Peg Solitaire implementation, complete with a random board
authorsimon <simon@cda61777-01e9-0310-a592-d414129be87e>
Mon, 4 Jul 2005 19:42:55 +0000 (19:42 +0000)
committersimon <simon@cda61777-01e9-0310-a592-d414129be87e>
Mon, 4 Jul 2005 19:42:55 +0000 (19:42 +0000)
generator. The generator is lacking in almost any kind of finesse,
but it produces puzzles which at least _I_ find plausibly puzzling.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6052 cda61777-01e9-0310-a592-d414129be87e

Recipe
list.c
pegs.c [new file with mode: 0644]

diff --git a/Recipe b/Recipe
index a6fde21..726e4ef 100644 (file)
--- a/Recipe
+++ b/Recipe
@@ -19,9 +19,10 @@ NET      = net tree234
 NETSLIDE = netslide tree234
 MINES    = mines tree234
 FLIP     = flip tree234
+PEGS     = pegs tree234
 
 ALL      = list NET NETSLIDE cube fifteen sixteen rect pattern solo twiddle
-         + MINES samegame FLIP guess
+         + MINES samegame FLIP guess PEGS
 
 net      : [X] gtk COMMON NET
 netslide : [X] gtk COMMON NETSLIDE
@@ -36,6 +37,7 @@ mines    : [X] gtk COMMON MINES
 samegame : [X] gtk COMMON samegame
 flip     : [X] gtk COMMON FLIP
 guess    : [X] gtk COMMON guess
+pegs     : [X] gtk COMMON PEGS
 
 # The Windows Net shouldn't be called `net.exe' since Windows
 # already has a reasonably important utility program by that name!
@@ -52,6 +54,7 @@ mines    : [G] WINDOWS COMMON MINES
 samegame : [G] WINDOWS COMMON samegame
 flip     : [G] WINDOWS COMMON FLIP
 guess    : [G] WINDOWS COMMON guess
+pegs     : [G] WINDOWS COMMON PEGS
 
 # Mac OS X unified application containing all the puzzles.
 Puzzles  : [MX] osx osx.icns osx-info.plist COMMON ALL
diff --git a/list.c b/list.c
index 6be39cc..5883a9a 100644 (file)
--- a/list.c
+++ b/list.c
@@ -25,6 +25,7 @@ extern const game mines;
 extern const game net;
 extern const game netslide;
 extern const game pattern;
+extern const game pegs;
 extern const game rect;
 extern const game samegame;
 extern const game sixteen;
@@ -40,6 +41,7 @@ const game *gamelist[] = {
     &net,
     &netslide,
     &pattern,
+    &pegs,
     &rect,
     &samegame,
     &sixteen,
diff --git a/pegs.c b/pegs.c
new file mode 100644 (file)
index 0000000..8672513
--- /dev/null
+++ b/pegs.c
@@ -0,0 +1,1105 @@
+/*
+ * pegs.c: the classic Peg Solitaire game.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+#include <ctype.h>
+#include <math.h>
+
+#include "puzzles.h"
+#include "tree234.h"
+
+#define GRID_HOLE 0
+#define GRID_PEG  1
+#define GRID_OBST 2
+
+enum {
+    COL_BACKGROUND,
+    COL_HIGHLIGHT,
+    COL_LOWLIGHT,
+    COL_PEG,
+    NCOLOURS
+};
+
+/*
+ * Grid shapes. I do some macro ickery here to ensure that my enum
+ * and the various forms of my name list always match up.
