(int n)
+{
+    int len = 1; /* Counting 0 as a bit of a special case */
+    int i;
+
+    for (i = 1; i < n; i *= 10) {
+        len += max(n - i, 0);
+    }
+
+    return len;
+}
+
+static char *encode_solve_move(const game_state *state)
+{
+    int len, i, j;
+    char *ret, *p;
+    /* This is going to return a string representing the moves needed to set
+     * every line in a grid to be the same as the ones in 'state'.  The exact
+     * length of this string is predictable. */
+
+    len = 1;  /* Count the 'S' prefix */
+    /* Numbers in horizontal lines */
+    /* Horizontal lines, x position */
+    len += len_0_to_n(state->w) * (state->h + 1);
+    /* Horizontal lines, y position */
+    len += len_0_to_n(state->h + 1) * (state->w);
+    /* Vertical lines, y position */
+    len += len_0_to_n(state->h) * (state->w + 1);
+    /* Vertical lines, x position */
+    len += len_0_to_n(state->w + 1) * (state->h);
+    /* For each line we also have two letters and a comma */
+    len += 3 * (LINE_COUNT(state));
+
+    ret = snewn(len + 1, char);
+    p = ret;
+
+    p += sprintf(p, "S");
+
+    FORALL_HL(state, i, j) {
+        switch (RIGHTOF_DOT(state, i, j)) {
+            case LINE_YES:
+                p += sprintf(p, "%d,%dhy", i, j);
+                break;
+            case LINE_NO:
+                p += sprintf(p, "%d,%dhn", i, j);
+                break;
+        }
+    }
+
+    FORALL_VL(state, i, j) {
+        switch (BELOW_DOT(state, i, j)) {
+            case LINE_YES:
+                p += sprintf(p, "%d,%dvy", i, j);
+                break;
+            case LINE_NO:
+                p += sprintf(p, "%d,%dvn", i, j);
+                break;
+        }
+    }
+
+    /* No point in doing sums like that if they're going to be wrong */
+    assert(strlen(ret) <= (size_t)len);
+    return ret;
+}
+
+static game_ui *new_ui(game_state *state)
+{
+    return NULL;
+}
+
+static void free_ui(game_ui *ui)
+{
+}
+
+static char *encode_ui(game_ui *ui)
+{
+    return NULL;
+}
+
+static void decode_ui(game_ui *ui, char *encoding)
+{
+}
+
+static void game_changed_state(game_ui *ui, game_state *oldstate,
+                               game_state *newstate)
+{
+}
+
+#define SIZE(d) ((d) * TILE_SIZE + 2 * BORDER + 1)
+
+static void game_compute_size(game_params *params, int tilesize,
+                              int *x, int *y)
+{
+    struct { int tilesize; } ads, *ds = &ads;
+    ads.tilesize = tilesize;
+
+    *x = SIZE(params->w);
+    *y = SIZE(params->h);
+}
+
+static void game_set_size(drawing *dr, game_drawstate *ds,
+              game_params *params, int tilesize)
+{
+    ds->tilesize = tilesize;
+    ds->linewidth = max(1,tilesize/16);
+}
+
+static float *game_colours(frontend *fe, int *ncolours)
+{
+    float *ret = snewn(4 * NCOLOURS, float);
+
+    frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);
+
+    ret[COL_FOREGROUND * 3 + 0] = 0.0F;
+    ret[COL_FOREGROUND * 3 + 1] = 0.0F;
+    ret[COL_FOREGROUND * 3 + 2] = 0.0F;
+
+    ret[COL_HIGHLIGHT * 3 + 0] = 1.0F;
+    ret[COL_HIGHLIGHT * 3 + 1] = 1.0F;
+    ret[COL_HIGHLIGHT * 3 + 2] = 1.0F;
+
+    ret[COL_MISTAKE * 3 + 0] = 1.0F;
+    ret[COL_MISTAKE * 3 + 1] = 0.0F;
+    ret[COL_MISTAKE * 3 + 2] = 0.0F;
+
+    *ncolours = NCOLOURS;
+    return ret;
+}
+
+static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
+{
+    struct game_drawstate *ds = snew(struct game_drawstate);
+
+    ds->tilesize = ds->linewidth = 0;
+    ds->started = 0;
+    ds->hl = snewn(HL_COUNT(state), char);
+    ds->vl = snewn(VL_COUNT(state), char);
+    ds->clue_error = snewn(SQUARE_COUNT(state), char);
+    ds->flashing = 0;
+
+    memset(ds->hl, LINE_UNKNOWN, HL_COUNT(state));
+    memset(ds->vl, LINE_UNKNOWN, VL_COUNT(state));
+    memset(ds->clue_error, 0, SQUARE_COUNT(state));
+
+    return ds;
+}
+
+static void game_free_drawstate(drawing *dr, game_drawstate *ds)
+{
+    sfree(ds->clue_error);
+    sfree(ds->hl);
+    sfree(ds->vl);
+    sfree(ds);
+}
+
+static int game_timing_state(game_state *state, game_ui *ui)
+{
+    return TRUE;
+}
+
+static float game_anim_length(game_state *oldstate, game_state *newstate,
+                              int dir, game_ui *ui)
+{
+    return 0.0F;
+}
+
+static char *game_text_format(game_state *state)
+{
+    int i, j;
+    int len;
+    char *ret, *rp;
+
+    len = (2 * state->w + 2) * (2 * state->h + 1);
+    rp = ret = snewn(len + 1, char);
+    
+#define DRAW_HL \
+    switch (ABOVE_SQUARE(state, i, j)) { \
+        case LINE_YES: \
+            rp += sprintf(rp, " -"); \
+            break; \
+        case LINE_NO: \
+            rp += sprintf(rp, " x"); \
+            break; \
+        case LINE_UNKNOWN: \
+            rp += sprintf(rp, "  "); \
+            break; \
+        default: \
+            assert(!"Illegal line state for HL"); \
+    }
+
+#define DRAW_VL \
+    switch (LEFTOF_SQUARE(state, i, j)) { \
+        case LINE_YES: \
+            rp += sprintf(rp, "|"); \
+            break; \
+        case LINE_NO: \
+            rp += sprintf(rp, "x"); \
+            break; \
+        case LINE_UNKNOWN: \
+            rp += sprintf(rp, " "); \
+            break; \
+        default: \
+            assert(!"Illegal line state for VL"); \
+    }
+    
+    for (j = 0; j < state->h; ++j) {
+        for (i = 0; i < state->w; ++i) {
+            DRAW_HL;
+        }
+        rp += sprintf(rp, " \n");
+        for (i = 0; i < state->w; ++i) {
+            DRAW_VL;
+            rp += sprintf(rp, "%c", (int)CLUE2CHAR(CLUE_AT(state, i, j)));
+        }
+        DRAW_VL;
+        rp += sprintf(rp, "\n");
+    }
+    for (i = 0; i < state->w; ++i) {
+        DRAW_HL;
+    }
+    rp += sprintf(rp, " \n");
+    
+    assert(strlen(ret) == len);
+    return ret;
+}
+
+/* ----------------------------------------------------------------------
+ * Debug code
+ */
+
+#ifdef DEBUG_CACHES
+static void check_caches(const solver_state* sstate)
+{
+    int i, j;
+    const game_state *state = sstate->state;
+
+    FORALL_DOTS(state, i, j) {
+#if 0
+        fprintf(stderr, "dot [%d,%d] y: %d %d n: %d %d\n", i, j,
+               dot_order(state, i, j, LINE_YES),
+               sstate->dot_yescount[i + (state->w + 1) * j],
+               dot_order(state, i, j, LINE_NO),
+               sstate->dot_nocount[i + (state->w + 1) * j]);
+#endif
+                    
+        assert(dot_order(state, i, j, LINE_YES) ==
+               DOT_YES_COUNT(sstate, i, j));
+        assert(dot_order(state, i, j, LINE_NO) ==
+               DOT_NO_COUNT(sstate, i, j));
+    }
+
+    FORALL_SQUARES(state, i, j) {
+#if 0
+        fprintf(stderr, "square [%d,%d] y: %d %d n: %d %d\n", i, j,
+               square_order(state, i, j, LINE_YES),
+               sstate->square_yescount[i + state->w * j],
+               square_order(state, i, j, LINE_NO),
+               sstate->square_nocount[i + state->w * j]);
+#endif
+                    
+        assert(square_order(state, i, j, LINE_YES) ==
+               SQUARE_YES_COUNT(sstate, i, j));
+        assert(square_order(state, i, j, LINE_NO) ==
+               SQUARE_NO_COUNT(sstate, i, j));
+    }
+}
+
+#if 0
+#define check_caches(s) \
+    do { \
+        fprintf(stderr, "check_caches at line %d\n", __LINE__); \
+        check_caches(s); \
+    } while (0)
+#endif
+#endif /* DEBUG_CACHES */
+
+/* ----------------------------------------------------------------------
+ * Solver utility functions
+ */
+
+static int set_line_bydot(solver_state *sstate, int x, int y, enum direction d,
+                          enum line_state line_new
+#ifdef SHOW_WORKING
+                          , const char *reason
+#endif
+                          ) 
+{
+    game_state *state = sstate->state;
+
+    /* This line borders at most two squares in our board.  We figure out the
+     * x and y positions of those squares so we can record that their yes or no
+     * counts have been changed */
+    int sq1_x=-1, sq1_y=-1, sq2_x=-1, sq2_y=-1;
+    int otherdot_x=-1, otherdot_y=-1;
+
+    int progress = FALSE;
+
+#if 0
+    fprintf(stderr, "set_line_bydot [%d,%d], %s, %d\n",
+            x, y, DIR2STR(d), line_new);
+#endif
+
+    assert(line_new != LINE_UNKNOWN);
+
+    check_caches(sstate);
+
+    switch (d) {
+        case LEFT:
+            assert(x > 0);
+
+            if (LEFTOF_DOT(state, x, y) != line_new) {
+                LV_LEFTOF_DOT(state, x, y) = line_new;
+
+                otherdot_x = x-1;
+                otherdot_y = y;
+
+                sq1_x = x-1;
+                sq1_y = y-1;
+                sq2_x = x-1;
+                sq2_y = y;
+
+                progress = TRUE;
+            }
+            break;
+        case RIGHT:
+            assert(x < state->w);
+            if (RIGHTOF_DOT(state, x, y) != line_new) {
+                LV_RIGHTOF_DOT(state, x, y) = line_new;
+
+                otherdot_x = x+1;
+                otherdot_y = y;
+
+                sq1_x = x;
+                sq1_y = y-1;
+                sq2_x = x;
+                sq2_y = y;
+
+                progress = TRUE;
+            }
+            break;
+        case UP:
+            assert(y > 0);
+            if (ABOVE_DOT(state, x, y) != line_new) {
+                LV_ABOVE_DOT(state, x, y) = line_new;
+
+                otherdot_x = x;
+                otherdot_y = y-1;
+
+                sq1_x = x-1;
+                sq1_y = y-1;
+                sq2_x = x;
+                sq2_y = y-1;
+
+                progress = TRUE;
+            }
+            break;
+        case DOWN:
+            assert(y < state->h);
+            if (BELOW_DOT(state, x, y) != line_new) {
+                LV_BELOW_DOT(state, x, y) = line_new;
+
+                otherdot_x = x;
+                otherdot_y = y+1;
+
+                sq1_x = x-1;
+                sq1_y = y;
+                sq2_x = x;
+                sq2_y = y;
+
+                progress = TRUE;
+            }
+            break;
+    }
+
+    if (!progress)
+        return progress;
+
+#ifdef SHOW_WORKING
+    fprintf(stderr, "set line [%d,%d] -> [%d,%d] to %s (%s)\n",
+            x, y, otherdot_x, otherdot_y, line_new == LINE_YES ? "YES" : "NO",
+            reason);
+#endif
+
+    /* Above we updated the cache for the dot that the line in question reaches
+     * from the dot we've been told about.  Here we update that for the dot
+     * named in our arguments. */
+    if (line_new == LINE_YES) {
+        if (sq1_x >= 0 && sq1_y >= 0)
+            ++SQUARE_YES_COUNT(sstate, sq1_x, sq1_y);
+        if (sq2_x < state->w && sq2_y < state->h)
+            ++SQUARE_YES_COUNT(sstate, sq2_x, sq2_y);
+        ++DOT_YES_COUNT(sstate, x, y);
+        ++DOT_YES_COUNT(sstate, otherdot_x, otherdot_y);
+    } else {
+        if (sq1_x >= 0 && sq1_y >= 0)
+            ++SQUARE_NO_COUNT(sstate, sq1_x, sq1_y);
+        if (sq2_x < state->w && sq2_y < state->h)
+            ++SQUARE_NO_COUNT(sstate, sq2_x, sq2_y);
+        ++DOT_NO_COUNT(sstate, x, y);
+        ++DOT_NO_COUNT(sstate, otherdot_x, otherdot_y);
+    }
+    
+    check_caches(sstate);
+    return progress;
+}
+
+#ifdef SHOW_WORKING
+#define set_line_bydot(a, b, c, d, e) \
+    set_line_bydot(a, b, c, d, e, __FUNCTION__)
+#endif
+
+/*
+ * Merge two dots due to the existence of an edge between them.
+ * Updates the dsf tracking equivalence classes, and keeps track of
+ * the length of path each dot is currently a part of.
+ * Returns TRUE if the dots were already linked, ie if they are part of a
+ * closed loop, and false otherwise.
+ */
+static int merge_dots(solver_state *sstate, int x1, int y1, int x2, int y2)
+{
+    int i, j, len;
+
+    i = y1 * (sstate->state->w + 1) + x1;
+    j = y2 * (sstate->state->w + 1) + x2;
+
+    i = dsf_canonify(sstate->dotdsf, i);
+    j = dsf_canonify(sstate->dotdsf, j);
+
+    if (i == j) {
+        return TRUE;
+    } else {
+        len = sstate->looplen[i] + sstate->looplen[j];
+        dsf_merge(sstate->dotdsf, i, j);
+        i = dsf_canonify(sstate->dotdsf, i);
+        sstate->looplen[i] = len;
+        return FALSE;
+    }
+}
+
+/* Seriously, these should be functions */
+
+#define LINEDSF_INDEX(state, x, y, d) \
+   ((d == UP)    ? ((y-1) * (state->w + 1) + x) : \
+    (d == DOWN)  ? ((y)   * (state->w + 1) + x) : \
+    (d == LEFT)  ? ((y) * (state->w) + x-1 + VL_COUNT(state)) : \
+    (d == RIGHT) ? ((y) * (state->w) + x   + VL_COUNT(state)) : \
+    (assert(!"bad direction value"), 0))
+
+static void linedsf_deindex(const game_state *state, int i, 
+                            int *px, int *py, enum direction *pd)
+{
+    int i_mod;
+    if (i < VL_COUNT(state)) {
+        *(pd) = DOWN;
+        *(px) = (i) % (state->w+1);
+        *(py) = (i) / (state->w+1);
+    } else {
+        i_mod = i - VL_COUNT(state);
+        *(pd) = RIGHT;
+        *(px) = (i_mod) % (state->w);
+        *(py) = (i_mod) / (state->w);
+    }
+}
+
+/* Merge two lines because the solver has deduced that they must be either
+ * identical or opposite.   Returns TRUE if this is new information, otherwise
+ * FALSE. */
+static int merge_lines(solver_state *sstate, 
+                       int x1, int y1, enum direction d1,
+                       int x2, int y2, enum direction d2,
+                       int inverse
+#ifdef SHOW_WORKING
+                       , const char *reason
+#endif
+                      )
+{
+    int i, j, inv_tmp;
+
+    i = LINEDSF_INDEX(sstate->state, x1, y1, d1);
+    j = LINEDSF_INDEX(sstate->state, x2, y2, d2);
+
+    assert(i < LINE_COUNT(sstate->state));
+    assert(j < LINE_COUNT(sstate->state));
+    
+    i = edsf_canonify(sstate->hard->linedsf, i, &inv_tmp);
+    inverse ^= inv_tmp;
+    j = edsf_canonify(sstate->hard->linedsf, j, &inv_tmp);
+    inverse ^= inv_tmp;
+
+    edsf_merge(sstate->hard->linedsf, i, j, inverse);
+
+#ifdef SHOW_WORKING
+    if (i != j) {
+        fprintf(stderr, "%s [%d,%d,%s] [%d,%d,%s] %s(%s)\n",
+                __FUNCTION__, 
+                x1, y1, DIR2STR(d1),
+                x2, y2, DIR2STR(d2),
+                inverse ? "inverse " : "", reason);
+    }
+#endif
+    return (i != j);
+}
+
+#ifdef SHOW_WORKING
+#define merge_lines(a, b, c, d, e, f, g, h) \
+    merge_lines(a, b, c, d, e, f, g, h, __FUNCTION__)
+#endif
+
+/* Return 0 if the given lines are not in the same equivalence class, 1 if they
+ * are known identical, or 2 if they are known opposite */
+#if 0
+static int lines_related(solver_state *sstate,
+                         int x1, int y1, enum direction d1, 
+                         int x2, int y2, enum direction d2)
+{
+    int i, j, inv1, inv2;
+
+    i = LINEDSF_INDEX(sstate->state, x1, y1, d1);
+    j = LINEDSF_INDEX(sstate->state, x2, y2, d2);
+  
+    i = edsf_canonify(sstate->hard->linedsf, i, &inv1);
+    j = edsf_canonify(sstate->hard->linedsf, j, &inv2);
+
+    if (i == j)
+        return (inv1 == inv2) ? 1 : 2;
+    else
+        return 0;
+}
+#endif
+
+/* Count the number of lines of a particular type currently going into the
+ * given dot.  Lines going off the edge of the board are assumed fixed no. */
+static int dot_order(const game_state* state, int i, int j, char line_type)
+{
+    int n = 0;
+
+    if (i > 0) {
+        if (line_type == LV_LEFTOF_DOT(state, i, j))
+            ++n;
+    } else {
+        if (line_type == LINE_NO)
+            ++n;
+    }
+    if (i < state->w) {
+        if (line_type == LV_RIGHTOF_DOT(state, i, j))
+            ++n;
+    } else {
+        if (line_type == LINE_NO)
+            ++n;
+    }
+    if (j > 0) {
+        if (line_type == LV_ABOVE_DOT(state, i, j))
+            ++n;
+    } else {
+        if (line_type == LINE_NO)
+            ++n;
+    }
+    if (j < state->h) {
+        if (line_type == LV_BELOW_DOT(state, i, j))
+            ++n;
+    } else {
+        if (line_type == LINE_NO)
+            ++n;
+    }
+
+    return n;
+}
+
+/* Count the number of lines of a particular type currently surrounding the
+ * given square */
+static int square_order(const game_state* state, int i, int j, char line_type)
+{
+    int n = 0;
+
+    if (ABOVE_SQUARE(state, i, j) == line_type)
+        ++n;
+    if (BELOW_SQUARE(state, i, j) == line_type)
+        ++n;
+    if (LEFTOF_SQUARE(state, i, j) == line_type)
+        ++n;
+    if (RIGHTOF_SQUARE(state, i, j) == line_type)
+        ++n;
+
+    return n;
+}
+
+/* Set all lines bordering a dot of type old_type to type new_type 
+ * Return value tells caller whether this function actually did anything */
+static int dot_setall(solver_state *sstate, int i, int j,
+                       char old_type, char new_type)
+{
+    int retval = FALSE, r;
+    game_state *state = sstate->state;
+    
+    if (old_type == new_type)
+        return FALSE;
+
+    if (i > 0        && LEFTOF_DOT(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, LEFT, new_type);
+        assert(r == TRUE);
+        retval = TRUE;
+    }
+
+    if (i < state->w && RIGHTOF_DOT(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, RIGHT, new_type);
+        assert(r == TRUE);
+        retval = TRUE;
+    }
+
+    if (j > 0        && ABOVE_DOT(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, UP, new_type);
+        assert(r == TRUE);
+        retval = TRUE;
+    }
+
+    if (j < state->h && BELOW_DOT(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, DOWN, new_type);
+        assert(r == TRUE);
+        retval = TRUE;
+    }
+
+    return retval;
+}
+
+/* Set all lines bordering a square of type old_type to type new_type */
+static int square_setall(solver_state *sstate, int i, int j,
+                         char old_type, char new_type)
+{
+    int r = FALSE;
+    game_state *state = sstate->state;
+
+#if 0
+    fprintf(stderr, "square_setall [%d,%d] from %d to %d\n", i, j,
+                    old_type, new_type);
+#endif
+    if (ABOVE_SQUARE(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, RIGHT, new_type);
+        assert(r == TRUE);
+    }
+    if (BELOW_SQUARE(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j+1, RIGHT, new_type);
+        assert(r == TRUE);
+    }
+    if (LEFTOF_SQUARE(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i, j, DOWN, new_type);
+        assert(r == TRUE);
+    }
+    if (RIGHTOF_SQUARE(state, i, j) == old_type) {
+        r = set_line_bydot(sstate, i+1, j, DOWN, new_type);
+        assert(r == TRUE);
+    }
+
+    return r;
+}
+
+/* ----------------------------------------------------------------------
+ * Loop generation and clue removal
+ */
+
+/* We're going to store a list of current candidate squares for lighting.
+ * Each square gets a 'score', which tells us how adding that square right
+ * now would affect the length of the solution loop.  We're trying to
+ * maximise that quantity so will bias our random selection of squares to
+ * light towards those with high scores */
+struct square { 
+    int score;
+    unsigned long random;
+    int x, y;
+};
+
+static int get_square_cmpfn(void *v1, void *v2) 
+{
+    struct square *s1 = v1;
+    struct square *s2 = v2;
+    int r;
+    
+    r = s1->x - s2->x;
+    if (r)
+        return r;
+
+    r = s1->y - s2->y;
+    if (r)
+        return r;
+
+    return 0;
+}
+
+static int square_sort_cmpfn(void *v1, void *v2)
 {
-    struct square *s1 = (struct square *)v1;
-    struct square *s2 = (struct square *)v2;
+    struct square *s1 = v1;
+    struct square *s2 = v2;
     int r;
 
