Replace my brute-force algorithm in face_text_pos with a more complex
[sgt/puzzles] / loopy.c
diff --git a/loopy.c b/loopy.c
index 488b507..7d03ed2 100644 (file)
--- a/loopy.c
+++ b/loopy.c
@@ -228,6 +228,7 @@ struct game_drawstate {
     int started;
     int tilesize;
     int flashing;
+    int *textx, *texty;
     char *lines;
     char *clue_error;
     char *clue_satisfied;
@@ -253,7 +254,10 @@ static void check_caches(const solver_state* sstate);
     A(Cairo,grid_new_cairo,3,4) \
     A(Great-Hexagonal,grid_new_greathexagonal,3,3) \
     A(Octagonal,grid_new_octagonal,3,3) \
-    A(Kites,grid_new_kites,3,3)
+    A(Kites,grid_new_kites,3,3) \
+    A(Floret,grid_new_floret,1,2) \
+    A(Dodecagonal,grid_new_dodecagonal,2,2) \
+    A(Great-Dodecagonal,grid_new_greatdodecagonal,2,2)
 
 #define GRID_NAME(title,fn,amin,omin) #title,
 #define GRID_CONFIG(title,fn,amin,omin) ":" #title
@@ -300,7 +304,7 @@ static void params_generate_grid(game_params *params)
                                ((field) &= ~(1<<(bit)), TRUE) : FALSE)
 
 #define CLUE2CHAR(c) \
-    ((c < 0) ? ' ' : c + '0')
+    ((c < 0) ? ' ' : c < 10 ? c + '0' : c - 10 + 'A')
 
 /* ----------------------------------------------------------------------
  * General struct manipulation and other straightforward code
@@ -504,6 +508,9 @@ static const game_params presets[] = {
     {  5,  4, DIFF_HARD, 5, NULL },
     {  5,  5, DIFF_HARD, 6, NULL },
     {  5,  5, DIFF_HARD, 7, NULL },
+    {  3,  3, DIFF_HARD, 8, NULL },
+    {  3,  3, DIFF_HARD, 9, NULL },
+    {  3,  3, DIFF_HARD, 10, NULL },
 #else
     {  7,  7, DIFF_EASY, 0, NULL },
     {  10,  10, DIFF_EASY, 0, NULL },
@@ -518,6 +525,9 @@ static const game_params presets[] = {
     {  5,  4, DIFF_HARD, 5, NULL },
     {  7,  7, DIFF_HARD, 6, NULL },
     {  5,  5, DIFF_HARD, 7, NULL },
+    {  5,  5, DIFF_HARD, 8, NULL },
+    {  5,  4, DIFF_HARD, 9, NULL },
+    {  5,  4, DIFF_HARD, 10, NULL },
 #endif
 };
 
@@ -702,7 +712,7 @@ static char *validate_desc(game_params *params, char *desc)
     g = params->game_grid;
 
     for (; *desc; ++desc) {
-        if (*desc >= '0' && *desc <= '9') {
+        if ((*desc >= '0' && *desc <= '9') || (*desc >= 'A' && *desc <= 'Z')) {
             count++;
             continue;
         }
@@ -829,8 +839,14 @@ static float *game_colours(frontend *fe, int *ncolours)
     ret[COL_FOREGROUND * 3 + 1] = 0.0F;
     ret[COL_FOREGROUND * 3 + 2] = 0.0F;
 
-    ret[COL_LINEUNKNOWN * 3 + 0] = 0.8F;
-    ret[COL_LINEUNKNOWN * 3 + 1] = 0.8F;
+    /*
+     * We want COL_LINEUNKNOWN to be a yellow which is a bit darker
+     * than the background. (I previously set it to 0.8,0.8,0, but
+     * found that this went badly with the 0.8,0.8,0.8 favoured as a
+     * background by the Java frontend.)
+     */
+    ret[COL_LINEUNKNOWN * 3 + 0] = ret[COL_BACKGROUND * 3 + 0] * 0.9F;
+    ret[COL_LINEUNKNOWN * 3 + 1] = ret[COL_BACKGROUND * 3 + 1] * 0.9F;
     ret[COL_LINEUNKNOWN * 3 + 2] = 0.0F;
 