+ */
+#define TYPELIST(A) \
+    A(CROSS,Cross,cross) \
+    A(OCTAGON,Octagon,octagon) \
+    A(RANDOM,Random,random)
+#define ENUM(upper,title,lower) TYPE_ ## upper,
+#define TITLE(upper,title,lower) #title,
+#define LOWER(upper,title,lower) #lower,
+#define CONFIG(upper,title,lower) ":" #title
+
+enum { TYPELIST(ENUM) TYPECOUNT };
+static char const *const pegs_titletypes[] = { TYPELIST(TITLE) };
+static char const *const pegs_lowertypes[] = { TYPELIST(LOWER) };
+#define TYPECONFIG TYPELIST(CONFIG)
+
+struct game_params {
+    int w, h;
+    int type;
+};
+
+struct game_state {
+    int w, h;
+    unsigned char *grid;
+};
+
+static game_params *default_params(void)
+{
+    game_params *ret = snew(game_params);
+
+    ret->w = ret->h = 7;
+    ret->type = TYPE_CROSS;
+
+    return ret;
+}
+
+static const struct game_params pegs_presets[] = {
+    {7, 7, TYPE_CROSS},
+    {7, 7, TYPE_OCTAGON},
+    {5, 5, TYPE_RANDOM},
+    {7, 7, TYPE_RANDOM},
+    {9, 9, TYPE_RANDOM},
+};
+
+static int game_fetch_preset(int i, char **name, game_params **params)
+{
+    game_params *ret;
+    char str[80];
+
+    if (i < 0 || i >= lenof(pegs_presets))
+        return FALSE;
+
+    ret = snew(game_params);
+    *ret = pegs_presets[i];
+
+    strcpy(str, pegs_titletypes[ret->type]);
+    if (ret->type == TYPE_RANDOM)
+       sprintf(str + strlen(str), " %dx%d", ret->w, ret->h);
+
+    *name = dupstr(str);
+    *params = ret;
+    return TRUE;
+}
+
+static void free_params(game_params *params)
+{
+    sfree(params);
+}
+
+static game_params *dup_params(game_params *params)
+{
+    game_params *ret = snew(game_params);
+    *ret = *params;                   /* structure copy */
+    return ret;
+}
+
+static void decode_params(game_params *params, char const *string)
+{
+    char const *p = string;
+    int i;
+
+    params->w = atoi(p);
+    while (*p && isdigit((unsigned char)*p)) p++;
+    if (*p == 'x') {
+        p++;
+        params->h = atoi(p);
+        while (*p && isdigit((unsigned char)*p)) p++;
+    } else {
+        params->h = params->w;
+    }
+
+    for (i = 0; i < lenof(pegs_lowertypes); i++)
+       if (!strcmp(p, pegs_lowertypes[i]))
+           params->type = i;
+}
+
+static char *encode_params(game_params *params, int full)
+{
+    char str[80];
+
+    sprintf(str, "%dx%d", params->w, params->h);
+    if (full) {
+       assert(params->type >= 0 && params->type < lenof(pegs_lowertypes));
+       strcat(str, pegs_lowertypes[params->type]);
+    }
+    return dupstr(str);
+}
+
+static config_item *game_configure(game_params *params)
+{
+    config_item *ret = snewn(4, config_item);
+    char buf[80];
+
+    ret[0].name = "Width";
+    ret[0].type = C_STRING;
+    sprintf(buf, "%d", params->w);
+    ret[0].sval = dupstr(buf);
+    ret[0].ival = 0;
+
+    ret[1].name = "Height";
+    ret[1].type = C_STRING;
+    sprintf(buf, "%d", params->h);
+    ret[1].sval = dupstr(buf);
+    ret[1].ival = 0;
+
+    ret[2].name = "Board type";
+    ret[2].type = C_CHOICES;
+    ret[2].sval = TYPECONFIG;
+    ret[2].ival = params->type;
+
+    ret[3].name = NULL;
+    ret[3].type = C_END;
+    ret[3].sval = NULL;
+    ret[3].ival = 0;
+
+    return ret;
+}
+
+static game_params *custom_params(config_item *cfg)
+{
+    game_params *ret = snew(game_params);
+
+    ret->w = atoi(cfg[0].sval);
+    ret->h = atoi(cfg[1].sval);
+    ret->type = cfg[2].ival;
+
+    return ret;
+}
+
+static char *validate_params(game_params *params)
+{
+    if (params->w <= 3 || params->h <= 3)
+       return "Width and height must both be greater than three";
+
+    /*
+     * It might be possible to implement generalisations of Cross
+     * and Octagon, but only if I can find a proof that they're all
+     * soluble. For the moment, therefore, I'm going to disallow
+     * them at any size other than the standard one.
+     */
+    if (params->type == TYPE_CROSS || params->type == TYPE_OCTAGON) {
+       if (params->w != 7 || params->h != 7)
+           return "This board type is only supported at 7x7";
+    }
+    return NULL;
+}
+
+/* ----------------------------------------------------------------------
+ * Beginning of code to generate random Peg Solitaire boards.
+ * 
+ * This procedure is done with no aesthetic judgment, no effort at
+ * symmetry, no difficulty grading and generally no finesse
+ * whatsoever. We simply begin with an empty board containing a
+ * single peg, and repeatedly make random reverse moves until it's
+ * plausibly full. This typically yields a scrappy haphazard mess
+ * with several holes, an uneven shape, and no redeeming features
+ * except guaranteed solubility.
+ *
+ * My only concessions to sophistication are (a) to repeat the
+ * generation process until I at least get a grid that touches
+ * every edge of the specified board size, and (b) to try when
+ * selecting moves to reuse existing space rather than expanding
+ * into new space (so that non-rectangular board shape becomes a
+ * factor during play).
+ */
+
+struct move {
+    /*
+     * x,y are the start point of the move during generation (hence
+     * its endpoint during normal play).
+     * 
+     * dx,dy are the direction of the move during generation.
+     * Absolute value 1. Hence, for example, x=3,y=5,dx=1,dy=0
+     * means that the move during generation starts at (3,5) and
+     * ends at (5,5), and vice versa during normal play.
+     */
+    int x, y, dx, dy;
+    /*
+     * cost is 0, 1 or 2, depending on how many GRID_OBSTs we must
+     * turn into GRID_HOLEs to play this move.