-    r = s2->score - s1->score;
-    if (r) {
-        return r;
+    r = s2->score - s1->score;
+    if (r) {
+        return r;
+    }
+
+    if (s1->random < s2->random)
+        return -1;
+    else if (s1->random > s2->random)
+        return 1;
+
+    /*
+     * It's _just_ possible that two squares might have been given
+     * the same random value. In that situation, fall back to
+     * comparing based on the coordinates. This introduces a tiny
+     * directional bias, but not a significant one.
+     */
+    return get_square_cmpfn(v1, v2);
+}
+
+enum { SQUARE_LIT, SQUARE_UNLIT };
+
+#define SQUARE_STATE(i, j) \
+    ( LEGAL_SQUARE(state, i, j) ? \
+        LV_SQUARE_STATE(i,j) : \
+        SQUARE_UNLIT )
+
+#define LV_SQUARE_STATE(i, j) board[SQUARE_INDEX(state, i, j)]
+
+/* Generate a new complete set of clues for the given game_state (respecting
+ * the dimensions provided by said game_state) */
+static void add_full_clues(game_state *state, random_state *rs)
+{
+    char *clues;
+    char *board;
+    int i, j, a, b, c;
+    int board_area = SQUARE_COUNT(state);
+    int t;
+
+    struct square *square, *tmpsquare, *sq;
+    struct square square_pos;
+
+    /* These will contain exactly the same information, sorted into different
+     * orders */
+    tree234 *lightable_squares_sorted, *lightable_squares_gettable;
+
+#define SQUARE_REACHABLE(i,j) \
+     (t = (SQUARE_STATE(i-1, j) == SQUARE_LIT || \
+           SQUARE_STATE(i+1, j) == SQUARE_LIT || \
+           SQUARE_STATE(i, j-1) == SQUARE_LIT || \
+           SQUARE_STATE(i, j+1) == SQUARE_LIT), \
+      t)
+
+    /* One situation in which we may not light a square is if that'll leave one
+     * square above/below and one left/right of us unlit, separated by a lit
+     * square diagnonal from us */
+#define SQUARE_DIAGONAL_VIOLATION(i, j, h, v) \
+    (t = (SQUARE_STATE((i)+(h), (j))     == SQUARE_UNLIT && \
+          SQUARE_STATE((i),     (j)+(v)) == SQUARE_UNLIT && \
+          SQUARE_STATE((i)+(h), (j)+(v)) == SQUARE_LIT), \
+     t)
+
+    /* We also may not light a square if it will form a loop of lit squares
+     * around some unlit squares, as then the game soln won't have a single
+     * loop */
+#define SQUARE_LOOP_VIOLATION(i, j, lit1, lit2) \
+    (SQUARE_STATE((i)+1, (j)) == lit1    && \
+     SQUARE_STATE((i)-1, (j)) == lit1    && \
+     SQUARE_STATE((i), (j)+1) == lit2    && \
+     SQUARE_STATE((i), (j)-1) == lit2)
+
+#define CAN_LIGHT_SQUARE(i, j) \
+    (SQUARE_REACHABLE(i, j)                                 && \
+     !SQUARE_DIAGONAL_VIOLATION(i, j, -1, -1)               && \
+     !SQUARE_DIAGONAL_VIOLATION(i, j, +1, -1)               && \
+     !SQUARE_DIAGONAL_VIOLATION(i, j, -1, +1)               && \
+     !SQUARE_DIAGONAL_VIOLATION(i, j, +1, +1)               && \
+     !SQUARE_LOOP_VIOLATION(i, j, SQUARE_LIT, SQUARE_UNLIT) && \
+     !SQUARE_LOOP_VIOLATION(i, j, SQUARE_UNLIT, SQUARE_LIT))
+
+#define IS_LIGHTING_CANDIDATE(i, j) \
+    (SQUARE_STATE(i, j) == SQUARE_UNLIT && \
+     CAN_LIGHT_SQUARE(i,j))
+
+    /* The 'score' of a square reflects its current desirability for selection
+     * as the next square to light.  We want to encourage moving into uncharted
+     * areas so we give scores according to how many of the square's neighbours
+     * are currently unlit. */
+
+   /* UNLIT    SCORE
+    *   3        2
+    *   2        0
+    *   1       -2
+    */
+#define SQUARE_SCORE(i,j) \
+    (2*((SQUARE_STATE(i-1, j) == SQUARE_UNLIT)  + \
+        (SQUARE_STATE(i+1, j) == SQUARE_UNLIT)  + \
+        (SQUARE_STATE(i, j-1) == SQUARE_UNLIT)  + \
+        (SQUARE_STATE(i, j+1) == SQUARE_UNLIT)) - 4)
+
+    /* When a square gets lit, this defines how far away from that square we
+     * need to go recomputing scores */
+#define SCORE_DISTANCE 1
+
+    board = snewn(board_area, char);
+    clues = state->clues;
+
+    /* Make a board */
+    memset(board, SQUARE_UNLIT, board_area);
+    
+    /* Seed the board with a single lit square near the middle */
+    i = state->w / 2;
+    j = state->h / 2;
+    if (state->w & 1 && random_bits(rs, 1))
+        ++i;
+    if (state->h & 1 && random_bits(rs, 1))
+        ++j;
+
+    LV_SQUARE_STATE(i, j) = SQUARE_LIT;
+
+    /* We need a way of favouring squares that will increase our loopiness.
+     * We do this by maintaining a list of all candidate squares sorted by
+     * their score and choose randomly from that with appropriate skew. 
+     * In order to avoid consistently biasing towards particular squares, we
+     * need the sort order _within_ each group of scores to be completely
+     * random.  But it would be abusing the hospitality of the tree234 data
+     * structure if our comparison function were nondeterministic :-).  So with
+     * each square we associate a random number that does not change during a
+     * particular run of the generator, and use that as a secondary sort key.
+     * Yes, this means we will be biased towards particular random squares in
+     * any one run but that doesn't actually matter. */
+    
+    lightable_squares_sorted   = newtree234(square_sort_cmpfn);
+    lightable_squares_gettable = newtree234(get_square_cmpfn);
+#define ADD_SQUARE(s) \
+    do { \
+        sq = add234(lightable_squares_sorted, s); \
+        assert(sq == s); \
+        sq = add234(lightable_squares_gettable, s); \
+        assert(sq == s); \
+    } while (0)
+
+#define REMOVE_SQUARE(s) \
+    do { \
+        sq = del234(lightable_squares_sorted, s); \
+        assert(sq); \
+        sq = del234(lightable_squares_gettable, s); \
+        assert(sq); \
+    } while (0)
+        
+#define HANDLE_DIR(a, b) \
+    square = snew(struct square); \
+    square->x = (i)+(a); \
+    square->y = (j)+(b); \
+    square->score = 2; \
+    square->random = random_bits(rs, 31); \
+    ADD_SQUARE(square);
+    HANDLE_DIR(-1, 0);
+    HANDLE_DIR( 1, 0);
+    HANDLE_DIR( 0,-1);
+    HANDLE_DIR( 0, 1);
+#undef HANDLE_DIR
+    
+    /* Light squares one at a time until the board is interesting enough */
+    while (TRUE)
+    {
+        /* We have count234(lightable_squares) possibilities, and in
+         * lightable_squares_sorted they are sorted with the most desirable
+         * first.  */
+        c = count234(lightable_squares_sorted);
+        if (c == 0)
+            break;
+        assert(c == count234(lightable_squares_gettable));
+
+        /* Check that the best square available is any good */
+        square = (struct square *)index234(lightable_squares_sorted, 0);
+        assert(square);
+
+        /*
+         * We never want to _decrease_ the loop's perimeter. Making
+         * moves that leave the perimeter the same is occasionally
+         * useful: if it were _never_ done then the user would be
+         * able to deduce illicitly that any degree-zero vertex was
+         * on the outside of the loop. So we do it sometimes but
+         * not always.
+         */
+        if (square->score < 0 || (square->score == 0 &&
+                                  random_upto(rs, 2) == 0)) {
+            break;
+        }
+
+        assert(square->score == SQUARE_SCORE(square->x, square->y));
+        assert(SQUARE_STATE(square->x, square->y) == SQUARE_UNLIT);
+        assert(square->x >= 0 && square->x < state->w);
+        assert(square->y >= 0 && square->y < state->h);
+
+        /* Update data structures */
+        LV_SQUARE_STATE(square->x, square->y) = SQUARE_LIT;
+        REMOVE_SQUARE(square);
+
+        /* We might have changed the score of any squares up to 2 units away in
+         * any direction */
+        for (b = -SCORE_DISTANCE; b <= SCORE_DISTANCE; b++) {
+            for (a = -SCORE_DISTANCE; a <= SCORE_DISTANCE; a++) {
+                if (!a && !b) 
+                    continue;
+                square_pos.x = square->x + a;
+                square_pos.y = square->y + b;
+                if (square_pos.x < 0 || square_pos.x >= state->w ||
+                    square_pos.y < 0 || square_pos.y >= state->h) {
+                   continue; 
+                }
+                tmpsquare = find234(lightable_squares_gettable, &square_pos,
+                                    NULL);
+                if (tmpsquare) {
+                    assert(tmpsquare->x == square_pos.x);
+                    assert(tmpsquare->y == square_pos.y);
+                    assert(SQUARE_STATE(tmpsquare->x, tmpsquare->y) == 
+                           SQUARE_UNLIT);
+                    REMOVE_SQUARE(tmpsquare);
+                } else {
+                    tmpsquare = snew(struct square);
+                    tmpsquare->x = square_pos.x;
+                    tmpsquare->y = square_pos.y;
+                    tmpsquare->random = random_bits(rs, 31);
+                }
+                tmpsquare->score = SQUARE_SCORE(tmpsquare->x, tmpsquare->y);
+
+                if (IS_LIGHTING_CANDIDATE(tmpsquare->x, tmpsquare->y)) {
+                    ADD_SQUARE(tmpsquare);
+                } else {
+                    sfree(tmpsquare);
+                }
+            }
+        }
+        sfree(square);
     }
 
-    if (s1->random < s2->random)
-        return -1;
-    else if (s1->random > s2->random)
-        return 1;
+    /* Clean up */
+    while ((square = delpos234(lightable_squares_gettable, 0)) != NULL)
+        sfree(square);
+    freetree234(lightable_squares_gettable);
+    freetree234(lightable_squares_sorted);
 
-    /*
-     * It's _just_ possible that two squares might have been given
-     * the same random value. In that situation, fall back to
-     * comparing based on the coordinates. This introduces a tiny
-     * directional bias, but not a significant one.
-     */
-    return get_square_cmpfn(v1, v2);
+    /* Copy out all the clues */
+    FORALL_SQUARES(state, i, j) {
+        c = SQUARE_STATE(i, j);
+        LV_CLUE_AT(state, i, j) = 0;
+        if (SQUARE_STATE(i-1, j) != c) ++LV_CLUE_AT(state, i, j);
+        if (SQUARE_STATE(i+1, j) != c) ++LV_CLUE_AT(state, i, j);
+        if (SQUARE_STATE(i, j-1) != c) ++LV_CLUE_AT(state, i, j);
+        if (SQUARE_STATE(i, j+1) != c) ++LV_CLUE_AT(state, i, j);
+    }
+
+    sfree(board);
 }
 
-static void print_tree(tree234 *tree)
+static int game_has_unique_soln(const game_state *state, int diff)
 {
-#if 0
-    int i = 0;
-    struct square *s;
-    printf("Print tree:\n");
-    while (i < count234(tree)) {
-        s = (struct square *)index234(tree, i);
-        assert(s);
-        printf("  [%d,%d], %d, %d\n", s->x, s->y, s->score, s->random);
-        ++i;
+    int ret;
+    solver_state *sstate_new;
+    solver_state *sstate = new_solver_state((game_state *)state, diff);
+    
+    sstate_new = solve_game_rec(sstate, diff);
+
+    assert(sstate_new->solver_status != SOLVER_MISTAKE);
+    ret = (sstate_new->solver_status == SOLVER_SOLVED);
+
+    free_solver_state(sstate_new);
+    free_solver_state(sstate);
+
+    return ret;
+}
+
+/* Remove clues one at a time at random. */
+static game_state *remove_clues(game_state *state, random_state *rs, 
+                                int diff)
+{
+    int *square_list, squares;
+    game_state *ret = dup_game(state), *saved_ret;
+    int n;
+#ifdef SHOW_WORKING
+    char *desc;
+#endif
+
+    /* We need to remove some clues.  We'll do this by forming a list of all
+     * available clues, shuffling it, then going along one at a
+     * time clearing each clue in turn for which doing so doesn't render the
+     * board unsolvable. */
+    squares = state->w * state->h;
+    square_list = snewn(squares, int);
+    for (n = 0; n < squares; ++n) {
+        square_list[n] = n;
     }
+
+    shuffle(square_list, squares, sizeof(int), rs);
+    
+    for (n = 0; n < squares; ++n) {
+        saved_ret = dup_game(ret);
+        LV_CLUE_AT(ret, square_list[n] % state->w,
+                   square_list[n] / state->w) = -1;
+
+#ifdef SHOW_WORKING
+        desc = state_to_text(ret);
+        fprintf(stderr, "%dx%d:%s\n", state->w, state->h, desc);
+        sfree(desc);
 #endif
+
+        if (game_has_unique_soln(ret, diff)) {
+            free_game(saved_ret);
+        } else {
+            free_game(ret);
+            ret = saved_ret;
+        }
+    }
+    sfree(square_list);
+
+    return ret;
 }
 
-enum { SQUARE_LIT, SQUARE_UNLIT };
+static char *new_game_desc(game_params *params, random_state *rs,
+                           char **aux, int interactive)
+{
+    /* solution and description both use run-length encoding in obvious ways */
+    char *retval;
+    game_state *state = snew(game_state), *state_new;
+
+    state->h = params->h;
+    state->w = params->w;
+
+    state->clues = snewn(SQUARE_COUNT(params), char);
+    state->hl = snewn(HL_COUNT(params), char);
+    state->vl = snewn(VL_COUNT(params), char);
+
+newboard_please:
+    memset(state->hl, LINE_UNKNOWN, HL_COUNT(params));
+    memset(state->vl, LINE_UNKNOWN, VL_COUNT(params));
+
+    state->solved = state->cheated = FALSE;
+    state->recursion_depth = params->rec;
+
+    /* Get a new random solvable board with all its clues filled in.  Yes, this
+     * can loop for ever if the params are suitably unfavourable, but
+     * preventing games smaller than 4x4 seems to stop this happening */
+
+    do {
+        add_full_clues(state, rs);
+    } while (!game_has_unique_soln(state, params->diff));
+
+    state_new = remove_clues(state, rs, params->diff);
+    free_game(state);
+    state = state_new;
 
-#define SQUARE_STATE(i, j)                 \
-    (((i) < 0 || (i) >= params->w ||       \
-      (j) < 0 || (j) >= params->h) ?       \
-     SQUARE_UNLIT :  LV_SQUARE_STATE(i,j))
+    if (params->diff > 0 && game_has_unique_soln(state, params->diff-1)) {
+#ifdef SHOW_WORKING
+        fprintf(stderr, "Rejecting board, it is too easy\n");
+#endif
+        goto newboard_please;
+    }
+
+    retval = state_to_text(state);
+
+    free_game(state);
+    
+    assert(!validate_desc(params, retval));
 
-#define LV_SQUARE_STATE(i, j) board[(i) + params->w * (j)]
+    return retval;
+}
 
-static void print_board(const game_params *params, const char *board)
+static game_state *new_game(midend *me, game_params *params, char *desc)
 {
-#if 0
     int i,j;
+    game_state *state = snew(game_state);
+    int empties_to_make = 0;
+    int n;
+    const char *dp = desc;
 
-    printf(" ");
-    for (i = 0; i < params->w; i++) {
-        printf("%d", i%10);
-    }
-    printf("\n");
-    for (j = 0; j < params->h; j++) {
-        printf("%d", j%10);
-        for (i = 0; i < params->w; i++) {
-            printf("%c", SQUARE_STATE(i, j) ? ' ' : 'O');
+    state->recursion_depth = 0; /* XXX pending removal, probably */
+    
+    state->h = params->h;
+    state->w = params->w;
+
+    state->clues = snewn(SQUARE_COUNT(params), char);
+    state->hl = snewn(HL_COUNT(params), char);
+    state->vl = snewn(VL_COUNT(params), char);
+
+    state->solved = state->cheated = FALSE;
+
+    FORALL_SQUARES(params, i, j) {
+        if (empties_to_make) {
+            empties_to_make--;
+            LV_CLUE_AT(state, i, j) = -1;
+            continue;
+        }
+
+        assert(*dp);
+        n = *dp - '0';
+        if (n >= 0 && n < 10) {
+            LV_CLUE_AT(state, i, j) = n;
+        } else {
+            n = *dp - 'a' + 1;
+            assert(n > 0);
+            LV_CLUE_AT(state, i, j) = -1;
+            empties_to_make = n - 1;
         }
-        printf("\n");
+        ++dp;
     }
-#endif
+
+    memset(state->hl, LINE_UNKNOWN, HL_COUNT(params));
+    memset(state->vl, LINE_UNKNOWN, VL_COUNT(params));
+
+    return state;
 }
 
-static char *new_fullyclued_board(game_params *params, random_state *rs)
+enum { LOOP_NONE=0, LOOP_SOLN, LOOP_NOT_SOLN };
+
+/* ----------------------------------------------------------------------
+ * Solver logic
+ *
+ * Our solver modes operate as follows.  Each mode also uses the modes above it.
+ *
+ *   Easy Mode
+ *   Just implement the rules of the game.
+ *
+ *   Normal Mode
+ *   For each pair of lines through each dot we store a bit for whether
+ *   at least one of them is on and whether at most one is on.  (If we know
+ *   both or neither is on that's already stored more directly.)  That's six
+ *   bits per dot.  Bit number n represents the lines shown in dline_desc.
+ *
+ *   Advanced Mode
+ *   Use edsf data structure to make equivalence classes of lines that are
+ *   known identical to or opposite to one another.
+ */
+
+/* The order the following are defined in is very important, see below.
+ * The last two fields may seem non-obvious: they specify that when talking
+ * about a square the dx and dy offsets should be added to the square coords to
+ * get to the right dot.  Where dx and dy are -1 this means that the dline
+ * doesn't make sense for a square. */
+/* XXX can this be done with a struct instead? */
+#define DLINES \
+    DLINE(DLINE_UD, UP,   DOWN,  -1, -1) \
+    DLINE(DLINE_LR, LEFT, RIGHT, -1, -1) \
+    DLINE(DLINE_UR, UP,   RIGHT,  0,  1) \
+    DLINE(DLINE_DL, DOWN, LEFT,   1,  0) \
+    DLINE(DLINE_UL, UP,   LEFT,   1,  1) \
+    DLINE(DLINE_DR, DOWN, RIGHT,  0,  0)
+
+#define OPP_DLINE(dline_desc) ((dline_desc) ^ 1)
+
+enum dline_desc {
+#define DLINE(desc, dir1, dir2, dx, dy) \
+    desc,
+    DLINES
+#undef DLINE
+};
+
+struct dline {
+    enum dline_desc desc;
+    enum direction dir1, dir2;
+    int dx, dy;
+};
+
+const static struct dline dlines[] =  {
+#define DLINE(desc, dir1, dir2, dx, dy) \
+    { desc, dir1, dir2, dx, dy },
+    DLINES
+#undef DLINE
+};
+
+#define FORALL_DOT_DLINES(dl_iter) \
+    for (dl_iter = 0; dl_iter < lenof(dlines); ++dl_iter)
+
+#define FORALL_SQUARE_DLINES(dl_iter) \
+    for (dl_iter = 2; dl_iter < lenof(dlines); ++dl_iter)
+
+#define DL2STR(d) \
+    ((d==DLINE_UD) ? "DLINE_UD": \
+     (d==DLINE_LR) ? "DLINE_LR": \
+     (d==DLINE_UR) ? "DLINE_UR": \
+     (d==DLINE_DL) ? "DLINE_DL": \
+     (d==DLINE_UL) ? "DLINE_UL": \
+     (d==DLINE_DR) ? "DLINE_DR": \
+     "oops")
+
+#define CHECK_DLINE_SENSIBLE(d) assert(dlines[(d)].dx != -1 && dlines[(d)].dy != -1)
+
+/* This will fail an assertion if the directions handed to it are the same, as
+ * no dline corresponds to that */
+static enum dline_desc dline_desc_from_dirs(enum direction dir1, 
+                                            enum direction dir2)
 {
-    char *clues;
-    char *board;
-    int i, j, a, b, c;
-    game_state s;
-    game_state *state = &s;
-    int board_area = SQUARE_COUNT(params);
-    int t;
+    int i;
 
-    struct square *square, *tmpsquare, *sq;
-    struct square square_pos;
+    assert (dir1 != dir2);
 
-    /* These will contain exactly the same information, sorted into different
-     * orders */
-    tree234 *lightable_squares_sorted, *lightable_squares_gettable;
+    for (i = 0; i < lenof(dlines); ++i) {
+        if ((dir1 == dlines[i].dir1 && dir2 == dlines[i].dir2) ||
+            (dir1 == dlines[i].dir2 && dir2 == dlines[i].dir1)) {
+            return dlines[i].desc;
+        }
+    }
 