     ret[COL_HIGHLIGHT * 3 + 0] = 1.0F;
@@ -862,17 +878,22 @@ static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
     struct game_drawstate *ds = snew(struct game_drawstate);
     int num_faces = state->game_grid->num_faces;
     int num_edges = state->game_grid->num_edges;
+    int i;
 
     ds->tilesize = 0;
     ds->started = 0;
     ds->lines = snewn(num_edges, char);
     ds->clue_error = snewn(num_faces, char);
     ds->clue_satisfied = snewn(num_faces, char);
+    ds->textx = snewn(num_faces, int);
+    ds->texty = snewn(num_faces, int);
     ds->flashing = 0;
 
     memset(ds->lines, LINE_UNKNOWN, num_edges);
     memset(ds->clue_error, 0, num_faces);
     memset(ds->clue_satisfied, 0, num_faces);
+    for (i = 0; i < num_faces; i++)
+        ds->textx[i] = ds->texty[i] = -1;
 
     return ds;
 }
@@ -1868,7 +1889,7 @@ static game_state *new_game(midend *me, game_params *params, char *desc)
     int i;
     game_state *state = snew(game_state);
     int empties_to_make = 0;
-    int n;
+    int n,n2;
     const char *dp = desc;
     grid *g;
     int num_faces, num_edges;
@@ -1896,8 +1917,11 @@ static game_state *new_game(midend *me, game_params *params, char *desc)
 
         assert(*dp);
         n = *dp - '0';
+        n2 = *dp - 'A' + 10;
         if (n >= 0 && n < 10) {
             state->clues[i] = n;
+       } else if (n2 >= 10 && n2 < 36) {
+            state->clues[i] = n2;
         } else {
             n = *dp - 'a' + 1;
             assert(n > 0);
@@ -2412,6 +2436,13 @@ static int trivial_deductions(solver_state *sstate)
         if (state->clues[i] < 0)
             continue;
 
+        /*
+         * This code checks whether the numeric clue on a face is so
+         * large as to permit all its remaining LINE_UNKNOWNs to be
+         * filled in as LINE_YES, or alternatively so small as to
+         * permit them all to be filled in as LINE_NO.
+         */
+
         if (state->clues[i] < current_yes) {
             sstate->solver_status = SOLVER_MISTAKE;
             return DIFF_EASY;
@@ -2433,6 +2464,57 @@ static int trivial_deductions(solver_state *sstate)
             sstate->face_solved[i] = TRUE;
             continue;
         }
+
+        if (f->order - state->clues[i] == current_no + 1 &&
+            f->order - current_yes - current_no > 2) {
+            /*
+             * One small refinement to the above: we also look for any
+             * adjacent pair of LINE_UNKNOWNs around the face with
+             * some LINE_YES incident on it from elsewhere. If we find
+             * one, then we know that pair of LINE_UNKNOWNs can't
+             * _both_ be LINE_YES, and hence that pushes us one line
+             * closer to being able to determine all the rest.
+             */
+            int j, k, e1, e2, e, d;
+
+            for (j = 0; j < f->order; j++) {
+                e1 = f->edges[j] - g->edges;
+                e2 = f->edges[j+1 < f->order ? j+1 : 0] - g->edges;
+
+                if (g->edges[e1].dot1 == g->edges[e2].dot1 ||
+                    g->edges[e1].dot1 == g->edges[e2].dot2) {
+                    d = g->edges[e1].dot1 - g->dots;
+                } else {
+                    assert(g->edges[e1].dot2 == g->edges[e2].dot1 ||
+                           g->edges[e1].dot2 == g->edges[e2].dot2);
+                    d = g->edges[e1].dot2 - g->dots;
+                }
+
+                if (state->lines[e1] == LINE_UNKNOWN &&
+                    state->lines[e2] == LINE_UNKNOWN) {
+                    for (k = 0; k < g->dots[d].order; k++) {
+                        int e = g->dots[d].edges[k] - g->edges;
+                        if (state->lines[e] == LINE_YES)
+                            goto found;    /* multi-level break */
+                    }
+                }
+            }
+            continue;
+
+          found:
+            /*
+             * If we get here, we've found such a pair of edges, and
+             * they're e1 and e2.
+             */
+            for (j = 0; j < f->order; j++) {
+                e = f->edges[j] - g->edges;
+                if (state->lines[e] == LINE_UNKNOWN && e != e1 && e != e2) {
+                    int r = solver_set_line(sstate, e, LINE_YES);
+                    assert(r);
+                    diff = min(diff, DIFF_EASY);
+                }
+            }
+        }
     }
 