+     */
+    int cost;
+};
+
+static int movecmp(void *av, void *bv)
+{
+    struct move *a = (struct move *)av;
+    struct move *b = (struct move *)bv;
+
+    if (a->y < b->y)
+       return -1;
+    else if (a->y > b->y)
+       return +1;
+
+    if (a->x < b->x)
+       return -1;
+    else if (a->x > b->x)
+       return +1;
+
+    if (a->dy < b->dy)
+       return -1;
+    else if (a->dy > b->dy)
+       return +1;
+
+    if (a->dx < b->dx)
+       return -1;
+    else if (a->dx > b->dx)
+       return +1;
+
+    return 0;
+}
+
+static int movecmpcost(void *av, void *bv)
+{
+    struct move *a = (struct move *)av;
+    struct move *b = (struct move *)bv;
+
+    if (a->cost < b->cost)
+       return -1;
+    else if (a->cost > b->cost)
+       return +1;
+
+    return movecmp(av, bv);
+}
+
+struct movetrees {
+    tree234 *bymove, *bycost;
+};
+
+static void update_moves(unsigned char *grid, int w, int h, int x, int y,
+                        struct movetrees *trees)
+{
+    struct move move;
+    int dir, pos;
+
+    /*
+     * There are twelve moves that can include (x,y): three in each
+     * of four directions. Check each one to see if it's possible.
+     */
+    for (dir = 0; dir < 4; dir++) {
+       int dx, dy;
+
+       if (dir & 1)
+           dx = 0, dy = dir - 2;
+       else
+           dy = 0, dx = dir - 1;
+
+       assert(abs(dx) + abs(dy) == 1);
+
+       for (pos = 0; pos < 3; pos++) {
+           int v1, v2, v3;
+
+           move.dx = dx;
+           move.dy = dy;
+           move.x = x - pos*dx;
+           move.y = y - pos*dy;
+
+           if (move.x < 0 || move.x >= w || move.y < 0 || move.y >= h)
+               continue;              /* completely invalid move */
+           if (move.x+2*move.dx < 0 || move.x+2*move.dx >= w ||
+               move.y+2*move.dy < 0 || move.y+2*move.dy >= h)
+               continue;              /* completely invalid move */
+
+           v1 = grid[move.y * w + move.x];
+           v2 = grid[(move.y+move.dy) * w + (move.x+move.dx)];
+           v3 = grid[(move.y+2*move.dy)*w + (move.x+2*move.dx)];
+           if (v1 == GRID_PEG && v2 != GRID_PEG && v3 != GRID_PEG) {
+               struct move *m;
+
+               move.cost = (v2 == GRID_OBST) + (v3 == GRID_OBST);
+
+               /*
+                * This move is possible. See if it's already in
+                * the tree.
+                */
+               m = find234(trees->bymove, &move, NULL);
+               if (m && m->cost != move.cost) {
+                   /*
+                    * It's in the tree but listed with the wrong
+                    * cost. Remove the old version.
+                    */
+#ifdef GENERATION_DIAGNOSTICS
+                   printf("correcting %d%+d,%d%+d at cost %d\n",
+                          m->x, m->dx, m->y, m->dy, m->cost);
+#endif
+                   del234(trees->bymove, m);
+                   del234(trees->bycost, m);
+                   sfree(m);
+                   m = NULL;
+               }
+               if (!m) {
+                   struct move *m, *m2;
+                   m = snew(struct move);
+                   *m = move;
+                   m2 = add234(trees->bymove, m);
+                   m2 = add234(trees->bycost, m);
+                   assert(m2 == m);
+#ifdef GENERATION_DIAGNOSTICS
+                   printf("adding %d%+d,%d%+d at cost %d\n",
+                          move.x, move.dx, move.y, move.dy, move.cost);
+#endif
+               } else {
+#ifdef GENERATION_DIAGNOSTICS
+                   printf("not adding %d%+d,%d%+d at cost %d\n",
+                          move.x, move.dx, move.y, move.dy, move.cost);
+#endif
+               }
+           } else {
+               /*
+                * This move is impossible. If it is already in the
+                * tree, delete it.
+                * 
+                * (We make use here of the fact that del234
+                * doesn't have to be passed a pointer to the
+                * _actual_ element it's deleting: it merely needs
+                * one that compares equal to it, and it will
+                * return the one it deletes.)