-#define SQUARE_REACHABLE(i,j)                      \
-     (t = (SQUARE_STATE(i-1, j) == SQUARE_LIT ||      \
-           SQUARE_STATE(i+1, j) == SQUARE_LIT ||      \
-           SQUARE_STATE(i, j-1) == SQUARE_LIT ||      \
-           SQUARE_STATE(i, j+1) == SQUARE_LIT),       \
-/*      printf("SQUARE_REACHABLE(%d,%d) = %d\n", i, j, t), */ \
-      t)
+    assert(!"dline not found");
+    return DLINE_UD; /* placate compiler */
+}
 
+/* The following functions allow you to get or set info about the selected
+ * dline corresponding to the dot or square at [i,j].  You'll get an assertion
+ * failure if you talk about a dline that doesn't exist, ie if you ask about
+ * non-touching lines around a square. */
+static int get_dot_dline(const game_state *state, const char *dline_array,
+                         int i, int j, enum dline_desc desc)
+{
+/*    fprintf(stderr, "get_dot_dline %p [%d,%d] %s\n", dline_array, i, j, DL2STR(desc)); */
+    return BIT_SET(dline_array[i + (state->w + 1) * j], desc);
+}
 
-    /* One situation in which we may not light a square is if that'll leave one
-     * square above/below and one left/right of us unlit, separated by a lit
-     * square diagnonal from us */
-#define SQUARE_DIAGONAL_VIOLATION(i, j, h, v)           \
-    (t = (SQUARE_STATE((i)+(h), (j))     == SQUARE_UNLIT && \
-          SQUARE_STATE((i),     (j)+(v)) == SQUARE_UNLIT && \
-          SQUARE_STATE((i)+(h), (j)+(v)) == SQUARE_LIT),    \
-/*     t ? printf("SQUARE_DIAGONAL_VIOLATION(%d, %d, %d, %d)\n",
-                  i, j, h, v) : 0,*/ \
-     t)
+static int set_dot_dline(game_state *state, char *dline_array,
+                         int i, int j, enum dline_desc desc
+#ifdef SHOW_WORKING
+                         , const char *reason
+#endif
+                         )
+{
+    int ret;
+    ret = SET_BIT(dline_array[i + (state->w + 1) * j], desc);
 
-    /* We also may not light a square if it will form a loop of lit squares
-     * around some unlit squares, as then the game soln won't have a single
-     * loop */
-#define SQUARE_LOOP_VIOLATION(i, j, lit1, lit2) \
-    (SQUARE_STATE((i)+1, (j)) == lit1    &&     \
-     SQUARE_STATE((i)-1, (j)) == lit1    &&     \
-     SQUARE_STATE((i), (j)+1) == lit2    &&     \
-     SQUARE_STATE((i), (j)-1) == lit2)
+#ifdef SHOW_WORKING
+    if (ret)
+        fprintf(stderr, "set_dot_dline %p [%d,%d] %s (%s)\n", dline_array, i, j, DL2STR(desc), reason);
+#endif
+    return ret;
+}
 
-#define CAN_LIGHT_SQUARE(i, j)                                 \
-    (SQUARE_REACHABLE(i, j)                                 && \
-     !SQUARE_DIAGONAL_VIOLATION(i, j, -1, -1)               && \
-     !SQUARE_DIAGONAL_VIOLATION(i, j, +1, -1)               && \
-     !SQUARE_DIAGONAL_VIOLATION(i, j, -1, +1)               && \
-     !SQUARE_DIAGONAL_VIOLATION(i, j, +1, +1)               && \
-     !SQUARE_LOOP_VIOLATION(i, j, SQUARE_LIT, SQUARE_UNLIT) && \
-     !SQUARE_LOOP_VIOLATION(i, j, SQUARE_UNLIT, SQUARE_LIT))
+static int get_square_dline(game_state *state, char *dline_array,
+                            int i, int j, enum dline_desc desc)
+{
+    CHECK_DLINE_SENSIBLE(desc);
+/*    fprintf(stderr, "get_square_dline %p [%d,%d] %s\n", dline_array, i, j, DL2STR(desc)); */
+    return BIT_SET(dline_array[(i+dlines[desc].dx) + (state->w + 1) * (j+dlines[desc].dy)], 
+                   desc);
+}
 
-#define IS_LIGHTING_CANDIDATE(i, j)        \
-    (SQUARE_STATE(i, j) == SQUARE_UNLIT && \
-     CAN_LIGHT_SQUARE(i,j))
+static int set_square_dline(game_state *state, char *dline_array,
+                            int i, int j, enum dline_desc desc
+#ifdef SHOW_WORKING
+                            , const char *reason
+#endif
+                            )
+{
+    int ret;
+    CHECK_DLINE_SENSIBLE(desc);
+    ret = SET_BIT(dline_array[(i+dlines[desc].dx) + (state->w + 1) * (j+dlines[desc].dy)], desc);
+#ifdef SHOW_WORKING
+    if (ret)
+        fprintf(stderr, "set_square_dline %p [%d,%d] %s (%s)\n", dline_array, i, j, DL2STR(desc), reason);
+#endif
+    return ret;
+}
 
-    /* The 'score' of a square reflects its current desirability for selection
-     * as the next square to light.  We want to encourage moving into uncharted
-     * areas so we give scores according to how many of the square's neighbours
-     * are currently unlit. */
+#ifdef SHOW_WORKING
+#define set_dot_dline(a, b, c, d, e) \
+        set_dot_dline(a, b, c, d, e, __FUNCTION__)
+#define set_square_dline(a, b, c, d, e) \
+        set_square_dline(a, b, c, d, e, __FUNCTION__)
+#endif
 
-   /* UNLIT    SCORE
-    *   3        2
-    *   2        0
-    *   1       -2
-    */
-#define SQUARE_SCORE(i,j)                  \
-    (2*((SQUARE_STATE(i-1, j) == SQUARE_UNLIT)  +   \
-        (SQUARE_STATE(i+1, j) == SQUARE_UNLIT)  +   \
-        (SQUARE_STATE(i, j-1) == SQUARE_UNLIT)  +   \
-        (SQUARE_STATE(i, j+1) == SQUARE_UNLIT)) - 4)
+static int set_dot_opp_dline(game_state *state, char *dline_array,
+                             int i, int j, enum dline_desc desc)
+{
+    return set_dot_dline(state, dline_array, i, j, OPP_DLINE(desc));
+}
 
-    /* When a square gets lit, this defines how far away from that square we
-     * need to go recomputing scores */
-#define SCORE_DISTANCE 1
+static int set_square_opp_dline(game_state *state, char *dline_array,
+                                int i, int j, enum dline_desc desc)
+{
+    return set_square_dline(state, dline_array, i, j, OPP_DLINE(desc));
+}
 
-    board = snewn(board_area, char);
-    clues = snewn(board_area, char);
+/* Find out if both the lines in the given dline are UNKNOWN */
+static int dline_both_unknown(const game_state *state, int i, int j,
+                              enum dline_desc desc)
+{
+    return 
+        (get_line_status_from_point(state, i, j, dlines[desc].dir1) == LINE_UNKNOWN) &&
+        (get_line_status_from_point(state, i, j, dlines[desc].dir2) == LINE_UNKNOWN);
+}
 
-    state->h = params->h;
-    state->w = params->w;
-    state->clues = clues;
+#define SQUARE_DLINES \
+                   HANDLE_DLINE(DLINE_UL, RIGHTOF_SQUARE, BELOW_SQUARE, 1, 1); \
+                   HANDLE_DLINE(DLINE_UR, LEFTOF_SQUARE,  BELOW_SQUARE, 0, 1); \
+                   HANDLE_DLINE(DLINE_DL, RIGHTOF_SQUARE, ABOVE_SQUARE, 1, 0); \
+                   HANDLE_DLINE(DLINE_DR, LEFTOF_SQUARE,  ABOVE_SQUARE, 0, 0); 
+
+#define DOT_DLINES \
+                   HANDLE_DLINE(DLINE_UD,    ABOVE_DOT,  BELOW_DOT); \
+                   HANDLE_DLINE(DLINE_LR,    LEFTOF_DOT, RIGHTOF_DOT); \
+                   HANDLE_DLINE(DLINE_UL,    ABOVE_DOT,  LEFTOF_DOT); \
+                   HANDLE_DLINE(DLINE_UR,    ABOVE_DOT,  RIGHTOF_DOT); \
+                   HANDLE_DLINE(DLINE_DL,    BELOW_DOT,  LEFTOF_DOT); \
+                   HANDLE_DLINE(DLINE_DR,    BELOW_DOT,  RIGHTOF_DOT); 
+
+static void array_setall(char *array, char from, char to, int len)
+{
+    char *p = array, *p_old = p;
+    int len_remaining = len;
+
+    while ((p = memchr(p, from, len_remaining))) {
+        *p = to;
+        len_remaining -= p - p_old;
+        p_old = p;
+    }
+}
 
-    /* Make a board */
-    memset(board, SQUARE_UNLIT, board_area);
-    
-    /* Seed the board with a single lit square near the middle */
-    i = params->w / 2;
-    j = params->h / 2;
-    if (params->w & 1 && random_bits(rs, 1))
-        ++i;
-    if (params->h & 1 && random_bits(rs, 1))
-        ++j;
 
-    LV_SQUARE_STATE(i, j) = SQUARE_LIT;
 
-    /* We need a way of favouring squares that will increase our loopiness.
-     * We do this by maintaining a list of all candidate squares sorted by
-     * their score and choose randomly from that with appropriate skew. 
-     * In order to avoid consistently biasing towards particular squares, we
-     * need the sort order _within_ each group of scores to be completely
-     * random.  But it would be abusing the hospitality of the tree234 data
-     * structure if our comparison function were nondeterministic :-).  So with
-     * each square we associate a random number that does not change during a
-     * particular run of the generator, and use that as a secondary sort key.
-     * Yes, this means we will be biased towards particular random squares in
-     * any one run but that doesn't actually matter. */
-    
-    lightable_squares_sorted   = newtree234(square_sort_cmpfn);
-    lightable_squares_gettable = newtree234(get_square_cmpfn);
-#define ADD_SQUARE(s)                                          \
-    do {                                                       \
-/*      printf("ADD SQUARE: [%d,%d], %d, %d\n",
-               s->x, s->y, s->score, s->random);*/ \
-        sq = add234(lightable_squares_sorted, s);              \
-        assert(sq == s);                                       \
-        sq = add234(lightable_squares_gettable, s);            \
-        assert(sq == s);                                       \
-    } while (0)
+static int get_line_status_from_point(const game_state *state,
+                                      int x, int y, enum direction d)
+{
+    switch (d) {
+        case LEFT:
+            return LEFTOF_DOT(state, x, y);
+        case RIGHT:
+            return RIGHTOF_DOT(state, x, y);
+        case UP:
+            return ABOVE_DOT(state, x, y);
+        case DOWN:
+            return BELOW_DOT(state, x, y);
+    }
 
-#define REMOVE_SQUARE(s)                                       \
-    do {                                                       \
-/*      printf("DELETE SQUARE: [%d,%d], %d, %d\n",
-               s->x, s->y, s->score, s->random);*/ \
-        sq = del234(lightable_squares_sorted, s);              \
-        assert(sq);                                            \
-        sq = del234(lightable_squares_gettable, s);            \
-        assert(sq);                                            \
-    } while (0)
-        
-#define HANDLE_DIR(a, b)                                       \
-    square = snew(struct square);                              \
-    square->x = (i)+(a);                                       \
-    square->y = (j)+(b);                                       \
-    square->score = 2;                                         \
-    square->random = random_bits(rs, 31);                      \
-    ADD_SQUARE(square);
-    HANDLE_DIR(-1, 0);
-    HANDLE_DIR( 1, 0);
-    HANDLE_DIR( 0,-1);
-    HANDLE_DIR( 0, 1);
-#undef HANDLE_DIR
-    
-    /* Light squares one at a time until the board is interesting enough */
-    while (TRUE)
-    {
-        /* We have count234(lightable_squares) possibilities, and in
-         * lightable_squares_sorted they are sorted with the most desirable
-         * first.  */
-        c = count234(lightable_squares_sorted);
-        if (c == 0)
-            break;
-        assert(c == count234(lightable_squares_gettable));
+    return 0;
+}
 
-        /* Check that the best square available is any good */
-        square = (struct square *)index234(lightable_squares_sorted, 0);
-        assert(square);
+/* First and second args are coord offset from top left of square to one end
+ * of line in question, third and fourth args are the direction from the first
+ * end of the line to the second.  Fifth arg is the direction of the line from
+ * the coord offset position.
+ * How confusing.  
+ */
+#define SQUARE_LINES \
+    SQUARE_LINE( 0,  0, RIGHT, RIGHTOF_DOT, UP); \
+    SQUARE_LINE( 0, +1, RIGHT, RIGHTOF_DOT, DOWN); \
+    SQUARE_LINE( 0,  0, DOWN,  BELOW_DOT,   LEFT); \
+    SQUARE_LINE(+1,  0, DOWN,  BELOW_DOT,   RIGHT); 
+
+/* Set pairs of lines around this square which are known to be identical to
+ * the given line_state */
+static int square_setall_identical(solver_state *sstate, int x, int y,
+                                   enum line_state line_new)
+{
+    /* can[dir] contains the canonical line associated with the line in
+     * direction dir from the square in question.  Similarly inv[dir] is
+     * whether or not the line in question is inverse to its canonical
+     * element. */
+    int can[4], inv[4], i, j;
+    int retval = FALSE;
 
-       /*
-        * We never want to _decrease_ the loop's perimeter. Making
-        * moves that leave the perimeter the same is occasionally
-        * useful: if it were _never_ done then the user would be
-        * able to deduce illicitly that any degree-zero vertex was
-        * on the outside of the loop. So we do it sometimes but
-        * not always.
-        */
-        if (square->score < 0 || (square->score == 0 &&
-                                 random_upto(rs, 2) == 0))
-            break;
+    i = 0;
 
-        print_tree(lightable_squares_sorted);
-        assert(square->score == SQUARE_SCORE(square->x, square->y));
-        assert(SQUARE_STATE(square->x, square->y) == SQUARE_UNLIT);
-        assert(square->x >= 0 && square->x < params->w);
-        assert(square->y >= 0 && square->y < params->h);
-/*        printf("LIGHT SQUARE: [%d,%d], score = %d\n", square->x, square->y, square->score); */
+#if 0
+    fprintf(stderr, "Setting all identical unknown lines around square "
+                    "[%d,%d] to %d:\n", x, y, line_new);                 
+#endif
 
-        /* Update data structures */
-        LV_SQUARE_STATE(square->x, square->y) = SQUARE_LIT;
-        REMOVE_SQUARE(square);
+#define SQUARE_LINE(dx, dy, linedir, dir_dot, sqdir) \
+    can[sqdir] = \
+        edsf_canonify(sstate->hard->linedsf, \
+                      LINEDSF_INDEX(sstate->state, x+(dx), y+(dy), linedir), \
+                      &inv[sqdir]);
+    
+    SQUARE_LINES;
 
-        print_board(params, board);
+#undef SQUARE_LINE
 
-        /* We might have changed the score of any squares up to 2 units away in
-         * any direction */
-        for (b = -SCORE_DISTANCE; b <= SCORE_DISTANCE; b++) {
-            for (a = -SCORE_DISTANCE; a <= SCORE_DISTANCE; a++) {
-                if (!a && !b) 
-                    continue;
-                square_pos.x = square->x + a;
-                square_pos.y = square->y + b;
-/*                printf("Refreshing score for [%d,%d]:\n", square_pos.x, square_pos.y); */
-                if (square_pos.x < 0 || square_pos.x >= params->w ||
-                    square_pos.y < 0 || square_pos.y >= params->h) {
-/*                    printf("  Out of bounds\n"); */
-                   continue; 
-                }
-                tmpsquare = find234(lightable_squares_gettable, &square_pos,
-                                    NULL);
-                if (tmpsquare) {
-/*                    printf(" Removing\n"); */
-                    assert(tmpsquare->x == square_pos.x);
-                    assert(tmpsquare->y == square_pos.y);
-                    assert(SQUARE_STATE(tmpsquare->x, tmpsquare->y) == 
-                           SQUARE_UNLIT);
-                    REMOVE_SQUARE(tmpsquare);
-                } else {
-/*                    printf(" Creating\n"); */
-                    tmpsquare = snew(struct square);
-                    tmpsquare->x = square_pos.x;
-                    tmpsquare->y = square_pos.y;
-                    tmpsquare->random = random_bits(rs, 31);
-                }
-                tmpsquare->score = SQUARE_SCORE(tmpsquare->x, tmpsquare->y);
+    for (j = 0; j < 4; ++j) {
+        for (i = 0; i < 4; ++i) {
+            if (i == j)
+                continue;
 
-                if (IS_LIGHTING_CANDIDATE(tmpsquare->x, tmpsquare->y)) {
-/*                    printf(" Adding\n"); */
-                    ADD_SQUARE(tmpsquare);
-                } else {
-/*                    printf(" Destroying\n"); */
-                    sfree(tmpsquare);
+            if (can[i] == can[j] && inv[i] == inv[j]) {
+
+                /* Lines in directions i and j are identical.
+                 * Only do j now, we'll do i when the loop causes us to
+                 * consider {i,j} in the opposite order. */
+#define SQUARE_LINE(dx, dy, dir, c, sqdir) \
+                if (j == sqdir) { \
+                    retval = set_line_bydot(sstate, x+(dx), y+(dy), dir, line_new); \
+                    if (retval) { \
+                        break; \
+                    } \
                 }
+                
+                SQUARE_LINES;
+
+#undef SQUARE_LINE
             }
         }
-        sfree(square);
-/*        printf("\n\n"); */
     }
 
-    while ((square = delpos234(lightable_squares_gettable, 0)) != NULL)
-        sfree(square);
-    freetree234(lightable_squares_gettable);
-    freetree234(lightable_squares_sorted);
+    return retval;
+}
+
+#if 0
+/* Set all identical lines passing through the current dot to the chosen line
+ * state.  (implicitly this only looks at UNKNOWN lines) */
+static int dot_setall_identical(solver_state *sstate, int x, int y,
+                                enum line_state line_new)
+{
+    /* The implementation of this is a little naughty but I can't see how to do
+     * it elegantly any other way */
+    int can[4], inv[4], i, j;
+    enum direction d;
+    int retval = FALSE;
+
+    for (d = 0; d < 4; ++d) {
+        can[d] = edsf_canonify(sstate->hard->linedsf, 
+                               LINEDSF_INDEX(sstate->state, x, y, d),
+                               inv+d);
+    }
+    
+    for (j = 0; j < 4; ++j) {
+next_j:
+        for (i = 0; i < j; ++i) {
+            if (can[i] == can[j] && inv[i] == inv[j]) {
+                /* Lines in directions i and j are identical */
+                if (get_line_status_from_point(sstate->state, x, y, j) ==
+                        LINE_UNKNOWN) {
+                    set_line_bydot(sstate->state, x, y, j, 
+                                               line_new);
+                    retval = TRUE;
+                    goto next_j;
+                }
+            }
 
-    /* Copy out all the clues */
-    for (j = 0; j < params->h; ++j) {
-        for (i = 0; i < params->w; ++i) {
-            c = SQUARE_STATE(i, j);
-            LV_CLUE_AT(state, i, j) = '0';
-            if (SQUARE_STATE(i-1, j) != c) ++LV_CLUE_AT(state, i, j);
-            if (SQUARE_STATE(i+1, j) != c) ++LV_CLUE_AT(state, i, j);
-            if (SQUARE_STATE(i, j-1) != c) ++LV_CLUE_AT(state, i, j);
-            if (SQUARE_STATE(i, j+1) != c) ++LV_CLUE_AT(state, i, j);
         }
     }
 
-    sfree(board);
-    return clues;
+    return retval;
 }
+#endif
+
+static int square_setboth_in_dline(solver_state *sstate, enum dline_desc dd,
+                                   int i, int j, enum line_state line_new)
+{
+    int retval = FALSE;
+    const struct dline dll = dlines[dd], *dl = &dll;
+    
+#if 0
+    fprintf(stderr, "square_setboth_in_dline %s [%d,%d] to %d\n",
+                    DL2STR(dd), i, j, line_new);
+#endif
 
-static solver_state *solve_game_rec(const solver_state *sstate);
+    CHECK_DLINE_SENSIBLE(dd);
+    
+    retval |=
+        set_line_bydot(sstate, i+dl->dx, j+dl->dy, dl->dir1, line_new);
+    retval |=
+        set_line_bydot(sstate, i+dl->dx, j+dl->dy, dl->dir2, line_new);
+
+    return retval;
+}
 
-static int game_has_unique_soln(const game_state *state)
+/* Call this function to register that the two unknown lines going into the dot
+ * [x,y] are identical or opposite (depending on the value of 'inverse').  This
+ * function will cause an assertion failure if anything other than exactly two
+ * lines into the dot are unknown. 
+ * As usual returns TRUE if any progress was made, otherwise FALSE. */
+static int dot_relate_2_unknowns(solver_state *sstate, int x, int y, int inverse)
 {
-    int ret;
-    solver_state *sstate_new;
-    solver_state *sstate = new_solver_state((game_state *)state);
+    enum direction d1=DOWN, d2=DOWN; /* Just to keep compiler quiet */
+    int dirs_set = 0;
+
+#define TRY_DIR(d) \
+              if (get_line_status_from_point(sstate->state, x, y, d) == \
+                      LINE_UNKNOWN) { \
+                  if (dirs_set == 0) \
+                      d1 = d; \
+                  else { \
+                      assert(dirs_set == 1); \
+                      d2 = d; \
+                  } \
+                  dirs_set++; \
+              } while (0)
     
-    sstate_new = solve_game_rec(sstate);
+    TRY_DIR(UP);
+    TRY_DIR(DOWN);
+    TRY_DIR(LEFT);
+    TRY_DIR(RIGHT);
+#undef TRY_DIR
 