     check_caches(sstate);
@@ -2531,7 +2613,7 @@ static int dline_deductions(solver_state *sstate)
      * on that.  We check this with an assertion, in case someone decides to
      * make a grid which has larger faces than this.  Note, this algorithm
      * could get quite expensive if there are many large faces. */
-#define MAX_FACE_SIZE 8
+#define MAX_FACE_SIZE 12
 
     for (i = 0; i < g->num_faces; i++) {
         int maxs[MAX_FACE_SIZE][MAX_FACE_SIZE];
@@ -3319,263 +3401,809 @@ static void grid_to_screen(const game_drawstate *ds, const grid *g,
     *y += BORDER(ds->tilesize);
 }
 
+static int solve_2x2_matrix(double mx[4], double vin[2], double vout[2])
+{
+    double inv[4];
+    double det;
+    det = (mx[0]*mx[3] - mx[1]*mx[2]);
+    if (det == 0)
+        return FALSE;
+
+    inv[0] = mx[3] / det;
+    inv[1] = -mx[1] / det;
+    inv[2] = -mx[2] / det;
+    inv[3] = mx[0] / det;
+
+    vout[0] = inv[0]*vin[0] + inv[1]*vin[1];
+    vout[1] = inv[2]*vin[0] + inv[3]*vin[1];
+
+    return TRUE;
+}
+
+static int solve_3x3_matrix(double mx[9], double vin[3], double vout[3])
+{
+    double inv[9];
+    double det;
+
+    det = (mx[0]*mx[4]*mx[8] + mx[1]*mx[5]*mx[6] + mx[2]*mx[3]*mx[7] -
+           mx[0]*mx[5]*mx[7] - mx[1]*mx[3]*mx[8] - mx[2]*mx[4]*mx[6]);
+    if (det == 0)
+        return FALSE;
+
+    inv[0] = (mx[4]*mx[8] - mx[5]*mx[7]) / det;
+    inv[1] = (mx[2]*mx[7] - mx[1]*mx[8]) / det;
+    inv[2] = (mx[1]*mx[5] - mx[2]*mx[4]) / det;
+    inv[3] = (mx[5]*mx[6] - mx[3]*mx[8]) / det;
+    inv[4] = (mx[0]*mx[8] - mx[2]*mx[6]) / det;
+    inv[5] = (mx[2]*mx[3] - mx[0]*mx[5]) / det;
+    inv[6] = (mx[3]*mx[7] - mx[4]*mx[6]) / det;
+    inv[7] = (mx[1]*mx[6] - mx[0]*mx[7]) / det;
+    inv[8] = (mx[0]*mx[4] - mx[1]*mx[3]) / det;
+
+    vout[0] = inv[0]*vin[0] + inv[1]*vin[1] + inv[2]*vin[2];
+    vout[1] = inv[3]*vin[0] + inv[4]*vin[1] + inv[5]*vin[2];
+    vout[2] = inv[6]*vin[0] + inv[7]*vin[1] + inv[8]*vin[2];
+
+    return TRUE;
+}
+
 /* Returns (into x,y) position of centre of face for rendering the text clue.
  */
 static void face_text_pos(const game_drawstate *ds, const grid *g,
-                          const grid_face *f, int *x, int *y)
+                          const grid_face *f, int *xret, int *yret)
 {
-    int i;
+    double xbest, ybest, bestdist;
+    int i, j, k, m;
+    grid_dot *edgedot1[3], *edgedot2[3];
+    grid_dot *dots[3];
+    int nedges, ndots;
+    int faceindex = f - g->faces;
 
-    /* Simplest solution is the centroid. Might not work in some cases. */
+    /*
+     * Return the cached position for this face, if we've already
+     * worked it out.
+     */
+    if (ds->textx[faceindex] >= 0) {
+        *xret = ds->textx[faceindex];
+        *yret = ds->texty[faceindex];
+        return;
+    }
 