+                */
+               struct move *m = del234(trees->bymove, &move);
+#ifdef GENERATION_DIAGNOSTICS
+               printf("%sdeleting %d%+d,%d%+d\n", m ? "" : "not ",
+                      move.x, move.dx, move.y, move.dy);
+#endif
+               if (m) {
+                   del234(trees->bycost, m);
+                   sfree(m);
+               }
+           }
+       }
+    }
+}
+
+static void pegs_genmoves(unsigned char *grid, int w, int h, random_state *rs)
+{
+    struct movetrees atrees, *trees = &atrees;
+    struct move *m;
+    int x, y, i, nmoves;
+
+    trees->bymove = newtree234(movecmp);
+    trees->bycost = newtree234(movecmpcost);
+
+    for (y = 0; y < h; y++)
+       for (x = 0; x < w; x++)
+           if (grid[y*w+x] == GRID_PEG)
+               update_moves(grid, w, h, x, y, trees);
+
+    nmoves = 0;
+
+    while (1) {
+       int limit, maxcost, index;
+       struct move mtmp, move, *m;
+
+       /*
+        * See how many moves we can make at zero cost. Make one,
+        * if possible. Failing that, make a one-cost move, and
+        * then a two-cost one.
+        * 
+        * After filling at least half the input grid, we no longer
+        * accept cost-2 moves: if that's our only option, we give
+        * up and finish.
+        */
+       mtmp.y = h+1;
+       maxcost = (nmoves < w*h/2 ? 2 : 1);
+       m = NULL;                      /* placate optimiser */
+       for (mtmp.cost = 0; mtmp.cost <= maxcost; mtmp.cost++) {
+           limit = -1;
+           m = findrelpos234(trees->bycost, &mtmp, NULL, REL234_LT, &limit);
+#ifdef GENERATION_DIAGNOSTICS
+           printf("%d moves available with cost %d\n", limit+1, mtmp.cost);
+#endif
+           if (m)
+               break;
+       }
+       if (!m)
+           break;
+
+       index = random_upto(rs, limit+1);
+       move = *(struct move *)index234(trees->bycost, index);
+
+#ifdef GENERATION_DIAGNOSTICS
+       printf("selecting move %d%+d,%d%+d at cost %d\n",
+              move.x, move.dx, move.y, move.dy, move.cost);
+#endif
+
+       grid[move.y * w + move.x] = GRID_HOLE;
+       grid[(move.y+move.dy) * w + (move.x+move.dx)] = GRID_PEG;
+       grid[(move.y+2*move.dy)*w + (move.x+2*move.dx)] = GRID_PEG;
+
+       for (i = 0; i <= 2; i++) {
+           int tx = move.x + i*move.dx;
+           int ty = move.y + i*move.dy;
+           update_moves(grid, w, h, tx, ty, trees);
+       }
+
+       nmoves++;
+    }
+
+    while ((m = delpos234(trees->bymove, 0)) != NULL) {
+       del234(trees->bycost, m);
+       sfree(m);
+    }
+    freetree234(trees->bymove);
+    freetree234(trees->bycost);
+}
+
+static void pegs_generate(unsigned char *grid, int w, int h, random_state *rs)
+{
+    while (1) {
+       int x, y, extremes;
+
+       memset(grid, GRID_OBST, w*h);
+       grid[(h/2) * w + (w/2)] = GRID_PEG;
+#ifdef GENERATION_DIAGNOSTICS
+       printf("beginning move selection\n");
+#endif
+       pegs_genmoves(grid, w, h, rs);
+#ifdef GENERATION_DIAGNOSTICS
+       printf("finished move selection\n");
+#endif
+
+       extremes = 0;
+       for (y = 0; y < h; y++) {
+           if (grid[y*w+0] != GRID_OBST)
+               extremes |= 1;
+           if (grid[y*w+w-1] != GRID_OBST)
+               extremes |= 2;
+       }
+       for (x = 0; x < w; x++) {
+           if (grid[0*w+x] != GRID_OBST)
+               extremes |= 4;
+           if (grid[(h-1)*w+x] != GRID_OBST)
+               extremes |= 8;
+       }
+
+       if (extremes == 15)
+           break;
+#ifdef GENERATION_DIAGNOSTICS
+       printf("insufficient extent; trying again\n");
+#endif
+    }
+    fflush(stdout);
+}
+
+/* ----------------------------------------------------------------------
+ * End of board generation code. Now for the client code which uses
+ * it as part of the puzzle.
+ */
+
+static char *new_game_desc(game_params *params, random_state *rs,
+                          char **aux, int interactive)
+{
+    int w = params->w, h = params->h;
+    unsigned char *grid;
+    char *ret;
+    int i;
+
+    grid = snewn(w*h, unsigned char);
+    if (params->type == TYPE_RANDOM) {
+       pegs_generate(grid, w, h, rs);
+    } else {
+       int x, y, cx, cy, v;
+
+       for (y = 0; y < h; y++)
+           for (x = 0; x < w; x++) {
+               v = GRID_OBST;         /* placate optimiser */
+               switch (params->type) {
+                 case TYPE_CROSS:
+                   cx = abs(x - w/2);
+                   cy = abs(y - h/2);
+                   if (cx == 0 && cy == 0)
+                       v = GRID_HOLE;
+                   else if (cx > 1 && cy > 1)
+                       v = GRID_OBST;
+                   else
+                       v = GRID_PEG;
+                   break;
+                 case TYPE_OCTAGON:
+                   cx = abs(x - w/2);
+                   cy = abs(y - h/2);
+                   if (cx == 0 && cy == 0)
+                       v = GRID_HOLE;
+                   else if (cx + cy > 1 + max(w,h)/2)
+                       v = GRID_OBST;
+                   else
+                       v = GRID_PEG;
+                   break;
+               }
+               grid[y*w+x] = v;
+           }
+    }
+
+    /*
+     * Encode a game description which is simply a long list of P
+     * for peg, H for hole or O for obstacle.