-    ret = (sstate_new->solver_status == SOLVER_SOLVED);
+    assert(dirs_set == 2);
+    assert(d1 != d2);
 
-    free_solver_state(sstate_new);
-    free_solver_state(sstate);
+#if 0
+    fprintf(stderr, "Lines in direction %s and %s from dot [%d,%d] are %s\n",
+            DIR2STR(d1), DIR2STR(d2), x, y, inverse?"opposite":"the same");
+#endif
 
-    return ret;
+    return merge_lines(sstate, x, y, d1, x, y, d2, inverse);
 }
 
-/* Remove clues one at a time at random. */
-static game_state *remove_clues(game_state *state, random_state *rs)
+/* Very similar to dot_relate_2_unknowns. */
+static int square_relate_2_unknowns(solver_state *sstate, int x, int y, int inverse)
 {
-    int *square_list, squares;
-    game_state *ret = dup_game(state), *saved_ret;
-    int n;
+    enum direction d1=DOWN, d2=DOWN;
+    int x1=-1, y1=-1, x2=-1, y2=-1;
+    int dirs_set = 0;
 
-    /* We need to remove some clues.  We'll do this by forming a list of all
-     * available equivalence classes, shuffling it, then going along one at a
-     * time clearing every member of each equivalence class, where removing a
-     * class doesn't render the board unsolvable. */
-    squares = state->w * state->h;
-    square_list = snewn(squares, int);
-    for (n = 0; n < squares; ++n) {
-        square_list[n] = n;
-    }
+#if 0
+    fprintf(stderr, "2 unknowns around square [%d,%d] are %s\n",
+                     x, y, inverse?"opposite":"the same");
+#endif
 
-    shuffle(square_list, squares, sizeof(int), rs);
+#define TRY_DIR(i, j, d, dir_sq) \
+          do { \
+              if (dir_sq(sstate->state, x, y) == LINE_UNKNOWN) { \
+                  if (dirs_set == 0) { \
+                      d1 = d; x1 = i; y1 = j; \
+                  } else { \
+                      assert(dirs_set == 1); \
+                      d2 = d; x2 = i; y2 = j; \
+                  } \
+                  dirs_set++; \
+              } \
+          } while (0)
     
-    for (n = 0; n < squares; ++n) {
-        saved_ret = dup_game(ret);
-       LV_CLUE_AT(ret, square_list[n] % state->w,
-                  square_list[n] / state->w) = ' ';
-        if (game_has_unique_soln(ret)) {
-           free_game(saved_ret);
-        } else {
-            free_game(ret);
-            ret = saved_ret;
-        }
-    }
-    sfree(square_list);
+    TRY_DIR(x,   y,   RIGHT, ABOVE_SQUARE);
+    TRY_DIR(x,   y,   DOWN, LEFTOF_SQUARE);
+    TRY_DIR(x+1, y,   DOWN, RIGHTOF_SQUARE);
+    TRY_DIR(x,   y+1, RIGHT, BELOW_SQUARE);
+#undef TRY_DIR
 
-    return ret;
-}
+    assert(dirs_set == 2);
 
-static char *validate_desc(game_params *params, char *desc);
+#if 0
+    fprintf(stderr, "Line in direction %s from dot [%d,%d] and line in direction %s from dot [%2d,%2d] are %s\n",
+            DIR2STR(d1), x1, y1, DIR2STR(d2), x2, y2, inverse?"opposite":"the same");
+#endif
 
-static char *new_game_desc(game_params *params, random_state *rs,
-                           char **aux, int interactive)
-{
-    /* solution and description both use run-length encoding in obvious ways */
-    char *retval;
-    char *description = snewn(SQUARE_COUNT(params) + 1, char);
-    char *dp = description;
-    int i, j;
-    int empty_count;
-    game_state *state = snew(game_state), *state_new;
+    return merge_lines(sstate, x1, y1, d1, x2, y2, d2, inverse);
+}
 
-    state->h = params->h;
-    state->w = params->w;
+/* Figure out if any dlines can be 'collapsed' (and do so if they can).  This
+ * can happen if one of the lines is known and due to the dline status this
+ * tells us state of the other, or if there's an interaction with the linedsf
+ * (ie if atmostone is set for a dline and the lines are known identical they
+ * must both be LINE_NO, etc).  XXX at the moment only the former is
+ * implemented, and indeed the latter should be implemented in the hard mode
+ * solver only.
+ */
+static int dot_collapse_dlines(solver_state *sstate, int i, int j)
+{
+    int progress = FALSE;
+    enum direction dir1, dir2;
+    int dir1st;
+    int dlset;
+    game_state *state = sstate->state;
+    enum dline_desc dd;
 
-    state->hl = snewn(HL_COUNT(params), char);
-    state->vl = snewn(VL_COUNT(params), char);
-    memset(state->hl, LINE_UNKNOWN, HL_COUNT(params));
-    memset(state->vl, LINE_UNKNOWN, VL_COUNT(params));
+    for (dir1 = 0; dir1 < 4; dir1++) {
+        dir1st = get_line_status_from_point(state, i, j, dir1);
+        if (dir1st == LINE_UNKNOWN)
+            continue;
+        /* dir2 iterates over the whole range rather than starting at dir1+1
+         * because test below is asymmetric */
+        for (dir2 = 0; dir2 < 4; dir2++) {
+            if (dir1 == dir2)
+                continue;
 
-    state->solved = state->cheated = FALSE;
-    state->recursion_depth = params->rec;
+            if ((i == 0        && (dir1 == LEFT  || dir2 == LEFT))  ||
+                (j == 0        && (dir1 == UP    || dir2 == UP))    ||
+                (i == state->w && (dir1 == RIGHT || dir2 == RIGHT)) ||
+                (j == state->h && (dir1 == DOWN  || dir2 == DOWN))) {
+                continue;
+            }
 
-    /* Get a new random solvable board with all its clues filled in.  Yes, this
-     * can loop for ever if the params are suitably unfavourable, but
-     * preventing games smaller than 4x4 seems to stop this happening */
-    do {
-        state->clues = new_fullyclued_board(params, rs);
-    } while (!game_has_unique_soln(state));
+#if 0
+        fprintf(stderr, "dot_collapse_dlines [%d,%d], %s %s\n", i, j,
+                    DIR2STR(dir1), DIR2STR(dir2));
+#endif
 
-    state_new = remove_clues(state, rs);
-    free_game(state);
-    state = state_new;
+            if (get_line_status_from_point(state, i, j, dir2) == 
+                LINE_UNKNOWN) {
+                dd = dline_desc_from_dirs(dir1, dir2);
 
-    empty_count = 0;
-    for (j = 0; j < params->h; ++j) {
-        for (i = 0; i < params->w; ++i) {
-            if (CLUE_AT(state, i, j) == ' ') {
-                if (empty_count > 25) {
-                    dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
-                    empty_count = 0;
+                dlset = get_dot_dline(state, sstate->normal->dot_atmostone, i, j, dd);
+                if (dlset && dir1st == LINE_YES) {
+/*                    fprintf(stderr, "setting %s to NO\n", DIR2STR(dir2)); */
+                    progress |= 
+                        set_line_bydot(sstate, i, j, dir2, LINE_NO);
                 }
-                empty_count++;
-            } else {
-                if (empty_count) {
-                    dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
-                    empty_count = 0;
+
+                dlset = get_dot_dline(state, sstate->normal->dot_atleastone, i, j, dd);
+                if (dlset && dir1st == LINE_NO) {
+/*                    fprintf(stderr, "setting %s to YES\n", DIR2STR(dir2)); */
+                    progress |=
+                        set_line_bydot(sstate, i, j, dir2, LINE_YES);
                 }
-                dp += sprintf(dp, "%c", (int)(CLUE_AT(state, i, j)));
             }
         }
     }
-    if (empty_count)
-        dp += sprintf(dp, "%c", (int)(empty_count + 'a' - 1));
-
-    free_game(state);
-    retval = dupstr(description);
-    sfree(description);
-    
-    assert(!validate_desc(params, retval));
 
-    return retval;
+    return progress;
 }
 
-/* We require that the params pass the test in validate_params and that the
- * description fills the entire game area */
-static char *validate_desc(game_params *params, char *desc)
+/*
+ * These are the main solver functions.  
+ *
+ * Their return values are diff values corresponding to the lowest mode solver
+ * that would notice the work that they have done.  For example if the normal
+ * mode solver adds actual lines or crosses, it will return DIFF_EASY as the
+ * easy mode solver might be able to make progress using that.  It doesn't make
+ * sense for one of them to return a diff value higher than that of the
+ * function itself.  
+ *
+ * Each function returns the lowest value it can, as early as possible, in
+ * order to try and pass as much work as possible back to the lower level
+ * solvers which progress more quickly.
+ */
+
+/* PROPOSED NEW DESIGN:
+ * We have a work queue consisting of 'events' notifying us that something has
+ * happened that a particular solver mode might be interested in.  For example
+ * the hard mode solver might do something that helps the normal mode solver at
+ * dot [x,y] in which case it will enqueue an event recording this fact.  Then
+ * we pull events off the work queue, and hand each in turn to the solver that
+ * is interested in them.  If a solver reports that it failed we pass the same
+ * event on to progressively more advanced solvers and the loop detector.  Once
+ * we've exhausted an event, or it has helped us progress, we drop it and
+ * continue to the next one.  The events are sorted first in order of solver
+ * complexity (easy first) then order of insertion (oldest first).
+ * Once we run out of events we loop over each permitted solver in turn
+ * (easiest first) until either a deduction is made (and an event therefore
+ * emerges) or no further deductions can be made (in which case we've failed).
+ *
+ * QUESTIONS: 
+ *    * How do we 'loop over' a solver when both dots and squares are concerned.
+ *      Answer: first all squares then all dots.
+ */
+
+static int easy_mode_deductions(solver_state *sstate)
 {
-    int count = 0;
+    int i, j, h, w, current_yes, current_no;
+    game_state *state;
+    int diff = DIFF_MAX;
 
-    for (; *desc; ++desc) {
-        if (*desc >= '0' && *desc <= '9') {
-            count++;
+    state = sstate->state;
+    h = state->h;
+    w = state->w;
+    
+    /* Per-square deductions */
+    FORALL_SQUARES(state, i, j) {
+        if (sstate->square_solved[SQUARE_INDEX(state, i, j)])
+            continue;
+
+        current_yes = SQUARE_YES_COUNT(sstate, i, j);
+        current_no  = SQUARE_NO_COUNT(sstate, i, j);
+
+        if (current_yes + current_no == 4)  {
+            sstate->square_solved[SQUARE_INDEX(state, i, j)] = TRUE;
+/*            diff = min(diff, DIFF_EASY); */
             continue;
         }
-        if (*desc >= 'a') {
-            count += *desc - 'a' + 1;
+
+        if (CLUE_AT(state, i, j) < 0)
+            continue;
+
+        if (CLUE_AT(state, i, j) < current_yes) {
+#if 0
+            fprintf(stderr, "detected error [%d,%d] in %s at line %d\n", i, j, __FUNCTION__, __LINE__);
+#endif
+            sstate->solver_status = SOLVER_MISTAKE;
+            return DIFF_EASY;
+        }
+        if (CLUE_AT(state, i, j) == current_yes) {
+            if (square_setall(sstate, i, j, LINE_UNKNOWN, LINE_NO))
+                diff = min(diff, DIFF_EASY);
+            sstate->square_solved[SQUARE_INDEX(state, i, j)] = TRUE;
             continue;
         }
-        return "Unknown character in description";
-    }
 
-    if (count < SQUARE_COUNT(params))
-        return "Description too short for board size";
-    if (count > SQUARE_COUNT(params))
-        return "Description too long for board size";
+        if (4 - CLUE_AT(state, i, j) < current_no) {
+#if 0
+            fprintf(stderr, "detected error [%d,%d] in %s at line %d\n", i, j, __FUNCTION__, __LINE__);
+#endif
+            sstate->solver_status = SOLVER_MISTAKE;
+            return DIFF_EASY;
+        }
+        if (4 - CLUE_AT(state, i, j) == current_no) {
+            if (square_setall(sstate, i, j, LINE_UNKNOWN, LINE_YES))
+                diff = min(diff, DIFF_EASY);
+            sstate->square_solved[SQUARE_INDEX(state, i, j)] = TRUE;
+            continue;
+        }
+    }
 
-    return NULL;
-}
+    check_caches(sstate);
 
-static game_state *new_game(midend *me, game_params *params, char *desc)
-{
-    int i,j;
-    game_state *state = snew(game_state);
-    int empties_to_make = 0;
-    int n;
-    const char *dp = desc;
+    /* Per-dot deductions */
+    FORALL_DOTS(state, i, j) {
+        if (sstate->dot_solved[DOT_INDEX(state, i, j)])
+            continue;
 
-    state->recursion_depth = params->rec;
-    
-    state->h = params->h;
-    state->w = params->w;
+        switch (DOT_YES_COUNT(sstate, i, j)) {
+            case 0:
+                switch (DOT_NO_COUNT(sstate, i, j)) {
+                    case 3:
+#if 0
+                        fprintf(stderr, "dot [%d,%d]: 0 yes, 3 no\n", i, j);
+#endif
+                        dot_setall(sstate, i, j, LINE_UNKNOWN, LINE_NO);
+                        diff = min(diff, DIFF_EASY);
+                        /* fall through */
+                    case 4:
+                        sstate->dot_solved[DOT_INDEX(state, i, j)] = TRUE;
+                        break;
+                }
+                break;
+            case 1:
+                switch (DOT_NO_COUNT(sstate, i, j)) {
+                    case 2: /* 1 yes, 2 no */
+#if 0
+                        fprintf(stderr, "dot [%d,%d]: 1 yes, 2 no\n", i, j);
+#endif
+                        dot_setall(sstate, i, j, LINE_UNKNOWN, LINE_YES);
+                        diff = min(diff, DIFF_EASY);
+                        sstate->dot_solved[DOT_INDEX(state, i, j)] = TRUE;
+                        break;
+                    case 3: /* 1 yes, 3 no */
+#if 0
+                        fprintf(stderr, "detected error [%d,%d] in %s at line %d\n", i, j, __FUNCTION__, __LINE__);
+#endif
+                        sstate->solver_status = SOLVER_MISTAKE;
+                        return DIFF_EASY;
+                }
+                break;
+            case 2:
+#if 0
+                fprintf(stderr, "dot [%d,%d]: 2 yes\n", i, j);
+#endif
+                dot_setall(sstate, i, j, LINE_UNKNOWN, LINE_NO);
+                diff = min(diff, DIFF_EASY);
+                sstate->dot_solved[DOT_INDEX(state, i, j)] = TRUE;
+                break;
+            case 3:
+            case 4:
+#if 0
+                fprintf(stderr, "detected error [%d,%d] in %s at line %d\n", i, j, __FUNCTION__, __LINE__);
+#endif
+                sstate->solver_status = SOLVER_MISTAKE;
+                return DIFF_EASY;
+        }
+    }
 
-    state->clues = snewn(SQUARE_COUNT(params), char);
-    state->hl    = snewn(HL_COUNT(params), char);
-    state->vl    = snewn(VL_COUNT(params), char);
+    check_caches(sstate);
 
-    state->solved = state->cheated = FALSE;
+    return diff;
+}
 
-    for (j = 0 ; j < params->h; ++j) {
-        for (i = 0 ; i < params->w; ++i) {
-            if (empties_to_make) {
-                empties_to_make--;
-                LV_CLUE_AT(state, i, j) = ' ';
-                continue;
-            }
+static int normal_mode_deductions(solver_state *sstate)
+{
+    int i, j;
+    game_state *state = sstate->state;
+    enum dline_desc dd;
+    int diff = DIFF_MAX;
 
-           assert(*dp);
-            n = *dp - '0';
-            if (n >=0 && n < 10) {
-                LV_CLUE_AT(state, i, j) = *dp;
-            } else {
-                n = *dp - 'a' + 1;
-                assert(n > 0);
-                LV_CLUE_AT(state, i, j) = ' ';
-                empties_to_make = n - 1;
-            }
-            ++dp;
-        }
-    }
+    FORALL_SQUARES(state, i, j) {
+        if (sstate->square_solved[SQUARE_INDEX(state, i, j)])
+            continue;
 
-    memset(state->hl, LINE_UNKNOWN, HL_COUNT(params));
-    memset(state->vl, LINE_UNKNOWN, VL_COUNT(params));
+        if (CLUE_AT(state, i, j) < 0)
+            continue;
 
-    return state;
-}
+        switch (CLUE_AT(state, i, j)) {
+            case 1:
+#if 0
+                fprintf(stderr, "clue [%d,%d] is 1, doing dline ops\n",
+                        i, j);
+#endif
+                FORALL_SQUARE_DLINES(dd) {
+                    /* At most one of any DLINE can be set */
+                    if (set_square_dline(state, 
+                                         sstate->normal->dot_atmostone, 
+                                         i, j, dd)) {
+                        diff = min(diff, DIFF_NORMAL);
+                    }
 
-enum { LOOP_NONE=0, LOOP_SOLN, LOOP_NOT_SOLN };
+                    if (get_square_dline(state,
+                                         sstate->normal->dot_atleastone, 
+                                         i, j, dd)) {
+                        /* This DLINE provides enough YESes to solve the clue */
+                        if (square_setboth_in_dline(sstate, OPP_DLINE(dd),
+                                                     i, j, LINE_NO)) {
+                            diff = min(diff, DIFF_EASY);
+                        }
+                    }
+                }
 
-/* Starting at dot [i,j] moves around 'state' removing lines until it's clear
- * whether or not the starting dot was on a loop.  Returns boolean specifying
- * whether a loop was found.  loop_status calls this and assumes that if state
- * has any lines set, this function will always remove at least one.  */
-static int destructively_find_loop(game_state *state)
-{
-    int a, b, i, j, new_i, new_j, n;
-    char *lp;
-
-    lp = (char *)memchr(state->hl, LINE_YES, HL_COUNT(state));
-    if (!lp) {
-        /* We know we're going to return false but we have to fulfil our
-         * contract */
-        lp = (char *)memchr(state->vl, LINE_YES, VL_COUNT(state));
-        if (lp)
-            *lp = LINE_NO;
-        
-        return FALSE;
-    }
+                break;
+            case 2:
+                /* If at least one of one DLINE is set, at most one
+                 * of the opposing one is and vice versa */
+#if 0
+                fprintf(stderr, "clue [%d,%d] is 2, doing dline ops\n",
+                               i, j);
+#endif
+                FORALL_SQUARE_DLINES(dd) {
+                    if (get_square_dline(state,
+                                         sstate->normal->dot_atmostone,
+                                         i, j, dd)) {
+                        if (set_square_opp_dline(state,
+                                                 sstate->normal->dot_atleastone,
+                                                 i, j, dd)) {
+                            diff = min(diff, DIFF_NORMAL);
+                        }
+                    }
+                    if (get_square_dline(state,
+                                         sstate->normal->dot_atleastone,
+                                         i, j, dd)) {
+                        if (set_square_opp_dline(state,
+                                                 sstate->normal->dot_atmostone,
+                                                 i, j, dd)) {
+                            diff = min(diff, DIFF_NORMAL);
+                        }
+                    }
+                }
+                break;
+            case 3:
+#if 0
+                fprintf(stderr, "clue [%d,%d] is 3, doing dline ops\n",
+                                i, j);
+#endif
+                FORALL_SQUARE_DLINES(dd) {
+                    /* At least one of any DLINE must be set */
+                    if (set_square_dline(state, 
+                                         sstate->normal->dot_atleastone, 
+                                         i, j, dd)) {
+                        diff = min(diff, DIFF_NORMAL);
+                    }
 
-    n = lp - state->hl;
-
-    i = n % state->w;
-    j = n / state->w;
-
-    assert(i + j * state->w == n); /* because I'm feeling stupid */
-    /* Save start position */
-    a = i;
-    b = j;
-
-    /* Delete one line from the potential loop */
-    if (LEFTOF_DOT(state, i, j) == LINE_YES) {
-        LV_LEFTOF_DOT(state, i, j) = LINE_NO;
-        i--;
-    } else if (ABOVE_DOT(state, i, j) == LINE_YES) {
-        LV_ABOVE_DOT(state, i, j) = LINE_NO;
-        j--;
-    } else if (RIGHTOF_DOT(state, i, j) == LINE_YES) {
-        LV_RIGHTOF_DOT(state, i, j) = LINE_NO;
-        i++;
-    } else if (BELOW_DOT(state, i, j) == LINE_YES) {
-        LV_BELOW_DOT(state, i, j) = LINE_NO;
-        j++;
-    } else {
-        return FALSE;
+                    if (get_square_dline(state,
+                                         sstate->normal->dot_atmostone, 
+                                         i, j, dd)) {
+                        /* This DLINE provides enough NOs to solve the clue */
+                        if (square_setboth_in_dline(sstate, OPP_DLINE(dd),
+                                                    i, j, LINE_YES)) {
+                            diff = min(diff, DIFF_EASY);
+                        }
+                    }
+                }
+                break;
+        }
     }
 
-    do {
-        /* From the current position of [i,j] there needs to be exactly one
-         * line */
-        new_i = new_j = -1;
-
-#define HANDLE_DIR(dir_dot, x, y)                    \
-        if (dir_dot(state, i, j) == LINE_YES) {      \
-            if (new_i != -1 || new_j != -1)          \
-                return FALSE;                        \
-            new_i = (i)+(x);                         \
-            new_j = (j)+(y);                         \
-            LV_##dir_dot(state, i, j) = LINE_NO;     \
-        }
-        HANDLE_DIR(ABOVE_DOT,    0, -1);
-        HANDLE_DIR(BELOW_DOT,    0, +1);
-        HANDLE_DIR(LEFTOF_DOT,  -1,  0);
-        HANDLE_DIR(RIGHTOF_DOT, +1,  0);
-#undef HANDLE_DIR
-        if (new_i == -1 || new_j == -1) {
-            return FALSE;
-        }
+    check_caches(sstate);
 