+     */
+    ret = snewn(w*h+1, char);
+    for (i = 0; i < w*h; i++)
+       ret[i] = (grid[i] == GRID_PEG ? 'P' :
+                 grid[i] == GRID_HOLE ? 'H' : 'O');
+    ret[w*h] = '\0';
+
+    sfree(grid);
+
+    return ret;
+}
+
+static char *validate_desc(game_params *params, char *desc)
+{
+    int len = params->w * params->h;
+
+    if (len != strlen(desc))
+       return "Game description is wrong length";
+    if (len != strspn(desc, "PHO"))
+       return "Invalid character in game description";
+
+    return NULL;
+}
+
+static game_state *new_game(midend_data *me, game_params *params, char *desc)
+{
+    int w = params->w, h = params->h;
+    game_state *state = snew(game_state);
+    int i;
+
+    state->w = w;
+    state->h = h;
+    state->grid = snewn(w*h, unsigned char);
+    for (i = 0; i < w*h; i++)
+       state->grid[i] = (desc[i] == 'P' ? GRID_PEG :
+                         desc[i] == 'H' ? GRID_HOLE : GRID_OBST);
+
+    return state;
+}
+
+static game_state *dup_game(game_state *state)
+{
+    int w = state->w, h = state->h;
+    game_state *ret = snew(game_state);
+
+    ret->w = state->w;
+    ret->h = state->h;
+    ret->grid = snewn(w*h, unsigned char);
+    memcpy(ret->grid, state->grid, w*h);
+
+    return ret;
+}
+
+static void free_game(game_state *state)
+{
+    sfree(state->grid);
+    sfree(state);
+}
+
+static char *solve_game(game_state *state, game_state *currstate,
+                       char *aux, char **error)
+{
+    return NULL;
+}
+
+static char *game_text_format(game_state *state)
+{
+    int w = state->w, h = state->h;
+    int x, y;
+    char *ret;
+
+    ret = snewn((w+1)*h + 1, char);
+
+    for (y = 0; y < h; y++) {
+       for (x = 0; x < w; x++)
+           ret[y*(w+1)+x] = (state->grid[y*w+x] == GRID_HOLE ? '-' :
+                             state->grid[y*w+x] == GRID_PEG ? '*' : ' ');
+       ret[y*(w+1)+w] = '\n';
+    }
+    ret[h*(w+1)] = '\0';
+
+    return ret;
+}
+
+struct game_ui {
+    int dragging;                     /* boolean: is a drag in progress? */
+    int sx, sy;                               /* grid coords of drag start cell */
+    int dx, dy;                               /* pixel coords of current drag posn */
+};
+
+static game_ui *new_ui(game_state *state)
+{
+    game_ui *ui = snew(game_ui);
+
+    ui->sx = ui->sy = ui->dx = ui->dy = 0;
+    ui->dragging = FALSE;
+
+    return ui;
+}
+
+static void free_ui(game_ui *ui)
+{
+    sfree(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)
+{
+    /*
+     * Cancel a drag, in case the source square has become
+     * unoccupied.
+     */
+    ui->dragging = FALSE;
+}
+
+#define PREFERRED_TILE_SIZE 33
+#define TILESIZE (ds->tilesize)
+#define BORDER (TILESIZE / 2)
+
+#define HIGHLIGHT_WIDTH (TILESIZE / 16)
+
+#define COORD(x)     ( BORDER + (x) * TILESIZE )
+#define FROMCOORD(x) ( ((x) + TILESIZE - BORDER) / TILESIZE - 1 )
+
+struct game_drawstate {
+    int tilesize;
+    blitter *drag_background;
+    int dragging, dragx, dragy;
+    int w, h;
+    unsigned char *grid;
+    int started;
+};
+
+static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
+                           int x, int y, int button)
+{
+    int w = state->w, h = state->h;
+
+    if (button == LEFT_BUTTON) {
+       int tx, ty;
+
+       /*
+        * Left button down: we attempt to start a drag.
+        */
+       
+       /*
+        * There certainly shouldn't be a current drag in progress,
+        * unless the midend failed to send us button events in
+        * order; it has a responsibility to always get that right,
+        * so we can legitimately punish it by failing an
+        * assertion.
+        */
+       assert(!ui->dragging);
+
+       tx = FROMCOORD(x);
+       ty = FROMCOORD(y);
+       if (tx >= 0 && tx < w && ty >= 0 && ty < h &&
+           state->grid[ty*w+tx] == GRID_PEG) {
+           ui->dragging = TRUE;
+           ui->sx = tx;
+           ui->sy = ty;
+           ui->dx = x;
+           ui->dy = y;
+           return "";                 /* ui modified */
+       }
+    } else if (button == LEFT_DRAG && ui->dragging) {
+       /*
+        * Mouse moved; just move the peg being dragged.
+        */
+       ui->dx = x;
+       ui->dy = y;
+       return "";                     /* ui modified */
+    } else if (button == LEFT_RELEASE && ui->dragging) {
+       char buf[80];
+       int tx, ty, dx, dy;
+
+       /*
+        * Button released. Identify the target square of the drag,
+        * see if it represents a valid move, and if so make it.
+        */
+       ui->dragging = FALSE;          /* cancel the drag no matter what */
+       tx = FROMCOORD(x);
+       ty = FROMCOORD(y);
+       if (tx < 0 || tx >= w || ty < 0 || ty >= h)
+           return "";                 /* target out of range */
+       dx = tx - ui->sx;
+       dy = ty - ui->sy;
+       if (max(abs(dx),abs(dy)) != 2 || min(abs(dx),abs(dy)) != 0)
+           return "";                 /* move length was wrong */
+       dx /= 2;
+       dy /= 2;
+
+       if (state->grid[ty*w+tx] != GRID_HOLE ||
+           state->grid[(ty-dy)*w+(tx-dx)] != GRID_PEG ||
+           state->grid[ui->sy*w+ui->sx] != GRID_PEG)
+           return "";                 /* grid contents were invalid */
+
+       /*
+        * We have a valid move. Encode it simply as source and
+        * destination coordinate pairs.
+        */
+       sprintf(buf, "%d,%d-%d,%d", ui->sx, ui->sy, tx, ty);
+       return dupstr(buf);
+    }
+    return NULL;
+}
+
+static game_state *execute_move(game_state *state, char *move)
+{
+    int w = state->w, h = state->h;
+    int sx, sy, tx, ty;
+    game_state *ret;
+
+    if (sscanf(move, "%d,%d-%d,%d", &sx, &sy, &tx, &ty)) {
+       int mx, my, dx, dy;
+
+       if (sx < 0 || sx >= w || sy < 0 || sy >= h)
+           return NULL;               /* source out of range */
+       if (tx < 0 || tx >= w || ty < 0 || ty >= h)
+           return NULL;               /* target out of range */
+
+       dx = tx - sx;
+       dy = ty - sy;
+       if (max(abs(dx),abs(dy)) != 2 || min(abs(dx),abs(dy)) != 0)
+           return NULL;               /* move length was wrong */
+       mx = sx + dx/2;
+       my = sy + dy/2;
+
+       if (state->grid[sy*w+sx] != GRID_PEG ||
+           state->grid[my*w+mx] != GRID_PEG ||
+           state->grid[ty*w+tx] != GRID_HOLE)
+           return NULL;               /* grid contents were invalid */
+
+       ret = dup_game(state);
+       ret->grid[sy*w+sx] = GRID_HOLE;
+       ret->grid[my*w+mx] = GRID_HOLE;
+       ret->grid[ty*w+tx] = GRID_PEG;
+
+       return ret;
+    }
+    return NULL;
+}
+
+/* ----------------------------------------------------------------------
+ * Drawing routines.
+ */
+
+static void game_size(game_params *params, game_drawstate *ds,
+                      int *x, int *y, int expand)
+{
+    double tsx, tsy, ts;
+    /*
+     * Each window dimension equals the tile size times one more
+     * than the grid dimension (the border is half the width of the
+     * tiles).
+     */
+    tsx = (double)*x / ((double)params->w + 1.0);
+    tsy = (double)*y / ((double)params->h + 1.0);
+    ts = min(tsx, tsy);
+    if (expand)
+        ds->tilesize = (int)(ts + 0.5);
+    else
+        ds->tilesize = min((int)ts, PREFERRED_TILE_SIZE);
+
+    *x = TILESIZE * params->w + 2 * BORDER;
+    *y = TILESIZE * params->h + 2 * BORDER;
+
+    if (ds->drag_background)
+       blitter_free(ds->drag_background);
+    ds->drag_background = blitter_new(TILESIZE, TILESIZE);
+}
+
+static float *game_colours(frontend *fe, game_state *state, int *ncolours)
+{
+    float *ret = snewn(3 * NCOLOURS, float);
+    int i;
+    float max;
+
+    frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);
+
+    /*
+     * Drop the background colour so that the highlight is
+     * noticeably brighter than it while still being under 1.