-        i = new_i;
-        j = new_j;
-    } while (i != a || j != b);
+    if (diff < DIFF_NORMAL)
+        return diff;
 
-    return TRUE;
-}
+    FORALL_DOTS(state, i, j) {
+        if (sstate->dot_solved[DOT_INDEX(state, i, j)])
+            continue;
 
-static int loop_status(game_state *state)
-{
-    int i, j, n;
-    game_state *tmpstate;
-    int loop_found = FALSE, non_loop_found = FALSE, any_lines_found = FALSE;
+#if 0
+        text = game_text_format(state);
+        fprintf(stderr, "-----------------\n%s", text);
+        sfree(text);
+#endif
 
-#define BAD_LOOP_FOUND \
-    do { free_game(tmpstate); return LOOP_NOT_SOLN; } while(0)
+        switch (DOT_YES_COUNT(sstate, i, j)) {
+        case 0:
+            switch (DOT_NO_COUNT(sstate, i, j)) {
+                case 1:
+                    /* Make note that at most one of each unknown DLINE
+                     * is YES */
+                    break;
+            }
+            break;
 
-    /* Repeatedly look for loops until we either run out of lines to consider
-     * or discover for sure that the board fails on the grounds of having no
-     * loop */
-    tmpstate = dup_game(state);
+        case 1:
+            switch (DOT_NO_COUNT(sstate, i, j)) {
+                case 1: 
+                    /* 1 yes, 1 no, so exactly one of unknowns is
+                     * yes */
+#if 0
+                    fprintf(stderr, "dot [%d,%d]: 1 yes, 1 no\n", i, j);
+#endif
+                    FORALL_DOT_DLINES(dd) {
+                        if (dline_both_unknown(state, 
+                                               i, j, dd)) {
+                            if (set_dot_dline(state,
+                                              sstate->normal->dot_atleastone,
+                                              i, j, dd)) {
+                                diff = min(diff, DIFF_NORMAL); 
+                            }
+                        }
+                    }
 
-    while (TRUE) {
-        if (!memchr(tmpstate->hl, LINE_YES, HL_COUNT(tmpstate)) &&
-            !memchr(tmpstate->vl, LINE_YES, VL_COUNT(tmpstate))) {
+                    /* fall through */
+                case 0: 
+#if 0
+                    fprintf(stderr, "dot [%d,%d]: 1 yes, 0 or 1 no\n", i, j);
+#endif
+                    /* 1 yes, fewer than 2 no, so at most one of
+                     * unknowns is yes */
+                    FORALL_DOT_DLINES(dd) {
+                        if (dline_both_unknown(state, 
+                                               i, j, dd)) {
+                            if (set_dot_dline(state,
+                                              sstate->normal->dot_atmostone,
+                                              i, j, dd)) {
+                                diff = min(diff, DIFF_NORMAL); 
+                            }
+                        }
+                    }
+                    break;
+            }
             break;
         }
-        any_lines_found = TRUE;
-
-        if (loop_found) 
-            BAD_LOOP_FOUND;
-        if (destructively_find_loop(tmpstate)) {
-            loop_found = TRUE;
-            if (non_loop_found)
-                BAD_LOOP_FOUND;
-        } else {
-            non_loop_found = TRUE;
-        }
-    }
-
-    free_game(tmpstate);
-
-    if (!any_lines_found)
-        return LOOP_NONE;
-    
-    if (non_loop_found) {
-        assert(!loop_found); /* should have dealt with this already */
-        return LOOP_NONE;
-    }
 
-    /* Check that every clue is satisfied */
-    for (j = 0; j < state->h; ++j) {
-        for (i = 0; i < state->w; ++i) {
-            n = CLUE_AT(state, i, j);
-            if (n != ' ') {
-                if (square_order(state, i, j, LINE_YES) != n - '0') {
-                    return LOOP_NOT_SOLN;
+        /* DLINE deductions that don't depend on the exact number of
+         * LINE_YESs or LINE_NOs */
+
+        /* If at least one of a dline in a dot is YES, at most one
+         * of the opposite dline to that dot must be YES. */
+        FORALL_DOT_DLINES(dd) {
+            if (get_dot_dline(state, 
+                              sstate->normal->dot_atleastone,
+                              i, j, dd)) {
+                if (set_dot_opp_dline(state,
+                                      sstate->normal->dot_atmostone,
+                                      i, j, dd)) {
+                    diff = min(diff, DIFF_NORMAL); 
                 }
             }
         }
+
+        if (dot_collapse_dlines(sstate, i, j))
+            diff = min(diff, DIFF_EASY);
     }
+    check_caches(sstate);
 
-    return LOOP_SOLN;
+    return diff;
 }
 
-/* Sums the lengths of the numbers in range [0,n) */
-/* See equivalent function in solo.c for justification of this. */
-static int len_0_to_n(int n)
+static int hard_mode_deductions(solver_state *sstate)
 {
-    int len = 1; /* Counting 0 as a bit of a special case */
-    int i;
+    int i, j, a, b, s;
+    game_state *state = sstate->state;
+    const int h=state->h, w=state->w;
+    enum direction dir1, dir2;
+    int can1, can2, inv1, inv2;
+    int diff = DIFF_MAX;
+    enum dline_desc dd;
+
+    FORALL_SQUARES(state, i, j) {
+        if (sstate->square_solved[SQUARE_INDEX(state, i, j)])
+            continue;
 
-    for (i = 1; i < n; i *= 10) {
-        len += max(n - i, 0);
+        switch (CLUE_AT(state, i, j)) {
+            case -1:
+                continue;
+
+            case 1:
+                if (square_setall_identical(sstate, i, j, LINE_NO)) 
+                    diff = min(diff, DIFF_EASY);
+                break;
+            case 3:
+                if (square_setall_identical(sstate, i, j, LINE_YES))
+                    diff = min(diff, DIFF_EASY);
+                break;
+        }
+
+        if (SQUARE_YES_COUNT(sstate, i, j) + 
+            SQUARE_NO_COUNT(sstate, i, j) == 2) {
+            /* There are exactly two unknown lines bordering this
+             * square. */
+            if (SQUARE_YES_COUNT(sstate, i, j) + 1 == 
+                CLUE_AT(state, i, j)) {
+                /* They must be different */
+                if (square_relate_2_unknowns(sstate, i, j, TRUE))
+                    diff = min(diff, DIFF_HARD);
+            }
+        }
     }
 
-    return len;
-}
+    check_caches(sstate);
 
-static char *encode_solve_move(const game_state *state)
-{
-    int len, i, j;
-    char *ret, *p;
-    /* This is going to return a string representing the moves needed to set
-     * every line in a grid to be the same as the ones in 'state'.  The exact
-     * length of this string is predictable. */
+    FORALL_DOTS(state, i, j) {
+        if (DOT_YES_COUNT(sstate, i, j) == 1 &&
+            DOT_NO_COUNT(sstate, i, j) == 1) {
+            if (dot_relate_2_unknowns(sstate, i, j, TRUE))
+                diff = min(diff, DIFF_HARD);
+            continue;
+        }
 
-    len = 1;  /* Count the 'S' prefix */
-    /* Numbers in horizontal lines */
-    /* Horizontal lines, x position */
-    len += len_0_to_n(state->w) * (state->h + 1);
-    /* Horizontal lines, y position */
-    len += len_0_to_n(state->h + 1) * (state->w);
-    /* Vertical lines, y position */
-    len += len_0_to_n(state->h) * (state->w + 1);
-    /* Vertical lines, x position */
-    len += len_0_to_n(state->w + 1) * (state->h);
-    /* For each line we also have two letters and a comma */
-    len += 3 * (HL_COUNT(state) + VL_COUNT(state));
+        if (DOT_YES_COUNT(sstate, i, j) == 0 &&
+            DOT_NO_COUNT(sstate, i, j) == 2) {
+            if (dot_relate_2_unknowns(sstate, i, j, FALSE))
+                diff = min(diff, DIFF_HARD);
+            continue;
+        }
+    }
 
-    ret = snewn(len + 1, char);
-    p = ret;
+    /* If two lines into a dot are related, the other two lines into that dot
+     * are related in the same way. */
 
-    p += sprintf(p, "S");
+    /* iter over points that aren't on edges */
+    for (i = 1; i < w; ++i) {
+        for (j = 1; j < h; ++j) {
+            if (sstate->dot_solved[DOT_INDEX(state, i, j)])
+                continue;
 
-    for (j = 0; j < state->h + 1; ++j) {
-        for (i = 0; i < state->w; ++i) {
-            switch (RIGHTOF_DOT(state, i, j)) {
-                case LINE_YES:
-                    p += sprintf(p, "%d,%dhy", i, j);
-                    break;
-                case LINE_NO:
-                    p += sprintf(p, "%d,%dhn", i, j);
-                    break;
-/*                default: */
-                    /* I'm going to forgive this because I think the results
-                     * are cute. */
-/*                    assert(!"Solver produced incomplete solution!"); */
+            /* iter over directions */
+            for (dir1 = 0; dir1 < 4; ++dir1) {
+                for (dir2 = dir1+1; dir2 < 4; ++dir2) {
+                    /* canonify both lines */
+                    can1 = edsf_canonify
+                        (sstate->hard->linedsf,
+                         LINEDSF_INDEX(state, i, j, dir1),
+                         &inv1);
+                    can2 = edsf_canonify
+                        (sstate->hard->linedsf,
+                         LINEDSF_INDEX(state, i, j, dir2),
+                         &inv2);
+                    /* merge opposite lines */
+                    if (can1 == can2) {
+                        if (merge_lines(sstate, 
+                                        i, j, OPP_DIR(dir1),
+                                        i, j, OPP_DIR(dir2),
+                                        inv1 ^ inv2)) {
+                            diff = min(diff, DIFF_HARD);
+                        }
+                    }
+                }
             }
         }
     }
 
-    for (j = 0; j < state->h; ++j) {
-        for (i = 0; i < state->w + 1; ++i) {
-            switch (BELOW_DOT(state, i, j)) {
-                case LINE_YES:
-                    p += sprintf(p, "%d,%dvy", i, j);
-                    break;
-                case LINE_NO:
-                    p += sprintf(p, "%d,%dvn", i, j);
-                    break;
-/*                default: */
-                    /* I'm going to forgive this because I think the results
-                     * are cute. */
-/*                    assert(!"Solver produced incomplete solution!"); */
+    /* If the state of a line is known, deduce the state of its canonical line
+     * too. */
+    FORALL_DOTS(state, i, j) {
+        /* Do this even if the dot we're on is solved */
+        if (i < w) {
+            can1 = edsf_canonify(sstate->hard->linedsf, 
+                                 LINEDSF_INDEX(state, i, j, RIGHT),
+                                 &inv1);
+            linedsf_deindex(state, can1, &a, &b, &dir1);
+            s = RIGHTOF_DOT(state, i, j);
+            if (s != LINE_UNKNOWN)
+            {
+                if (set_line_bydot(sstate, a, b, dir1, inv1 ? OPP(s) : s))
+                    diff = min(diff, DIFF_EASY);
+            }
+        }
+        if (j < h) {
+            can1 = edsf_canonify(sstate->hard->linedsf, 
+                                 LINEDSF_INDEX(state, i, j, DOWN),
+                                 &inv1);
+            linedsf_deindex(state, can1, &a, &b, &dir1);
+            s = BELOW_DOT(state, i, j);
+            if (s != LINE_UNKNOWN)
+            {
+                if (set_line_bydot(sstate, a, b, dir1, inv1 ? OPP(s) : s))
+                    diff = min(diff, DIFF_EASY);
             }
         }
     }
 
-    /* No point in doing sums like that if they're going to be wrong */
-    assert(strlen(ret) <= (size_t)len);
-    return ret;
-}
-
-/* BEGIN SOLVER IMPLEMENTATION */
-
-   /* For each pair of lines through each dot we store a bit for whether
-    * exactly one of those lines is ON, and in separate arrays we store whether
-    * at least one is on and whether at most 1 is on.  (If we know both or
-    * neither is on that's already stored more directly.)  That's six bits per
-    * dot.  Bit number n represents the lines shown in dot_type_dirs[n]. */
-
-enum dline {
-    DLINE_VERT  = 0,
-    DLINE_HORIZ = 1,
-    DLINE_UL    = 2,
-    DLINE_DR    = 3,
-    DLINE_UR    = 4,
-    DLINE_DL    = 5
-};
-
-#define OPP_DLINE(dline) (dline ^ 1)
-   
-
-#define SQUARE_DLINES                                                          \
-                   HANDLE_DLINE(DLINE_UL, RIGHTOF_SQUARE, BELOW_SQUARE, 1, 1); \
-                   HANDLE_DLINE(DLINE_UR, LEFTOF_SQUARE,  BELOW_SQUARE, 0, 1); \
-                   HANDLE_DLINE(DLINE_DL, RIGHTOF_SQUARE, ABOVE_SQUARE, 1, 0); \
-                   HANDLE_DLINE(DLINE_DR, LEFTOF_SQUARE,  ABOVE_SQUARE, 0, 0); 
+    /* Interactions between dline and linedsf */
+    FORALL_DOTS(state, i, j) {
+        if (sstate->dot_solved[DOT_INDEX(state, i, j)])
+            continue;
 
-#define DOT_DLINES                                                       \
-                   HANDLE_DLINE(DLINE_VERT,  ABOVE_DOT,  BELOW_DOT);     \
-                   HANDLE_DLINE(DLINE_HORIZ, LEFTOF_DOT, RIGHTOF_DOT);   \
-                   HANDLE_DLINE(DLINE_UL,    ABOVE_DOT,  LEFTOF_DOT);    \
-                   HANDLE_DLINE(DLINE_UR,    ABOVE_DOT,  RIGHTOF_DOT);   \
-                   HANDLE_DLINE(DLINE_DL,    BELOW_DOT,  LEFTOF_DOT);    \
-                   HANDLE_DLINE(DLINE_DR,    BELOW_DOT,  RIGHTOF_DOT); 
+        FORALL_DOT_DLINES(dd) {
+            const struct dline dll = dlines[dd], *dl = &dll;
+            if (i == 0 && (dl->dir1 == LEFT || dl->dir2 == LEFT))
+                continue;
+            if (i == w && (dl->dir1 == RIGHT || dl->dir2 == RIGHT))
+                continue;
+            if (j == 0 && (dl->dir1 == UP || dl->dir2 == UP))
+                continue;
+            if (j == h && (dl->dir1 == DOWN || dl->dir2 == DOWN))
+                continue;
 
-static void array_setall(char *array, char from, char to, int len)
-{
-    char *p = array, *p_old = p;
-    int len_remaining = len;
+            if (get_dot_dline(state, sstate->normal->dot_atleastone,
+                              i, j, dd) &&
+                get_dot_dline(state, sstate->normal->dot_atmostone,
+                              i, j, dd)) {
+                /* atleastone && atmostone => inverse */
+                if (merge_lines(sstate, i, j, dl->dir1, i, j, dl->dir2, 1)) {
+                    diff = min(diff, DIFF_HARD);
+                }
+            } else {
+                /* don't have atleastone and atmostone for this dline */
+                can1 = edsf_canonify(sstate->hard->linedsf,
+                                     LINEDSF_INDEX(state, i, j, dl->dir1),
+                                     &inv1);
+                can2 = edsf_canonify(sstate->hard->linedsf,
+                                     LINEDSF_INDEX(state, i, j, dl->dir2),
+                                     &inv2);
+                if (can1 == can2) {
+                    if (inv1 == inv2) {
+                        /* identical => collapse dline */
+                        if (get_dot_dline(state, 
+                                          sstate->normal->dot_atleastone,
+                                          i, j, dd)) {
+                            if (set_line_bydot(sstate, i, j, 
+                                               dl->dir1, LINE_YES)) {
+                                diff = min(diff, DIFF_EASY);
+                            }
+                            if (set_line_bydot(sstate, i, j, 
+                                               dl->dir2, LINE_YES)) {
+                                diff = min(diff, DIFF_EASY);
+                            }
+                        } else if (get_dot_dline(state, 
+                                                 sstate->normal->dot_atmostone,
+                                                 i, j, dd)) {
+                            if (set_line_bydot(sstate, i, j, 
+                                               dl->dir1, LINE_NO)) {
+                                diff = min(diff, DIFF_EASY);
+                            }
+                            if (set_line_bydot(sstate, i, j, 
+                                               dl->dir2, LINE_NO)) {
+                                diff = min(diff, DIFF_EASY);
+                            }
+                        }
+                    } else {
+                        /* inverse => atleastone && atmostone */
+                        if (set_dot_dline(state, 
+                                          sstate->normal->dot_atleastone,
+                                          i, j, dd)) {
+                            diff = min(diff, DIFF_NORMAL);
+                        }
+                        if (set_dot_dline(state, 
+                                          sstate->normal->dot_atmostone,
+                                          i, j, dd)) {
+                            diff = min(diff, DIFF_NORMAL);
+                        }
+                    }
+                }
+            }
+        }
+    }
+    
+    /* If the state of the canonical line for line 'l' is known, deduce the
+     * state of 'l' */
+    FORALL_DOTS(state, i, j) {
+        if (sstate->dot_solved[DOT_INDEX(state, i, j)])
+            continue;
 
-    while ((p = memchr(p, from, len_remaining))) {
-        *p = to;
-        len_remaining -= p - p_old;
-        p_old = p;
+        if (i < w) {
+            can1 = edsf_canonify(sstate->hard->linedsf, 
+                                 LINEDSF_INDEX(state, i, j, RIGHT),
+                                 &inv1);
+            linedsf_deindex(state, can1, &a, &b, &dir1);
+            s = get_line_status_from_point(state, a, b, dir1);
+            if (s != LINE_UNKNOWN)
+            {
+                if (set_line_bydot(sstate, i, j, RIGHT, inv1 ? OPP(s) : s))
+                    diff = min(diff, DIFF_EASY);
+            }
+        }
+        if (j < h) {
+            can1 = edsf_canonify(sstate->hard->linedsf, 
+                                 LINEDSF_INDEX(state, i, j, DOWN),
+                                 &inv1);
+            linedsf_deindex(state, can1, &a, &b, &dir1);
+            s = get_line_status_from_point(state, a, b, dir1);
+            if (s != LINE_UNKNOWN)
+            {
+                if (set_line_bydot(sstate, i, j, DOWN, inv1 ? OPP(s) : s))
+                    diff = min(diff, DIFF_EASY);
+            }
+        }
     }
-}
 
+    return diff;
+}
 
-static int game_states_equal(const game_state *state1,
-                             const game_state *state2) 
+static int loop_deductions(solver_state *sstate)
 {
-    /* This deliberately doesn't check _all_ fields, just the ones that make a
-     * game state 'interesting' from the POV of the solver */
-    /* XXX review this */
-    if (state1 == state2)
-        return 1;
-
-    if (!state1 || !state2)
-        return 0;
-
-    if (state1->w != state2->w || state1->h != state2->h)
-        return 0;
-
-    if (memcmp(state1->hl, state2->hl, HL_COUNT(state1)))
-        return 0;
-
-    if (memcmp(state1->vl, state2->vl, VL_COUNT(state1)))
-        return 0;
+    int edgecount = 0, clues = 0, satclues = 0, sm1clues = 0;
+    game_state *state = sstate->state;
+    int shortest_chainlen = DOT_COUNT(state);
+    int loop_found = FALSE;
+    int d;
+    int dots_connected;
+    int progress = FALSE;
+    int i, j;
 
-    return 1;
-}
+    /*
+     * Go through the grid and update for all the new edges.
+     * Since merge_dots() is idempotent, the simplest way to
+     * do this is just to update for _all_ the edges.
+     * 
+     * Also, while we're here, we count the edges, count the
+     * clues, count the satisfied clues, and count the
+     * satisfied-minus-one clues.
+     */
+    FORALL_DOTS(state, i, j) {
+        if (RIGHTOF_DOT(state, i, j) == LINE_YES) {
+            loop_found |= merge_dots(sstate, i, j, i+1, j);
+            edgecount++;
+        }
+        if (BELOW_DOT(state, i, j) == LINE_YES) {
+            loop_found |= merge_dots(sstate, i, j, i, j+1);
+            edgecount++;
+        }
 
-static int solver_states_equal(const solver_state *sstate1,
-                               const solver_state *sstate2)
-{
-    if (!sstate1) {
-        if (!sstate2)
-            return TRUE;
-        else
-            return FALSE;
+        if (CLUE_AT(state, i, j) >= 0) {
+            int c = CLUE_AT(state, i, j);
+            int o = SQUARE_YES_COUNT(sstate, i, j);
+            if (o == c)
+                satclues++;
+            else if (o == c-1)
+                sm1clues++;
+            clues++;
+        }
     }
-    
-    if (!game_states_equal(sstate1->state, sstate2->state)) {
-        return 0;
+
+    for (i = 0; i < DOT_COUNT(state); ++i) {
+        dots_connected = 
+            sstate->looplen[dsf_canonify(sstate->dotdsf, i)];
+        if (dots_connected > 1)
+            shortest_chainlen = min(shortest_chainlen, dots_connected);
     }
 
-    /* XXX fields missing, needs review */
-    /* XXX we're deliberately not looking at solver_state as it's only a cache */
+    assert(sstate->solver_status == SOLVER_INCOMPLETE);
 
-    if (memcmp(sstate1->dot_atleastone, sstate2->dot_atleastone,
-               DOT_COUNT(sstate1->state))) {
-        return 0;
+    if (satclues == clues && shortest_chainlen == edgecount) {
+        sstate->solver_status = SOLVER_SOLVED;
+        /* This discovery clearly counts as progress, even if we haven't
+         * just added any lines or anything */
+        progress = TRUE; 
+        goto finished_loop_deductionsing;
     }
 