+     */
+    max = ret[COL_BACKGROUND*3];
+    for (i = 1; i < 3; i++)
+        if (ret[COL_BACKGROUND*3+i] > max)
+            max = ret[COL_BACKGROUND*3+i];
+    if (max * 1.2F > 1.0F) {
+        for (i = 0; i < 3; i++)
+            ret[COL_BACKGROUND*3+i] /= (max * 1.2F);
+    }
+
+    for (i = 0; i < 3; i++) {
+        ret[COL_HIGHLIGHT * 3 + i] = ret[COL_BACKGROUND * 3 + i] * 1.2F;
+        ret[COL_LOWLIGHT * 3 + i] = ret[COL_BACKGROUND * 3 + i] * 0.8F;
+    }
+
+    ret[COL_PEG * 3 + 0] = 0.0F;
+    ret[COL_PEG * 3 + 1] = 0.0F;
+    ret[COL_PEG * 3 + 2] = 1.0F;
+
+    *ncolours = NCOLOURS;
+    return ret;
+}
+
+static game_drawstate *game_new_drawstate(game_state *state)
+{
+    int w = state->w, h = state->h;
+    struct game_drawstate *ds = snew(struct game_drawstate);
+
+    ds->tilesize = 0;                 /* not decided yet */
+
+    /* We can't allocate the blitter rectangle for the drag background
+     * until we know what size to make it. */
+    ds->drag_background = NULL;
+    ds->dragging = FALSE;
+
+    ds->w = w;
+    ds->h = h;
+    ds->grid = snewn(w*h, unsigned char);
+    memset(ds->grid, 255, w*h);
+
+    ds->started = FALSE;
+
+    return ds;
+}
+
+static void game_free_drawstate(game_drawstate *ds)
+{
+    if (ds->drag_background)
+       blitter_free(ds->drag_background);
+    sfree(ds->grid);
+    sfree(ds);
+}
+
+static void draw_tile(frontend *fe, game_drawstate *ds,
+                     int x, int y, int v, int erasebg)
+{
+    if (erasebg) {
+       draw_rect(fe, x, y, TILESIZE, TILESIZE, COL_BACKGROUND);
+    }
+
+    if (v == GRID_HOLE) {
+       draw_circle(fe, x+TILESIZE/2, y+TILESIZE/2, TILESIZE/4,
+                   COL_LOWLIGHT, COL_LOWLIGHT);
+    } else if (v == GRID_PEG) {
+       draw_circle(fe, x+TILESIZE/2, y+TILESIZE/2, TILESIZE/3,
+                   COL_PEG, COL_PEG);
+    }
+
+    draw_update(fe, x, y, TILESIZE, TILESIZE);
+}
+
+static void game_redraw(frontend *fe, game_drawstate *ds, game_state *oldstate,
+                       game_state *state, int dir, game_ui *ui,
+                       float animtime, float flashtime)
+{
+    int w = state->w, h = state->h;
+    int x, y;
+
+    /*
+     * Erase the sprite currently being dragged, if any.
+     */
+    if (ds->dragging) {
+       assert(ds->drag_background);
+        blitter_load(fe, ds->drag_background, ds->dragx, ds->dragy);
+        draw_update(fe, ds->dragx, ds->dragy, TILESIZE, TILESIZE);
+       ds->dragging = FALSE;
+    }
+
+    if (!ds->started) {
+       draw_rect(fe, 0, 0,
+                 TILESIZE * state->w + 2 * BORDER,
+                 TILESIZE * state->h + 2 * BORDER, COL_BACKGROUND);
+
+       /*
+        * Draw relief marks around all the squares that aren't
+        * GRID_OBST.
+        */
+       for (y = 0; y < h; y++)
+           for (x = 0; x < w; x++)
+               if (state->grid[y*w+x] != GRID_OBST) {
+                   /*
+                    * First pass: draw the full relief square.
+                    */
+                   int coords[6];
+                   coords[0] = COORD(x+1) + HIGHLIGHT_WIDTH - 1;
+                   coords[1] = COORD(y) - HIGHLIGHT_WIDTH;
+                   coords[2] = COORD(x) - HIGHLIGHT_WIDTH;
+                   coords[3] = COORD(y+1) + HIGHLIGHT_WIDTH - 1;
+                   coords[4] = COORD(x) - HIGHLIGHT_WIDTH;
+                   coords[5] = COORD(y) - HIGHLIGHT_WIDTH;
+                   draw_polygon(fe, coords, 3, COL_HIGHLIGHT, COL_HIGHLIGHT);
+                   coords[4] = COORD(x+1) + HIGHLIGHT_WIDTH - 1;
+                   coords[5] = COORD(y+1) + HIGHLIGHT_WIDTH - 1;
+                   draw_polygon(fe, coords, 3, COL_LOWLIGHT, COL_LOWLIGHT);
+               }
+       for (y = 0; y < h; y++)
+           for (x = 0; x < w; x++)
+               if (state->grid[y*w+x] != GRID_OBST) {
+                   /*
+                    * Second pass: draw everything but the two
+                    * diagonal corners.