-    if (memcmp(sstate1->dot_atmostone, sstate2->dot_atmostone,
-               DOT_COUNT(sstate1->state))) {
-        return 0;
-    }
+    /*
+     * Now go through looking for LINE_UNKNOWN edges which
+     * connect two dots that are already in the same
+     * equivalence class. If we find one, test to see if the
+     * loop it would create is a solution.
+     */
+    FORALL_DOTS(state, i, j) {
+        for (d = 0; d < 2; d++) {
+            int i2, j2, eqclass, val;
 
-    /* handle dline_identical here */
+            if (d == 0) {
+                if (RIGHTOF_DOT(state, i, j) !=
+                        LINE_UNKNOWN)
+                    continue;
+                i2 = i+1;
+                j2 = j;
+            } else {
+                if (BELOW_DOT(state, i, j) !=
+                    LINE_UNKNOWN) {
+                    continue;
+                }
+                i2 = i;
+                j2 = j+1;
+            }
 
-    return 1;
-}
+            eqclass = dsf_canonify(sstate->dotdsf, j * (state->w+1) + i);
+            if (eqclass != dsf_canonify(sstate->dotdsf,
+                                        j2 * (state->w+1) + i2)) {
+                continue;
+            }
 
-static void dot_setall_dlines(solver_state *sstate, enum dline dl, int i, int j,
-                              enum line_state line_old, enum line_state line_new) 
-{
-    game_state *state = sstate->state;
+            val = LINE_NO;  /* loop is bad until proven otherwise */
+
+            /*
+             * This edge would form a loop. Next
+             * question: how long would the loop be?
+             * Would it equal the total number of edges
+             * (plus the one we'd be adding if we added
+             * it)?
+             */
+            if (sstate->looplen[eqclass] == edgecount + 1) {
+                int sm1_nearby;
+                int cx, cy;
+
+                /*
+                 * This edge would form a loop which
+                 * took in all the edges in the entire
+                 * grid. So now we need to work out
+                 * whether it would be a valid solution
+                 * to the puzzle, which means we have to
+                 * check if it satisfies all the clues.
+                 * This means that every clue must be
+                 * either satisfied or satisfied-minus-
+                 * 1, and also that the number of
+                 * satisfied-minus-1 clues must be at
+                 * most two and they must lie on either
+                 * side of this edge.
+                 */
+                sm1_nearby = 0;
+                cx = i - (j2-j);
+                cy = j - (i2-i);
+                if (CLUE_AT(state, cx,cy) >= 0 &&
+                        square_order(state, cx,cy, LINE_YES) ==
+                        CLUE_AT(state, cx,cy) - 1) {
+                    sm1_nearby++;
+                }
+                if (CLUE_AT(state, i, j) >= 0 &&
+                        SQUARE_YES_COUNT(sstate, i, j) ==
+                        CLUE_AT(state, i, j) - 1) {
+                    sm1_nearby++;
+                }
+                if (sm1clues == sm1_nearby &&
+                    sm1clues + satclues == clues) {
+                    val = LINE_YES;  /* loop is good! */
+                }
+            }
 
-    /* First line in dline */
-    switch (dl) {                                             
-        case DLINE_UL:                                                  
-        case DLINE_UR:                                                  
-        case DLINE_VERT:                                                  
-            if (j > 0 && ABOVE_DOT(state, i, j) == line_old)            
-                LV_ABOVE_DOT(state, i, j) = line_new;                   
-            break;                                                          
-        case DLINE_DL:                                                  
-        case DLINE_DR:                                                  
-            if (j <= (state)->h && BELOW_DOT(state, i, j) == line_old)  
-                LV_BELOW_DOT(state, i, j) = line_new;                   
-            break;
-        case DLINE_HORIZ:                                                  
-            if (i > 0 && LEFTOF_DOT(state, i, j) == line_old)           
-                LV_LEFTOF_DOT(state, i, j) = line_new;                  
-            break;                                                          
-    }
-
-    /* Second line in dline */
-    switch (dl) {                                             
-        case DLINE_UL:                                                  
-        case DLINE_DL:                                                  
-            if (i > 0 && LEFTOF_DOT(state, i, j) == line_old)           
-                LV_LEFTOF_DOT(state, i, j) = line_new;                  
-            break;                                                          
-        case DLINE_UR:                                                  
-        case DLINE_DR:                                                  
-        case DLINE_HORIZ:                                                  
-            if (i <= (state)->w && RIGHTOF_DOT(state, i, j) == line_old)
-                LV_RIGHTOF_DOT(state, i, j) = line_new;                 
-            break;                                                          
-        case DLINE_VERT:                                                  
-            if (j <= (state)->h && BELOW_DOT(state, i, j) == line_old)  
-                LV_BELOW_DOT(state, i, j) = line_new;                   
-            break;                                                          
-    }
-}
-
-static void update_solver_status(solver_state *sstate)
-{
-    if (sstate->solver_status == SOLVER_INCOMPLETE) {
-        switch (loop_status(sstate->state)) {
-            case LOOP_NONE:
-                sstate->solver_status = SOLVER_INCOMPLETE;
-                break;
-            case LOOP_SOLN:
-                if (sstate->solver_status != SOLVER_AMBIGUOUS)
-                    sstate->solver_status = SOLVER_SOLVED;
-                break;
-            case LOOP_NOT_SOLN:
-                sstate->solver_status = SOLVER_MISTAKE;
-                break;
+            /*
+             * Right. Now we know that adding this edge
+             * would form a loop, and we know whether
+             * that loop would be a viable solution or
+             * not.
+             * 
+             * If adding this edge produces a solution,
+             * then we know we've found _a_ solution but
+             * we don't know that it's _the_ solution -
+             * if it were provably the solution then
+             * we'd have deduced this edge some time ago
+             * without the need to do loop detection. So
+             * in this state we return SOLVER_AMBIGUOUS,
+             * which has the effect that hitting Solve
+             * on a user-provided puzzle will fill in a
+             * solution but using the solver to
+             * construct new puzzles won't consider this
+             * a reasonable deduction for the user to
+             * make.
+             */
+            if (d == 0) {
+                progress = set_line_bydot(sstate, i, j, RIGHT, val);
+                assert(progress == TRUE);
+            } else {
+                progress = set_line_bydot(sstate, i, j, DOWN, val);
+                assert(progress == TRUE);
+            }
+            if (val == LINE_YES) {
+                sstate->solver_status = SOLVER_AMBIGUOUS;
+                goto finished_loop_deductionsing;
+            }
         }
     }
-}
 
+finished_loop_deductionsing:
+    return progress ? DIFF_EASY : DIFF_MAX;
+}
 
 /* This will return a dynamically allocated solver_state containing the (more)
  * solved grid */
-static solver_state *solve_game_rec(const solver_state *sstate_start)
+static solver_state *solve_game_rec(const solver_state *sstate_start, 
+                                    int diff)
 {
-   int i, j;
-   int current_yes, current_no, desired;
-   solver_state *sstate, *sstate_saved, *sstate_tmp;
-   int t;
-/*   char *text; */
-   solver_state *sstate_rec_solved;
-   int recursive_soln_count;
+    int i, j;
+    int w, h;
+    solver_state *sstate, *sstate_saved, *sstate_tmp;
+    solver_state *sstate_rec_solved;
+    int recursive_soln_count;
+    int solver_progress;
+    game_state *state;
+
+    /* Indicates which solver we should call next.  This is a sensible starting
+     * point */
+    int current_solver = DIFF_EASY, next_solver;
+#ifdef SHOW_WORKING
+    char *text;
+#endif
 
 #if 0
-   printf("solve_game_rec: recursion_remaining = %d\n", 
-          sstate_start->recursion_remaining);
+    printf("solve_game_rec: recursion_remaining = %d\n", 
+           sstate_start->recursion_remaining);
 #endif
 
-   sstate = dup_solver_state((solver_state *)sstate_start);
+    sstate = dup_solver_state(sstate_start);
+    /* Cache the values of some variables for readability */
+    state = sstate->state;
+    h = state->h;
+    w = state->w;
 
-#if 0
-   text = game_text_format(sstate->state);
-   printf("%s\n", text);
-   sfree(text);
+    sstate_saved = NULL;
+
+nonrecursive_solver:
+    solver_progress = FALSE;
+
+    check_caches(sstate);
+
+    do {
+#ifdef SHOW_WORKING
+        text = game_text_format(state);
+        fprintf(stderr, "-----------------\n%s", text);
+        sfree(text);
 #endif
-   
-#define RETURN_IF_SOLVED                                 \
-   do {                                                  \
-       update_solver_status(sstate);                     \
-       if (sstate->solver_status != SOLVER_INCOMPLETE) { \
-           free_solver_state(sstate_saved);              \
-           return sstate;                                \
-       }                                                 \
-   } while (0)
-
-   sstate_saved = NULL;
-   RETURN_IF_SOLVED;
 
-nonrecursive_solver:
-   
-   while (1) {
-       sstate_saved = dup_solver_state(sstate);
-
-       /* First we do the 'easy' work, that might cause concrete results */
-
-       /* Per-square deductions */
-       for (j = 0; j < sstate->state->h; ++j) {
-           for (i = 0; i < sstate->state->w; ++i) {
-               /* Begin rules that look at the clue (if there is one) */
-               desired = CLUE_AT(sstate->state, i, j);
-               if (desired == ' ')
-                   continue;
-               desired = desired - '0';
-               current_yes = square_order(sstate->state, i, j, LINE_YES);
-               current_no  = square_order(sstate->state, i, j, LINE_NO);
-
-               if (desired <= current_yes) {
-                   square_setall(sstate->state, i, j, LINE_UNKNOWN, LINE_NO);
-                   continue;
-               }
+        if (sstate->solver_status == SOLVER_MISTAKE)
+            return sstate;
+
+/*        fprintf(stderr, "Invoking solver %d\n", current_solver); */
+        next_solver = solver_fns[current_solver](sstate);
+
+        if (next_solver == DIFF_MAX) {
+/*            fprintf(stderr, "Current solver failed\n"); */
+            if (current_solver < diff && current_solver + 1 < DIFF_MAX) {
+                /* Try next beefier solver */
+                next_solver = current_solver + 1;
+            } else {
+/*                fprintf(stderr, "Doing loop deductions\n"); */
+                next_solver = loop_deductions(sstate);
+            }
+        }
+
+        if (sstate->solver_status == SOLVER_SOLVED || 
+            sstate->solver_status == SOLVER_AMBIGUOUS) {
+/*            fprintf(stderr, "Solver completed\n"); */
+            break;
+        }
+
+        /* Once we've looped over all permitted solvers then the loop
+         * deductions without making any progress, we'll exit this while loop */
+        current_solver = next_solver;
+    } while (current_solver < DIFF_MAX);
+
+    if (sstate->solver_status == SOLVER_SOLVED ||
+        sstate->solver_status == SOLVER_AMBIGUOUS) {
+        /* s/LINE_UNKNOWN/LINE_NO/g */
+        array_setall(sstate->state->hl, LINE_UNKNOWN, LINE_NO, 
+                     HL_COUNT(sstate->state));
+        array_setall(sstate->state->vl, LINE_UNKNOWN, LINE_NO, 
+                     VL_COUNT(sstate->state));
+        return sstate;
+    }
 
-               if (4 - desired <= current_no) {
-                   square_setall(sstate->state, i, j, LINE_UNKNOWN, LINE_YES);
-               }
-           }
-       }
+    /* Perform recursive calls */
+    if (sstate->recursion_remaining) {
+        sstate_saved = dup_solver_state(sstate);
 
-       RETURN_IF_SOLVED;
-
-       /* Per-dot deductions */
-       for (j = 0; j < sstate->state->h + 1; ++j) {
-           for (i = 0; i < sstate->state->w + 1; ++i) {
-               switch (dot_order(sstate->state, i, j, LINE_YES)) {
-               case 0:
-                   if (dot_order(sstate->state, i, j, LINE_NO) == 3) {
-                       dot_setall(sstate->state, i, j, LINE_UNKNOWN, LINE_NO);
-                   }
-                   break;
-               case 1:
-                   switch (dot_order(sstate->state, i, j, LINE_NO)) {
-#define H1(dline, dir1_dot, dir2_dot, dot_howmany)                             \
-                       if (dir1_dot(sstate->state, i, j) == LINE_UNKNOWN) {    \
-                           if (dir2_dot(sstate->state, i, j) == LINE_UNKNOWN){ \
-                               sstate->dot_howmany                             \
-                                 [i + (sstate->state->w + 1) * j] |= 1<<dline; \
-                           }                                                   \
-                       }
-                       case 1: 
-#define HANDLE_DLINE(dline, dir1_dot, dir2_dot)                               \
-                           H1(dline, dir1_dot, dir2_dot, dot_atleastone)
-                           /* 1 yes, 1 no, so exactly one of unknowns is yes */
-                           DOT_DLINES;
-#undef HANDLE_DLINE
-                           /* fall through */
-                       case 0: 
-#define HANDLE_DLINE(dline, dir1_dot, dir2_dot)                               \
-                           H1(dline, dir1_dot, dir2_dot, dot_atmostone)
-                           /* 1 yes, fewer than 2 no, so at most one of
-                            * unknowns is yes */
-                           DOT_DLINES;
-#undef HANDLE_DLINE
-#undef H1
-                           break;
-                       case 2: /* 1 yes, 2 no */
-                           dot_setall(sstate->state, i, j, 
-                                      LINE_UNKNOWN, LINE_YES);
-                           break;
-                   }
-                   break;
-               case 2:
-               case 3:
-                   dot_setall(sstate->state, i, j, LINE_UNKNOWN, LINE_NO);
-               }
-#define HANDLE_DLINE(dline, dir1_dot, dir2_dot)                               \
-               if (sstate->dot_atleastone                                     \
-                     [i + (sstate->state->w + 1) * j] & 1<<dline) {           \
-                   sstate->dot_atmostone                                      \
-                     [i + (sstate->state->w + 1) * j] |= 1<<OPP_DLINE(dline); \
-               }
-               /* If at least one of a dline in a dot is YES, at most one of
-                * the opposite dline to that dot must be YES. */
-               DOT_DLINES;
-#undef HANDLE_DLINE
-           }
-       }
-       
-       /* More obscure per-square operations */
-       for (j = 0; j < sstate->state->h; ++j) {
-           for (i = 0; i < sstate->state->w; ++i) {
-#define H1(dline, dir1_sq, dir2_sq, a, b, dot_howmany, line_query, line_set)  \
-               if (sstate->dot_howmany[i+a + (sstate->state->w + 1) * (j+b)] &\
-                       1<<dline) {                                            \
-                   t = dir1_sq(sstate->state, i, j);                          \
-                   if (t == line_query)                                       \
-                       dir2_sq(sstate->state, i, j) = line_set;               \
-                   else {                                                     \
-                       t = dir2_sq(sstate->state, i, j);                      \
-                       if (t == line_query)                                   \
-                           dir1_sq(sstate->state, i, j) = line_set;           \
-                   }                                                          \
-               }
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b)                 \
-               H1(dline, dir1_sq, dir2_sq, a, b, dot_atmostone,     \
-                  LINE_YES, LINE_NO)
-               /* If at most one of the DLINE is on, and one is definitely on,
-                * set the other to definitely off */
-               SQUARE_DLINES;
-#undef HANDLE_DLINE
+        sstate->recursion_remaining--;
 
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b)                 \
-               H1(dline, dir1_sq, dir2_sq, a, b, dot_atleastone,    \
-                  LINE_NO, LINE_YES)
-               /* If at least one of the DLINE is on, and one is definitely
-                * off, set the other to definitely on */
-               SQUARE_DLINES;
-#undef HANDLE_DLINE
-#undef H1
-
-               switch (CLUE_AT(sstate->state, i, j)) {
-                   case '0':
-                   case '1':
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b)                          \
-                       /* At most one of any DLINE can be set */             \
-                       sstate->dot_atmostone                                 \
-                         [i+a + (sstate->state->w + 1) * (j+b)] |= 1<<dline; \
-                       /* This DLINE provides enough YESes to solve the clue */\
-                       if (sstate->dot_atleastone                            \
-                             [i+a + (sstate->state->w + 1) * (j+b)] &        \
-                           1<<dline) {                                       \
-                           dot_setall_dlines(sstate, OPP_DLINE(dline),       \
-                                             i+(1-a), j+(1-b),               \
-                                             LINE_UNKNOWN, LINE_NO);         \
-                       }
-                       SQUARE_DLINES;
-#undef HANDLE_DLINE
-                       break;
-                   case '2':
-#define H1(dline, dot_at1one, dot_at2one, a, b)                          \
-               if (sstate->dot_at1one                                    \
-                     [i+a + (sstate->state->w + 1) * (j+b)] &            \
-                   1<<dline) {                                           \
-                   sstate->dot_at2one                                    \
-                     [i+(1-a) + (sstate->state->w + 1) * (j+(1-b))] |=   \
-                       1<<OPP_DLINE(dline);                              \
-               }
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b)             \
-            H1(dline, dot_atleastone, dot_atmostone, a, b);     \
-            H1(dline, dot_atmostone, dot_atleastone, a, b); 
-                       /* If at least one of one DLINE is set, at most one of
-                        * the opposing one is and vice versa */
-                       SQUARE_DLINES;
-#undef HANDLE_DLINE
-#undef H1
-                       break;
-                   case '3':
-                   case '4':
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b)                           \
-                       /* At least one of any DLINE can be set */             \
-                       sstate->dot_atleastone                                 \
-                         [i+a + (sstate->state->w + 1) * (j+b)] |= 1<<dline;  \
-                       /* This DLINE provides enough NOs to solve the clue */ \
-                       if (sstate->dot_atmostone                              \
-                             [i+a + (sstate->state->w + 1) * (j+b)] &         \
-                           1<<dline) {                                        \
-                           dot_setall_dlines(sstate, OPP_DLINE(dline),        \
-                                             i+(1-a), j+(1-b),                \
-                                             LINE_UNKNOWN, LINE_YES);         \
-                       }
-                       SQUARE_DLINES;
-#undef HANDLE_DLINE
-                       break;
-               }
-           }
-       }
+        recursive_soln_count = 0;
+        sstate_rec_solved = NULL;
 
-       if (solver_states_equal(sstate, sstate_saved)) {
-          int edgecount = 0, clues = 0, satclues = 0, sm1clues = 0;
-          int d;
-
-          /*
-           * Go through the grid and update for all the new edges.
-           * Since merge_dots() is idempotent, the simplest way to
-           * do this is just to update for _all_ the edges.
-           * 
-           * Also, while we're here, we count the edges, count the
-           * clues, count the satisfied clues, and count the
-           * satisfied-minus-one clues.
-           */
-          for (j = 0; j <= sstate->state->h; ++j) {
-              for (i = 0; i <= sstate->state->w; ++i) {
-                  if (RIGHTOF_DOT(sstate->state, i, j) == LINE_YES) {
-                      merge_dots(sstate, i, j, i+1, j);
-                      edgecount++;
-                  }
-                  if (BELOW_DOT(sstate->state, i, j) == LINE_YES) {
-                      merge_dots(sstate, i, j, i, j+1);
-                      edgecount++;
-                  }
-
-                  if (CLUE_AT(sstate->state, i, j) != ' ') {
-                      int c = CLUE_AT(sstate->state, i, j) - '0';
-                      int o = square_order(sstate->state, i, j, LINE_YES);
-                      if (o == c)
-                          satclues++;
-                      else if (o == c-1)
-                          sm1clues++;
-                      clues++;
-                  }
-              }
-          }
-
-          /*
-           * Now go through looking for LINE_UNKNOWN edges which
-           * connect two dots that are already in the same
-           * equivalence class. If we find one, test to see if the
-           * loop it would create is a solution.
-           */
-          for (j = 0; j <= sstate->state->h; ++j) {
-              for (i = 0; i <= sstate->state->w; ++i) {
-                  for (d = 0; d < 2; d++) {
-                      int i2, j2, eqclass, val;
-
-                      if (d == 0) {
-                          if (RIGHTOF_DOT(sstate->state, i, j) !=
-                              LINE_UNKNOWN)
-                              continue;
-                          i2 = i+1;
-                          j2 = j;
-                      } else {
-                          if (BELOW_DOT(sstate->state, i, j) !=
-                              LINE_UNKNOWN)
-                              continue;
-                          i2 = i;
-                          j2 = j+1;
-                      }
-
-                      eqclass = dsf_canonify(sstate->dotdsf,
-                                             j * (sstate->state->w+1) + i);
-                      if (eqclass != dsf_canonify(sstate->dotdsf,
-                                                  j2 * (sstate->state->w+1) +
-                                                  i2))
-                          continue;
-
-                      val = LINE_NO;  /* loop is bad until proven otherwise */
-
-                      /*
-                       * This edge would form a loop. Next
-                       * question: how long would the loop be?
-                       * Would it equal the total number of edges
-                       * (plus the one we'd be adding if we added
-                       * it)?
-                       */
-                      if (sstate->looplen[eqclass] == edgecount + 1) {
-                          int sm1_nearby;
-                          int cx, cy;
-
-                          /*
-                           * This edge would form a loop which
-                           * took in all the edges in the entire
-                           * grid. So now we need to work out
-                           * whether it would be a valid solution
-                           * to the puzzle, which means we have to
-                           * check if it satisfies all the clues.
-                           * This means that every clue must be
-                           * either satisfied or satisfied-minus-
-                           * 1, and also that the number of
-                           * satisfied-minus-1 clues must be at
-                           * most two and they must lie on either
-                           * side of this edge.
-                           */
-                          sm1_nearby = 0;
-                          cx = i - (j2-j);
-                          cy = j - (i2-i);
-                          if (CLUE_AT(sstate->state, cx,cy) != ' ' &&
-                              square_order(sstate->state, cx,cy, LINE_YES) ==
-                              CLUE_AT(sstate->state, cx,cy) - '0' - 1)
-                              sm1_nearby++;
-                          if (CLUE_AT(sstate->state, i, j) != ' ' &&
-                              square_order(sstate->state, i, j, LINE_YES) ==
-                              CLUE_AT(sstate->state, i, j) - '0' - 1)
-                              sm1_nearby++;
-                          if (sm1clues == sm1_nearby &&
-                              sm1clues + satclues == clues)
-                              val = LINE_YES;  /* loop is good! */
-                      }
-
-                      /*
-                       * Right. Now we know that adding this edge
-                       * would form a loop, and we know whether
-                       * that loop would be a viable solution or
-                       * not.
-                       * 
-                       * If adding this edge produces a solution,
-                       * then we know we've found _a_ solution but
-                       * we don't know that it's _the_ solution -
-                       * if it were provably the solution then
-                       * we'd have deduced this edge some time ago
-                       * without the need to do loop detection. So
-                       * in this state we return SOLVER_AMBIGUOUS,
-                       * which has the effect that hitting Solve
-                       * on a user-provided puzzle will fill in a
-                       * solution but using the solver to
-                       * construct new puzzles won't consider this
-                       * a reasonable deduction for the user to
-                       * make.
-                       */
-                      if (d == 0)
-                          LV_RIGHTOF_DOT(sstate->state, i, j) = val;
-                      else
-                          LV_BELOW_DOT(sstate->state, i, j) = val;
-                      if (val == LINE_YES) {
-                           sstate->solver_status = SOLVER_AMBIGUOUS;
-                          goto finished_loop_checking;
-                      }
-                  }
-              }
-          }
-
-          finished_loop_checking:
-
-           RETURN_IF_SOLVED;
-       }
+        /* Memory management: 
+         * sstate_saved won't be modified but needs to be freed when we have
+         * finished with it.
+         * sstate is expected to contain our 'best' solution by the time we
+         * finish this section of code.  It's the thing we'll try adding lines
+         * to, seeing if they make it more solvable.
+         * If sstate_rec_solved is non-NULL, it will supersede sstate
+         * eventually.  sstate_tmp should not hold a value persistently.
+         */
 