+                    */
+                   draw_rect(fe, COORD(x) - HIGHLIGHT_WIDTH,
+                             COORD(y) - HIGHLIGHT_WIDTH,
+                             TILESIZE + HIGHLIGHT_WIDTH,
+                             TILESIZE + HIGHLIGHT_WIDTH, COL_HIGHLIGHT);
+                   draw_rect(fe, COORD(x),
+                             COORD(y),
+                             TILESIZE + HIGHLIGHT_WIDTH,
+                             TILESIZE + HIGHLIGHT_WIDTH, COL_LOWLIGHT);
+               }
+       for (y = 0; y < h; y++)
+           for (x = 0; x < w; x++)
+               if (state->grid[y*w+x] != GRID_OBST) {
+                   /*
+                    * Third pass: draw a trapezium on each edge.
+                    */
+                   int coords[8];
+                   int dx, dy, s, sn, c;
+
+                   for (dx = 0; dx < 2; dx++) {
+                       dy = 1 - dx;
+                       for (s = 0; s < 2; s++) {
+                           sn = 2*s - 1;
+                           c = s ? COL_LOWLIGHT : COL_HIGHLIGHT;
+
+                           coords[0] = COORD(x) + (s*dx)*(TILESIZE-1);
+                           coords[1] = COORD(y) + (s*dy)*(TILESIZE-1);
+                           coords[2] = COORD(x) + (s*dx+dy)*(TILESIZE-1);
+                           coords[3] = COORD(y) + (s*dy+dx)*(TILESIZE-1);
+                           coords[4] = coords[2] - HIGHLIGHT_WIDTH * (dy-sn*dx);
+                           coords[5] = coords[3] - HIGHLIGHT_WIDTH * (dx-sn*dy);
+                           coords[6] = coords[0] + HIGHLIGHT_WIDTH * (dy+sn*dx);
+                           coords[7] = coords[1] + HIGHLIGHT_WIDTH * (dx+sn*dy);
+                           draw_polygon(fe, coords, 4, c, c);
+                       }
+                   }
+               }
+       for (y = 0; y < h; y++)
+           for (x = 0; x < w; x++)
+               if (state->grid[y*w+x] != GRID_OBST) {
+                   /*
+                    * Second pass: draw everything but the two
+                    * diagonal corners.
+                    */
+                   draw_rect(fe, COORD(x),
+                             COORD(y),
+                             TILESIZE,
+                             TILESIZE, COL_BACKGROUND);
+               }
+
+       ds->started = TRUE;
+
+       draw_update(fe, 0, 0,
+                   TILESIZE * state->w + 2 * BORDER,
+                   TILESIZE * state->h + 2 * BORDER);
+    }
+
+    /*
+     * Loop over the grid redrawing anything that looks as if it
+     * needs it.
+     */
+    for (y = 0; y < h; y++)
+       for (x = 0; x < w; x++) {
+           int v;
+
+           v = state->grid[y*w+x];
+           /*
+            * Blank the source of a drag so it looks as if the
+            * user picked the peg up physically.
+            */
+           if (ui->dragging && ui->sx == x && ui->sy == y && v == GRID_PEG)
+               v = GRID_HOLE;
+           if (v != ds->grid[y*w+x] && v != GRID_OBST) {
+               draw_tile(fe, ds, COORD(x), COORD(y), v, TRUE);
+           }
+       }
+
+    /*
+     * Draw the dragging sprite if any.
+     */
+    if (ui->dragging) {
+       ds->dragging = TRUE;
+       ds->dragx = ui->dx - TILESIZE/2;
+       ds->dragy = ui->dy - TILESIZE/2;
+       blitter_save(fe, ds->drag_background, ds->dragx, ds->dragy);
+       draw_tile(fe, ds, ds->dragx, ds->dragy, GRID_PEG, FALSE);
+    }
+}
+
+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)
+{
+    return 0.0F;
+}
+
+static int game_wants_statusbar(void)
+{
+    return FALSE;
+}
+
+static int game_timing_state(game_state *state)
+{
+    return TRUE;
+}
+
+#ifdef COMBINED
+#define thegame pegs
+#endif
+
+const struct game thegame = {
+    "Pegs", NULL,
+    default_params,
+    game_fetch_preset,
+    decode_params,
+    encode_params,
+    free_params,
+    dup_params,
+    TRUE, game_configure, custom_params,
+    validate_params,
+    new_game_desc,
+    validate_desc,
+    new_game,
+    dup_game,
+    free_game,
+    FALSE, solve_game,
+    TRUE, game_text_format,
+    new_ui,
+    free_ui,
+    encode_ui,
+    decode_ui,
+    game_changed_state,
+    interpret_move,
+    execute_move,
+    game_size,
+    game_colours,
+    game_new_drawstate,
+    game_free_drawstate,
+    game_redraw,
+    game_anim_length,
+    game_flash_length,
+    game_wants_statusbar,
+    FALSE, game_timing_state,
+    0,                                /* mouse_priorities */
+};