-       if (solver_states_equal(sstate, sstate_saved)) {
-           /* Solver has stopped making progress so we terminate */
-           free_solver_state(sstate_saved); 
-           break;
-       }
+        /* NB SOLVER_AMBIGUOUS is like SOLVER_SOLVED except the solver is aware
+         * of the possibility of additional solutions.  So as soon as we have a
+         * SOLVER_AMBIGUOUS we can safely propagate it back to our caller, but
+         * if we get a SOLVER_SOLVED we want to keep trying in case we find
+         * further solutions and have to mark it ambiguous.
+         */
 
-       free_solver_state(sstate_saved); 
-   }
-
-   if (sstate->solver_status == SOLVER_SOLVED ||
-       sstate->solver_status == SOLVER_AMBIGUOUS) {
-       /* s/LINE_UNKNOWN/LINE_NO/g */
-       array_setall(sstate->state->hl, LINE_UNKNOWN, LINE_NO, 
-               HL_COUNT(sstate->state));
-       array_setall(sstate->state->vl, LINE_UNKNOWN, LINE_NO, 
-               VL_COUNT(sstate->state));
-       return sstate;
-   }
-
-   /* Perform recursive calls */
-   if (sstate->recursion_remaining) {
-       sstate->recursion_remaining--;
-   
-       sstate_saved = dup_solver_state(sstate);
-
-       recursive_soln_count = 0;
-       sstate_rec_solved = NULL;
-
-       /* Memory management: 
-        * sstate_saved won't be modified but needs to be freed when we have
-        * finished with it.
-        * sstate is expected to contain our 'best' solution by the time we
-        * finish this section of code.  It's the thing we'll try adding lines
-        * to, seeing if they make it more solvable.
-        * If sstate_rec_solved is non-NULL, it will supersede sstate
-        * eventually.  sstate_tmp should not hold a value persistently.
-        */
-
-       /* NB SOLVER_AMBIGUOUS is like SOLVER_SOLVED except the solver is aware
-        * of the possibility of additional solutions.  So as soon as we have a
-        * SOLVER_AMBIGUOUS we can safely propagate it back to our caller, but
-        * if we get a SOLVER_SOLVED we want to keep trying in case we find
-        * further solutions and have to mark it ambiguous.
-        */
-
-#define DO_RECURSIVE_CALL(dir_dot)                                         \
-       if (dir_dot(sstate->state, i, j) == LINE_UNKNOWN) {                 \
-           debug(("Trying " #dir_dot " at [%d,%d]\n", i, j));               \
-           LV_##dir_dot(sstate->state, i, j) = LINE_YES;                   \
-           sstate_tmp = solve_game_rec(sstate);                            \
-           switch (sstate_tmp->solver_status) {                            \
-               case SOLVER_AMBIGUOUS:                                      \
-                   debug(("Solver ambiguous, returning\n"));                \
-                   sstate_rec_solved = sstate_tmp;                         \
-                   goto finished_recursion;                                \
-               case SOLVER_SOLVED:                                         \
-                   switch (++recursive_soln_count) {                       \
-                       case 1:                                             \
-                           debug(("One solution found\n"));                 \
-                           sstate_rec_solved = sstate_tmp;                 \
-                           break;                                          \
-                       case 2:                                             \
-                           debug(("Ambiguous solutions found\n"));          \
-                           free_solver_state(sstate_tmp);                  \
-                           sstate_rec_solved->solver_status = SOLVER_AMBIGUOUS;\
-                           goto finished_recursion;                        \
-                       default:                                            \
-                           assert(!"recursive_soln_count out of range");   \
-                           break;                                          \
-                   }                                                       \
-                   break;                                                  \
-               case SOLVER_MISTAKE:                                        \
-                   debug(("Non-solution found\n"));                         \
-                   free_solver_state(sstate_tmp);                          \
-                   free_solver_state(sstate_saved);                        \
-                   LV_##dir_dot(sstate->state, i, j) = LINE_NO;            \
-                   goto nonrecursive_solver;                               \
-               case SOLVER_INCOMPLETE:                                     \
-                   debug(("Recursive step inconclusive\n"));                \
-                   free_solver_state(sstate_tmp);                          \
-                   break;                                                  \
-           }                                                               \
-           free_solver_state(sstate);                                      \
-           sstate = dup_solver_state(sstate_saved);                        \
-       }
+#define DO_RECURSIVE_CALL(dir_dot) \
+    if (dir_dot(sstate->state, i, j) == LINE_UNKNOWN) { \
+        debug(("Trying " #dir_dot " at [%d,%d]\n", i, j)); \
+        LV_##dir_dot(sstate->state, i, j) = LINE_YES; \
+        sstate_tmp = solve_game_rec(sstate, diff); \
+        switch (sstate_tmp->solver_status) { \
+            case SOLVER_AMBIGUOUS: \
+                debug(("Solver ambiguous, returning\n")); \
+                sstate_rec_solved = sstate_tmp; \
+                goto finished_recursion; \
+            case SOLVER_SOLVED: \
+                switch (++recursive_soln_count) { \
+                    case 1: \
+                        debug(("One solution found\n")); \
+                        sstate_rec_solved = sstate_tmp; \
+                        break; \
+                    case 2: \
+                        debug(("Ambiguous solutions found\n")); \
+                        free_solver_state(sstate_tmp); \
+                        sstate_rec_solved->solver_status = SOLVER_AMBIGUOUS; \
+                        goto finished_recursion; \
+                    default: \
+                        assert(!"recursive_soln_count out of range"); \
+                        break; \
+                } \
+                break; \
+            case SOLVER_MISTAKE: \
+                debug(("Non-solution found\n")); \
+                free_solver_state(sstate_tmp); \
+                free_solver_state(sstate_saved); \
+                LV_##dir_dot(sstate->state, i, j) = LINE_NO; \
+                goto nonrecursive_solver; \
+            case SOLVER_INCOMPLETE: \
+                debug(("Recursive step inconclusive\n")); \
+                free_solver_state(sstate_tmp); \
+                break; \
+        } \
+        free_solver_state(sstate); \
+        sstate = dup_solver_state(sstate_saved); \
+    }
        
-       for (j = 0; j < sstate->state->h + 1; ++j) {
-           for (i = 0; i < sstate->state->w + 1; ++i) {
-               /* Only perform recursive calls on 'loose ends' */
-               if (dot_order(sstate->state, i, j, LINE_YES) == 1) {
-                   if (LEFTOF_DOT(sstate->state, i, j) == LINE_UNKNOWN)
-                       DO_RECURSIVE_CALL(LEFTOF_DOT);
-                   if (RIGHTOF_DOT(sstate->state, i, j) == LINE_UNKNOWN)
-                       DO_RECURSIVE_CALL(RIGHTOF_DOT);
-                   if (ABOVE_DOT(sstate->state, i, j) == LINE_UNKNOWN)
-                       DO_RECURSIVE_CALL(ABOVE_DOT);
-                   if (BELOW_DOT(sstate->state, i, j) == LINE_UNKNOWN)
-                       DO_RECURSIVE_CALL(BELOW_DOT);
-               }
+       FORALL_DOTS(state, i, j) {
+           /* Only perform recursive calls on 'loose ends' */
+           if (DOT_YES_COUNT(sstate, i, j) == 1) {
+               DO_RECURSIVE_CALL(LEFTOF_DOT);
+               DO_RECURSIVE_CALL(RIGHTOF_DOT);
+               DO_RECURSIVE_CALL(ABOVE_DOT);
+               DO_RECURSIVE_CALL(BELOW_DOT);
            }
        }
 
@@ -1896,89 +3214,35 @@ finished_recursion:
            free_solver_state(sstate);
            sstate = sstate_rec_solved;
        } 
-   }
+    }
 
-   return sstate;
+    return sstate;
 }
 
-/* XXX bits of solver that may come in handy one day */
-#if 0
-#define HANDLE_DLINE(dline, dir1_dot, dir2_dot)                         \
-                   /* dline from this dot that's entirely unknown must have 
-                    * both lines identical */                           \
-                   if (dir1_dot(sstate->state, i, j) == LINE_UNKNOWN &&       \
-                       dir2_dot(sstate->state, i, j) == LINE_UNKNOWN) {       \
-                       sstate->dline_identical[i + (sstate->state->w + 1) * j] |= \
-                           1<<dline;                                    \
-                   } else if (sstate->dline_identical[i +
-                                                      (sstate->state->w + 1) * j] &\
-                              1<<dline) {                                   \
-                       /* If they're identical and one is known do the obvious 
-                        * thing */                                      \
-                       t = dir1_dot(sstate->state, i, j);                     \
-                       if (t != LINE_UNKNOWN)                           \
-                           dir2_dot(sstate->state, i, j) = t;                 \
-                       else {                                           \
-                           t = dir2_dot(sstate->state, i, j);                 \
-                           if (t != LINE_UNKNOWN)                       \
-                               dir1_dot(sstate->state, i, j) = t;             \
-                       }                                                \
-                   }                                                    \
-                   DOT_DLINES;
-#undef HANDLE_DLINE
-#endif
-
-#if 0
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b) \
-                       if (sstate->dline_identical[i+a +                     \
-                                                   (sstate->state->w + 1) * (j+b)] &\
-                           1<<dline) {                                       \
-                           dir1_sq(sstate->state, i, j) = LINE_YES;                \
-                           dir2_sq(sstate->state, i, j) = LINE_YES;                \
-                       }
-                       /* If two lines are the same they must be on */
-                       SQUARE_DLINES;
-#undef HANDLE_DLINE
-#endif
-
-
 #if 0
 #define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b) \
-               if (sstate->dot_atmostone[i+a + (sstate->state->w + 1) * (j+b)] &  \
-                   1<<dline) {                                   \
-                   if (square_order(sstate->state, i, j,  LINE_UNKNOWN) - 1 ==  \
-                       CLUE_AT(sstate->state, i, j) - '0') {      \
+               if (sstate->normal->dot_atmostone[i+a + (sstate->state->w + 1) * (j+b)] & \
+                   1<<dline) { \
+                   if (square_order(sstate->state, i, j,  LINE_UNKNOWN) - 1 == \
+                       CLUE_AT(sstate->state, i, j) - '0') { \
                        square_setall(sstate->state, i, j, LINE_UNKNOWN, LINE_YES); \
                            /* XXX the following may overwrite known data! */ \
-                       dir1_sq(sstate->state, i, j) = LINE_UNKNOWN;  \
-                       dir2_sq(sstate->state, i, j) = LINE_UNKNOWN;  \
-                   }                                  \
+                       dir1_sq(sstate->state, i, j) = LINE_UNKNOWN; \
+                       dir2_sq(sstate->state, i, j) = LINE_UNKNOWN; \
+                   } \
                }
                SQUARE_DLINES;
 #undef HANDLE_DLINE
 #endif
 
-#if 0
-#define HANDLE_DLINE(dline, dir1_sq, dir2_sq, a, b) \
-                       if (sstate->dline_identical[i+a + 
-                                                   (sstate->state->w + 1) * (j+b)] &\
-                           1<<dline) {                                       \
-                           dir1_sq(sstate->state, i, j) = LINE_NO;                 \
-                           dir2_sq(sstate->state, i, j) = LINE_NO;                 \
-                       }
-                       /* If two lines are the same they must be off */
-                       SQUARE_DLINES;
-#undef HANDLE_DLINE
-#endif
-
 static char *solve_game(game_state *state, game_state *currstate,
                         char *aux, char **error)
 {
     char *soln = NULL;
     solver_state *sstate, *new_sstate;
 
-    sstate = new_solver_state(state);
-    new_sstate = solve_game_rec(sstate);
+    sstate = new_solver_state(state, DIFF_MAX);
+    new_sstate = solve_game_rec(sstate, DIFF_MAX);
 
     if (new_sstate->solver_status == SOLVER_SOLVED) {
         soln = encode_solve_move(new_sstate->state);
@@ -1996,95 +3260,9 @@ static char *solve_game(game_state *state, game_state *currstate,
     return soln;
 }
 
-static char *game_text_format(game_state *state)
-{
-    int i, j;
-    int len;
-    char *ret, *rp;
-
-    len = (2 * state->w + 2) * (2 * state->h + 1);
-    rp = ret = snewn(len + 1, char);
-    
-#define DRAW_HL                          \
-    switch (ABOVE_SQUARE(state, i, j)) { \
-        case LINE_YES:                   \
-            rp += sprintf(rp, " -");     \
-            break;                       \
-        case LINE_NO:                    \
-            rp += sprintf(rp, " x");     \
-            break;                       \
-        case LINE_UNKNOWN:               \
-            rp += sprintf(rp, "  ");     \
-            break;                       \
-        default:                         \
-            assert(!"Illegal line state for HL");\
-    }
-
-#define DRAW_VL                          \
-    switch (LEFTOF_SQUARE(state, i, j)) {\
-        case LINE_YES:                   \
-            rp += sprintf(rp, "|");      \
-            break;                       \
-        case LINE_NO:                    \
-            rp += sprintf(rp, "x");      \
-            break;                       \
-        case LINE_UNKNOWN:               \
-            rp += sprintf(rp, " ");      \
-            break;                       \
-        default:                         \
-            assert(!"Illegal line state for VL");\
-    }
-    
-    for (j = 0; j < state->h; ++j) {
-        for (i = 0; i < state->w; ++i) {
-            DRAW_HL;
-        }
-        rp += sprintf(rp, " \n");
-        for (i = 0; i < state->w; ++i) {
-            DRAW_VL;
-            rp += sprintf(rp, "%c", (int)(CLUE_AT(state, i, j)));
-        }
-        DRAW_VL;
-        rp += sprintf(rp, "\n");
-    }
-    for (i = 0; i < state->w; ++i) {
-        DRAW_HL;
-    }
-    rp += sprintf(rp, " \n");
-    
-    assert(strlen(ret) == len);
-    return ret;
-}
-
-static game_ui *new_ui(game_state *state)
-{
-    return NULL;
-}
-
-static void free_ui(game_ui *ui)
-{
-}
-
-static char *encode_ui(game_ui *ui)
-{
-    return NULL;
-}
-
-static void decode_ui(game_ui *ui, char *encoding)
-{
-}
-
-static void game_changed_state(game_ui *ui, game_state *oldstate,
-                               game_state *newstate)
-{
-}
-
-struct game_drawstate {
-    int started;
-    int tilesize;
-    int flashing;
-    char *hl, *vl;
-};
+/* ----------------------------------------------------------------------
+ * Drawing and mouse-handling
+ */
 
 static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
                             int x, int y, int button)
@@ -2247,91 +3425,89 @@ static game_state *execute_move(game_state *state, char *move)
     /*
      * Check for completion.
      */
-    i = 0;                            /* placate optimiser */
+    i = 0;                   /* placate optimiser */
     for (j = 0; j <= newstate->h; j++) {
-       for (i = 0; i < newstate->w; i++)
-           if (LV_RIGHTOF_DOT(newstate, i, j) == LINE_YES)
-               break;
-       if (i < newstate->w)
-           break;
+        for (i = 0; i < newstate->w; i++)
+            if (LV_RIGHTOF_DOT(newstate, i, j) == LINE_YES)
+                break;
+        if (i < newstate->w)
+            break;
     }
     if (j <= newstate->h) {
-       int prevdir = 'R';
-       int x = i, y = j;
-       int looplen, count;
-
-       /*
-        * We've found a horizontal edge at (i,j). Follow it round
-        * to see if it's part of a loop.
-        */
-       looplen = 0;
-       while (1) {
-           int order = dot_order(newstate, x, y, LINE_YES);
-           if (order != 2)
-               goto completion_check_done;
-
-           if (LEFTOF_DOT(newstate, x, y) == LINE_YES && prevdir != 'L') {
-               x--;
-               prevdir = 'R';
-           } else if (RIGHTOF_DOT(newstate, x, y) == LINE_YES &&
-                      prevdir != 'R') {
-               x++;
-               prevdir = 'L';
-           } else if (ABOVE_DOT(newstate, x, y) == LINE_YES &&
-                      prevdir != 'U') {
-               y--;
-               prevdir = 'D';
-           } else if (BELOW_DOT(newstate, x, y) == LINE_YES &&
-                      prevdir != 'D') {
-               y++;
-               prevdir = 'U';
-           } else {
-               assert(!"Can't happen");   /* dot_order guarantees success */
-           }
-
-           looplen++;
-
-           if (x == i && y == j)
-               break;
-       }
-
-       if (x != i || y != j || looplen == 0)
-           goto completion_check_done;
-
-       /*
-        * We've traced our way round a loop, and we know how many
-        * line segments were involved. Count _all_ the line
-        * segments in the grid, to see if the loop includes them
-        * all.
-        */
-       count = 0;
-       for (j = 0; j <= newstate->h; j++)
-           for (i = 0; i <= newstate->w; i++)
-               count += ((RIGHTOF_DOT(newstate, i, j) == LINE_YES) +
-                         (BELOW_DOT(newstate, i, j) == LINE_YES));
-       assert(count >= looplen);
-       if (count != looplen)
-           goto completion_check_done;
-
-       /*
-        * The grid contains one closed loop and nothing else.
-        * Check that all the clues are satisfied.
-        */
-       for (j = 0; j < newstate->h; ++j) {
-           for (i = 0; i < newstate->w; ++i) {
-               int n = CLUE_AT(newstate, i, j);
-               if (n != ' ') {
-                   if (square_order(newstate, i, j, LINE_YES) != n - '0') {
-                       goto completion_check_done;
-                   }
-               }
-           }
-       }
-
-       /*
-        * Completed!
-        */
-       newstate->solved = TRUE;
+        int prevdir = 'R';
+        int x = i, y = j;
+        int looplen, count;
+
+        /*
+         * We've found a horizontal edge at (i,j). Follow it round
+         * to see if it's part of a loop.
+         */
+        looplen = 0;
+        while (1) {
+            int order = dot_order(newstate, x, y, LINE_YES);
+            if (order != 2)
+                goto completion_check_done;
+
+            if (LEFTOF_DOT(newstate, x, y) == LINE_YES && prevdir != 'L') {
+                x--;
+                prevdir = 'R';
+            } else if (RIGHTOF_DOT(newstate, x, y) == LINE_YES &&
+                       prevdir != 'R') {
+                x++;
+                prevdir = 'L';
+            } else if (ABOVE_DOT(newstate, x, y) == LINE_YES &&
+                       prevdir != 'U') {
+                y--;
+                prevdir = 'D';
+            } else if (BELOW_DOT(newstate, x, y) == LINE_YES && 
+                       prevdir != 'D') {
+                y++;
+                prevdir = 'U';
+            } else {
+                assert(!"Can't happen");   /* dot_order guarantees success */
+            }
+
+            looplen++;
+
+            if (x == i && y == j)
+                break;
+        }
+
+        if (x != i || y != j || looplen == 0)
+            goto completion_check_done;
+
+        /*
+         * We've traced our way round a loop, and we know how many
+         * line segments were involved. Count _all_ the line
+         * segments in the grid, to see if the loop includes them
+         * all.
+         */
+        count = 0;
+        FORALL_DOTS(newstate, i, j) {
+            count += ((RIGHTOF_DOT(newstate, i, j) == LINE_YES) +
+                      (BELOW_DOT(newstate, i, j) == LINE_YES));
+        }
+        assert(count >= looplen);
+        if (count != looplen)
+            goto completion_check_done;
+
+        /*
+         * The grid contains one closed loop and nothing else.
+         * Check that all the clues are satisfied.
+         */
+        FORALL_SQUARES(newstate, i, j) {
+            if (CLUE_AT(newstate, i, j) >= 0) {
+                if (square_order(newstate, i, j, LINE_YES) != 
+                    CLUE_AT(newstate, i, j)) {
+                    goto completion_check_done;
+                }
+            }
+        }
+
+        /*
+         * Completed!
+         */
+        newstate->solved = TRUE;
     }
 
 completion_check_done:
@@ -2345,74 +3521,14 @@ fail:
 /* ----------------------------------------------------------------------
  * Drawing routines.
  */
-
-#define SIZE(d) ((d) * TILE_SIZE + 2 * BORDER + 1)
-
-static void game_compute_size(game_params *params, int tilesize,
-                              int *x, int *y)
-{
-    struct { int tilesize; } ads, *ds = &ads;
-    ads.tilesize = tilesize;
-
-    *x = SIZE(params->w);
-    *y = SIZE(params->h);
-}
-
-static void game_set_size(drawing *dr, game_drawstate *ds,
-                         game_params *params, int tilesize)
-{
-    ds->tilesize = tilesize;
-}
-
-static float *game_colours(frontend *fe, game_state *state, int *ncolours)
-{
-    float *ret = snewn(4 * NCOLOURS, float);
-
-    frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);
-
-    ret[COL_FOREGROUND * 3 + 0] = 0.0F;
-    ret[COL_FOREGROUND * 3 + 1] = 0.0F;
-    ret[COL_FOREGROUND * 3 + 2] = 0.0F;
-
-    ret[COL_HIGHLIGHT * 3 + 0] = 1.0F;
-    ret[COL_HIGHLIGHT * 3 + 1] = 1.0F;
-    ret[COL_HIGHLIGHT * 3 + 2] = 1.0F;
-
-    *ncolours = NCOLOURS;
-    return ret;
-}
-
-static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
-{
-    struct game_drawstate *ds = snew(struct game_drawstate);
-
-    ds->tilesize = 0;
-    ds->started = 0;
-    ds->hl = snewn(HL_COUNT(state), char);
-    ds->vl = snewn(VL_COUNT(state), char);
-    ds->flashing = 0;
-
-    memset(ds->hl, LINE_UNKNOWN, HL_COUNT(state));
-    memset(ds->vl, LINE_UNKNOWN, VL_COUNT(state));
-
-    return ds;
-}
-
-static void game_free_drawstate(drawing *dr, game_drawstate *ds)
-{
-    sfree(ds->hl);
-    sfree(ds->vl);
-    sfree(ds);
-}
-
 static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
                         game_state *state, int dir, game_ui *ui,
                         float animtime, float flashtime)
 {
-    int i, j;
-    int w = state->w, h = state->h;
+    int i, j, n;
     char c[2];
     int line_colour, flash_changed;
+    int clue_mistake;
 
     if (!ds->started) {
         /*
@@ -2424,26 +3540,22 @@ static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
         draw_rect(dr, 0, 0, SIZE(state->w), SIZE(state->h), COL_BACKGROUND);
 
         /* Draw dots */
-        for (j = 0; j < h + 1; ++j) {
-            for (i = 0; i < w + 1; ++i) {
-                draw_rect(dr, 
-                          BORDER + i * TILE_SIZE - LINEWIDTH/2,
-                          BORDER + j * TILE_SIZE - LINEWIDTH/2,
-                          LINEWIDTH, LINEWIDTH, COL_FOREGROUND);
-            }
+        FORALL_DOTS(state, i, j) {
+            draw_rect(dr, 
+                      BORDER + i * TILE_SIZE - LINEWIDTH/2,
+                      BORDER + j * TILE_SIZE - LINEWIDTH/2,
+                      LINEWIDTH, LINEWIDTH, COL_FOREGROUND);
         }
 
         /* Draw clues */
-        for (j = 0; j < h; ++j) {
-            for (i = 0; i < w; ++i) {
-                c[0] = CLUE_AT(state, i, j);
-                c[1] = '\0';
-                draw_text(dr, 
-                          BORDER + i * TILE_SIZE + TILE_SIZE/2,
-                          BORDER + j * TILE_SIZE + TILE_SIZE/2,
-                          FONT_VARIABLE, TILE_SIZE/2, 
-                          ALIGN_VCENTRE | ALIGN_HCENTRE, COL_FOREGROUND, c);
-            }
+        FORALL_SQUARES(state, i, j) {
+            c[0] = CLUE2CHAR(CLUE_AT(state, i, j));
+            c[1] = '\0';
+            draw_text(dr, 
+                      BORDER + i * TILE_SIZE + TILE_SIZE/2,
+                      BORDER + j * TILE_SIZE + TILE_SIZE/2,
+                      FONT_VARIABLE, TILE_SIZE/2, 
+                      ALIGN_VCENTRE | ALIGN_HCENTRE, COL_FOREGROUND, c);
         }
         draw_update(dr, 0, 0,
                     state->w * TILE_SIZE + 2*BORDER + 1,
@@ -2465,125 +3577,154 @@ static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
 
 #define CROSS_SIZE (3 * LINEWIDTH / 2)
     
-#define CLEAR_VL(i, j) do {                                                \
-                           draw_rect(dr,                                   \
-                                 BORDER + i * TILE_SIZE - CROSS_SIZE,      \
-                                 BORDER + j * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,     \
-                                 CROSS_SIZE * 2,                           \
-                                 TILE_SIZE - LINEWIDTH,                    \
-                                 COL_BACKGROUND);                          \
-                           draw_update(dr,                                 \
-                                      BORDER + i * TILE_SIZE - CROSS_SIZE, \
-                                      BORDER + j * TILE_SIZE - CROSS_SIZE, \
-                                      CROSS_SIZE*2,                       \
-                                      TILE_SIZE + CROSS_SIZE*2);          \
-                        } while (0)
-
-#define CLEAR_HL(i, j) do {                                                \
-                           draw_rect(dr,                                   \
-                                 BORDER + i * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,     \
-                                 BORDER + j * TILE_SIZE - CROSS_SIZE,      \
-                                 TILE_SIZE - LINEWIDTH,                    \
-                                 CROSS_SIZE * 2,                           \
-                                 COL_BACKGROUND);                          \
-                           draw_update(dr,                                 \
-                                      BORDER + i * TILE_SIZE - CROSS_SIZE, \
-                                      BORDER + j * TILE_SIZE - CROSS_SIZE, \
-                                      TILE_SIZE + CROSS_SIZE*2,           \
-                                      CROSS_SIZE*2);                      \
-                        } while (0)
+    /* Redraw clue colours if necessary */
+    FORALL_SQUARES(state, i, j) {
+        n = CLUE_AT(state, i, j);
+        if (n < 0)
+            continue;
+
+        assert(n >= 0 && n <= 4);
+
+        c[0] = CLUE2CHAR(CLUE_AT(state, i, j));
+        c[1] = '\0';
+
+        clue_mistake = (square_order(state, i, j, LINE_YES) > n ||
+                        square_order(state, i, j, LINE_NO ) > (4-n));
+
+        if (clue_mistake != ds->clue_error[SQUARE_INDEX(state, i, j)]) {
+            draw_rect(dr, 
+                      BORDER + i * TILE_SIZE + CROSS_SIZE,
+                      BORDER + j * TILE_SIZE + CROSS_SIZE,
+                      TILE_SIZE - CROSS_SIZE * 2, TILE_SIZE - CROSS_SIZE * 2,
+                      COL_BACKGROUND);
+            draw_text(dr, 
+                      BORDER + i * TILE_SIZE + TILE_SIZE/2,
+                      BORDER + j * TILE_SIZE + TILE_SIZE/2,
+                      FONT_VARIABLE, TILE_SIZE/2, 
+                      ALIGN_VCENTRE | ALIGN_HCENTRE, 
+                      clue_mistake ? COL_MISTAKE : COL_FOREGROUND, c);
+            draw_update(dr, i * TILE_SIZE + BORDER, j * TILE_SIZE + BORDER,
+                        TILE_SIZE, TILE_SIZE);
+
+            ds->clue_error[SQUARE_INDEX(state, i, j)] = clue_mistake;
+        }
+    }
+
+    /* I've also had a request to colour lines red if they make a non-solution
+     * loop, or if more than two lines go into any point.  I think that would
+     * be good some time. */
+
+#define CLEAR_VL(i, j) \
+    do { \
+       draw_rect(dr, \
+                 BORDER + i * TILE_SIZE - CROSS_SIZE, \
+                 BORDER + j * TILE_SIZE + LINEWIDTH - LINEWIDTH/2, \
+                 CROSS_SIZE * 2, \
+                 TILE_SIZE - LINEWIDTH, \
+                 COL_BACKGROUND); \
+        draw_update(dr, \
+                    BORDER + i * TILE_SIZE - CROSS_SIZE, \
+                    BORDER + j * TILE_SIZE - CROSS_SIZE, \
+                    CROSS_SIZE*2, \
+                    TILE_SIZE + CROSS_SIZE*2); \
+    } while (0)
+
+#define CLEAR_HL(i, j) \
+    do { \
+       draw_rect(dr, \
+                 BORDER + i * TILE_SIZE + LINEWIDTH - LINEWIDTH/2, \
+                 BORDER + j * TILE_SIZE - CROSS_SIZE, \
+                 TILE_SIZE - LINEWIDTH, \
+                 CROSS_SIZE * 2, \
+                 COL_BACKGROUND); \
+       draw_update(dr, \
+                   BORDER + i * TILE_SIZE - CROSS_SIZE, \
+                   BORDER + j * TILE_SIZE - CROSS_SIZE, \
+                   TILE_SIZE + CROSS_SIZE*2, \
+                   CROSS_SIZE*2); \
+    } while (0)
 
     /* Vertical lines */
-    for (j = 0; j < h; ++j) {
-        for (i = 0; i < w + 1; ++i) {
-            switch (BELOW_DOT(state, i, j)) {
-                case LINE_UNKNOWN:
-                    if (ds->vl[i + (w + 1) * j] != BELOW_DOT(state, i, j)) {
-                        CLEAR_VL(i, j);
-                    }
-                    break;
-                case LINE_YES:
-                    if (ds->vl[i + (w + 1) * j] != BELOW_DOT(state, i, j) ||
-                        flash_changed) {
-                        CLEAR_VL(i, j);
-                        draw_rect(dr,
-                                  BORDER + i * TILE_SIZE - LINEWIDTH/2,
-                                  BORDER + j * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,
-                                  LINEWIDTH, TILE_SIZE - LINEWIDTH, 
-                                  line_colour);
-                    }
-                    break;
-                case LINE_NO:
-                    if (ds->vl[i + (w + 1) * j] != BELOW_DOT(state, i, j)) {
-                        CLEAR_VL(i, j);
-                        draw_line(dr,
-                                 BORDER + i * TILE_SIZE - CROSS_SIZE,
-                                 BORDER + j * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
-                                 BORDER + i * TILE_SIZE + CROSS_SIZE - 1,
-                                 BORDER + j * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
-                                  COL_FOREGROUND);
-                        draw_line(dr,
-                                 BORDER + i * TILE_SIZE + CROSS_SIZE - 1,
-                                 BORDER + j * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
-                                 BORDER + i * TILE_SIZE - CROSS_SIZE,
-                                 BORDER + j * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
-                                  COL_FOREGROUND);
-                    }
-                    break;
-            }
-            ds->vl[i + (w + 1) * j] = BELOW_DOT(state, i, j);
+    FORALL_VL(state, i, j) {
+        switch (BELOW_DOT(state, i, j)) {
+            case LINE_UNKNOWN:
+                if (ds->vl[VL_INDEX(state, i, j)] != BELOW_DOT(state, i, j)) {
+                    CLEAR_VL(i, j);
+                }
+                break;
+            case LINE_YES:
+                if (ds->vl[VL_INDEX(state, i, j)] != BELOW_DOT(state, i, j) ||
+                    flash_changed) {
+                    CLEAR_VL(i, j);
+                    draw_rect(dr,
+                              BORDER + i * TILE_SIZE - LINEWIDTH/2,
+                              BORDER + j * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,
+                              LINEWIDTH, TILE_SIZE - LINEWIDTH, 
+                              line_colour);
+                }
+                break;
+            case LINE_NO:
+                if (ds->vl[VL_INDEX(state, i, j)] != BELOW_DOT(state, i, j)) {
+                    CLEAR_VL(i, j);
+                    draw_line(dr,
+                              BORDER + i * TILE_SIZE - CROSS_SIZE,
+                              BORDER + j * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
+                              BORDER + i * TILE_SIZE + CROSS_SIZE - 1,
+                              BORDER + j * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
+                              COL_FOREGROUND);
+                    draw_line(dr,
+                              BORDER + i * TILE_SIZE + CROSS_SIZE - 1,
+                              BORDER + j * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
+                              BORDER + i * TILE_SIZE - CROSS_SIZE,
+                              BORDER + j * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
+                              COL_FOREGROUND);
+                }
+                break;
         }
+        ds->vl[VL_INDEX(state, i, j)] = BELOW_DOT(state, i, j);
     }
 
     /* Horizontal lines */
-    for (j = 0; j < h + 1; ++j) {
-        for (i = 0; i < w; ++i) {
-            switch (RIGHTOF_DOT(state, i, j)) {
-                case LINE_UNKNOWN:
-                    if (ds->hl[i + w * j] != RIGHTOF_DOT(state, i, j)) {
-                        CLEAR_HL(i, j);
+    FORALL_HL(state, i, j) {
+        switch (RIGHTOF_DOT(state, i, j)) {
+            case LINE_UNKNOWN:
+                if (ds->hl[HL_INDEX(state, i, j)] != RIGHTOF_DOT(state, i, j)) {
+                    CLEAR_HL(i, j);
+                }
+                break;
+            case LINE_YES:
+                if (ds->hl[HL_INDEX(state, i, j)] != RIGHTOF_DOT(state, i, j) ||
+                    flash_changed) {
+                    CLEAR_HL(i, j);
+                    draw_rect(dr,
+                              BORDER + i * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,
+                              BORDER + j * TILE_SIZE - LINEWIDTH/2,
+                              TILE_SIZE - LINEWIDTH, LINEWIDTH, 
+                              line_colour);
+                }
+                break; 
+            case LINE_NO:
+                if (ds->hl[HL_INDEX(state, i, j)] != RIGHTOF_DOT(state, i, j)) {
+                    CLEAR_HL(i, j);
+                    draw_line(dr,
+                              BORDER + i * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
+                              BORDER + j * TILE_SIZE + CROSS_SIZE - 1,
+                              BORDER + i * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
+                              BORDER + j * TILE_SIZE - CROSS_SIZE,
+                              COL_FOREGROUND);
+                    draw_line(dr,
+                              BORDER + i * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
+                              BORDER + j * TILE_SIZE - CROSS_SIZE,
+                              BORDER + i * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
+                              BORDER + j * TILE_SIZE + CROSS_SIZE - 1,
+                              COL_FOREGROUND);
+                    break;
                 }
-                        break;
-                case LINE_YES:
-                    if (ds->hl[i + w * j] != RIGHTOF_DOT(state, i, j) ||
-                        flash_changed) {
-                        CLEAR_HL(i, j);
-                        draw_rect(dr,
-                                  BORDER + i * TILE_SIZE + LINEWIDTH - LINEWIDTH/2,
-                                  BORDER + j * TILE_SIZE - LINEWIDTH/2,
-                                  TILE_SIZE - LINEWIDTH, LINEWIDTH, 
-                                  line_colour);
-                        break;
-                    }
-                case LINE_NO:
-                    if (ds->hl[i + w * j] != RIGHTOF_DOT(state, i, j)) {
-                        CLEAR_HL(i, j);
-                        draw_line(dr,
-                                 BORDER + i * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
-                                 BORDER + j * TILE_SIZE + CROSS_SIZE - 1,
-                                 BORDER + i * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
-                                 BORDER + j * TILE_SIZE - CROSS_SIZE,
-                                  COL_FOREGROUND);
-                        draw_line(dr,
-                                 BORDER + i * TILE_SIZE + TILE_SIZE/2 - CROSS_SIZE,
-                                 BORDER + j * TILE_SIZE - CROSS_SIZE,
-                                 BORDER + i * TILE_SIZE + TILE_SIZE/2 + CROSS_SIZE - 1,
-                                 BORDER + j * TILE_SIZE + CROSS_SIZE - 1,
-                                  COL_FOREGROUND);
-                        break;
-                    }
-            }
-            ds->hl[i + w * j] = RIGHTOF_DOT(state, i, j);
         }
+        ds->hl[HL_INDEX(state, i, j)] = RIGHTOF_DOT(state, i, j);
     }
 }
 
-static float game_anim_length(game_state *oldstate, game_state *newstate,
-                              int dir, game_ui *ui)
-{
-    return 0.0F;
-}
-
 static float game_flash_length(game_state *oldstate, game_state *newstate,
                                int dir, game_ui *ui)
 {
@@ -2595,16 +3736,6 @@ static float game_flash_length(game_state *oldstate, game_state *newstate,
     return 0.0F;
 }
 
-static int game_wants_statusbar(void)
-{
-    return FALSE;
-}
-
-static int game_timing_state(game_state *state, game_ui *ui)
-{
-    return TRUE;
-}
-
 static void game_print_size(game_params *params, float *x, float *y)
 {
     int pw, ph;
@@ -2619,54 +3750,55 @@ static void game_print_size(game_params *params, float *x, float *y)
 
 static void game_print(drawing *dr, game_state *state, int tilesize)
 {
-    int w = state->w, h = state->h;
     int ink = print_mono_colour(dr, 0);
     int x, y;
     game_drawstate ads, *ds = &ads;
-    ds->tilesize = tilesize;
+
+    game_set_size(dr, ds, NULL, tilesize);
 
     /*
      * Dots. I'll deliberately make the dots a bit wider than the
      * lines, so you can still see them. (And also because it's
      * annoyingly tricky to make them _exactly_ the same size...)
      */
-    for (y = 0; y <= h; y++)
-       for (x = 0; x <= w; x++)
-           draw_circle(dr, BORDER + x * TILE_SIZE, BORDER + y * TILE_SIZE,
-                       LINEWIDTH, ink, ink);
+    FORALL_DOTS(state, x, y) {
+        draw_circle(dr, BORDER + x * TILE_SIZE, BORDER + y * TILE_SIZE,
+                    LINEWIDTH, ink, ink);
+    }
 
     /*
      * Clues.
      */
-    for (y = 0; y < h; y++)
-       for (x = 0; x < w; x++)
-           if (CLUE_AT(state, x, y) != ' ') {
-               char c[2];
-
-                c[0] = CLUE_AT(state, x, y);
-                c[1] = '\0';
-                draw_text(dr, 
-                          BORDER + x * TILE_SIZE + TILE_SIZE/2,
-                          BORDER + y * TILE_SIZE + TILE_SIZE/2,
-                          FONT_VARIABLE, TILE_SIZE/2, 
-                          ALIGN_VCENTRE | ALIGN_HCENTRE, ink, c);
-           }
+    FORALL_SQUARES(state, x, y) {
+        if (CLUE_AT(state, x, y) >= 0) {
+            char c[2];
+
+            c[0] = CLUE2CHAR(CLUE_AT(state, x, y));
+            c[1] = '\0';
+            draw_text(dr, 
+                      BORDER + x * TILE_SIZE + TILE_SIZE/2,
+                      BORDER + y * TILE_SIZE + TILE_SIZE/2,
+                      FONT_VARIABLE, TILE_SIZE/2, 
+                      ALIGN_VCENTRE | ALIGN_HCENTRE, ink, c);
+        }
+    }
 
     /*
      * Lines. (At the moment, I'm not bothering with crosses.)
      */
-    for (y = 0; y <= h; y++)
-       for (x = 0; x < w; x++)
-           if (RIGHTOF_DOT(state, x, y) == LINE_YES)
-               draw_rect(dr, BORDER + x * TILE_SIZE,
-                         BORDER + y * TILE_SIZE - LINEWIDTH/2,
-                         TILE_SIZE, (LINEWIDTH/2) * 2 + 1, ink);
-    for (y = 0; y < h; y++)
-       for (x = 0; x <= w; x++)
-           if (BELOW_DOT(state, x, y) == LINE_YES)
-               draw_rect(dr, BORDER + x * TILE_SIZE - LINEWIDTH/2,
-                         BORDER + y * TILE_SIZE,
-                         (LINEWIDTH/2) * 2 + 1, TILE_SIZE, ink);
+    FORALL_HL(state, x, y) {
+        if (RIGHTOF_DOT(state, x, y) == LINE_YES)
+        draw_rect(dr, BORDER + x * TILE_SIZE,
+                  BORDER + y * TILE_SIZE - LINEWIDTH/2,
+                  TILE_SIZE, (LINEWIDTH/2) * 2 + 1, ink);
+    }
+
+    FORALL_VL(state, x, y) {
+        if (BELOW_DOT(state, x, y) == LINE_YES)
+        draw_rect(dr, BORDER + x * TILE_SIZE - LINEWIDTH/2,
+                  BORDER + y * TILE_SIZE,
+                  (LINEWIDTH/2) * 2 + 1, TILE_SIZE, ink);
+    }
 }
 
 #ifdef COMBINED
@@ -2674,7 +3806,7 @@ static void game_print(drawing *dr, game_state *state, int tilesize)
 #endif
 
 const struct game thegame = {
-    "Loopy", "games.loopy",
+    "Loopy", "games.loopy", "loopy",
     default_params,
     game_fetch_preset,
     decode_params,
@@ -2705,7 +3837,7 @@ const struct game thegame = {
     game_anim_length,
     game_flash_length,
     TRUE, FALSE, game_print_size, game_print,
-    game_wants_statusbar,
+    FALSE /* wants_statusbar */,
     FALSE, game_timing_state,
     0,                                       /* mouse_priorities */
 };