-    /* Another algorithm to look into:
-     * Find the midpoints of the sides, find the bounding-box,
-     * then take the centre of that. */
+    /*
+     * Otherwise, try to find the point in the polygon with the
+     * maximum distance to any edge or corner.
+     *
+     * This point must be in contact with at least three edges and/or
+     * vertices; so we iterate through all combinations of three of
+     * those, and find candidate points in each set.
+     *
+     * We don't actually iterate literally over _edges_, in the sense
+     * of grid_edge structures. Instead, we fill in edgedot1[] and
+     * edgedot2[] with a pair of dots adjacent in the face's list of
+     * vertices. This ensures that we get the edges in consistent
+     * orientation, which we could not do from the grid structure
+     * alone. (A moment's consideration of an order-3 vertex should
+     * make it clear that if a notional arrow was written on each
+     * edge, _at least one_ of the three faces bordering that vertex
+     * would have to have the two arrows tip-to-tip or tail-to-tail
+     * rather than tip-to-tail.)
+     */
+    nedges = ndots = 0;
+    bestdist = 0;
+    xbest = ybest = 0;
+
+    for (i = 0; i+2 < 2*f->order; i++) {
+        if (i < f->order) {
+            edgedot1[nedges] = f->dots[i];
+            edgedot2[nedges++] = f->dots[(i+1)%f->order];
+        } else
+            dots[ndots++] = f->dots[i - f->order];
+
+        for (j = i+1; j+1 < 2*f->order; j++) {
+            if (j < f->order) {
+                edgedot1[nedges] = f->dots[j];
+                edgedot2[nedges++] = f->dots[(j+1)%f->order];
+            } else
+                dots[ndots++] = f->dots[j - f->order];
+
+            for (k = j+1; k < 2*f->order; k++) {
+                double cx[2], cy[2];   /* candidate positions */
+                int cn = 0;            /* number of candidates */
+
+                if (k < f->order) {
+                    edgedot1[nedges] = f->dots[k];
+                    edgedot2[nedges++] = f->dots[(k+1)%f->order];
+                } else
+                    dots[ndots++] = f->dots[k - f->order];
+
+                /*
+                 * Find a point, or pair of points, equidistant from
+                 * all the specified edges and/or vertices.
+                 */
+                if (nedges == 3) {
+                    /*
+                     * Three edges. This is a linear matrix equation:
+                     * each row of the matrix represents the fact that
+                     * the point (x,y) we seek is at distance r from
+                     * that edge, and we solve three of those
+                     * simultaneously to obtain x,y,r. (We ignore r.)
+                     */
+                    double matrix[9], vector[3], vector2[3];
+                    int m;
+
+                    for (m = 0; m < 3; m++) {
+                        int x1 = edgedot1[m]->x, x2 = edgedot2[m]->x;
+                        int y1 = edgedot1[m]->y, y2 = edgedot2[m]->y;
+                        int dx = x2-x1, dy = y2-y1;
+
+                        /*
+                         * ((x,y) - (x1,y1)) . (dy,-dx) = r |(dx,dy)|
+                         *
+                         * => x dy - y dx - r |(dx,dy)| = (x1 dy - y1 dx)
+                         */
+                        matrix[3*m+0] = dy;
+                        matrix[3*m+1] = -dx;
+                        matrix[3*m+2] = -sqrt((double)dx*dx+(double)dy*dy);
+                        vector[m] = (double)x1*dy - (double)y1*dx;
+                    }
 
-    /* Best solution probably involves incentres (inscribed circles) */
+                    if (solve_3x3_matrix(matrix, vector, vector2)) {
+                        cx[cn] = vector2[0];
+                        cy[cn] = vector2[1];
+                        cn++;
+                    }
+                } else if (nedges == 2) {
+                    /*
+                     * Two edges and a dot. This will end up in a
+                     * quadratic equation.
+                     *
+                     * First, look at the two edges. Having our point
+                     * be some distance r from both of them gives rise
+                     * to a pair of linear equations in x,y,r of the
+                     * form
+                     *
+                     *   (x-x1) dy - (y-y1) dx = r sqrt(dx^2+dy^2)
+                     *
+                     * We eliminate r between those equations to give
+                     * us a single linear equation in x,y describing
+                     * the locus of points equidistant from both lines
+                     * - i.e. the angle bisector. 
+                     *
+                     * We then choose one of x,y to be a parameter t,
+                     * and derive linear formulae for x,y,r in terms
+                     * of t. This enables us to write down the
+                     * circular equation (x-xd)^2+(y-yd)^2=r^2 as a
+                     * quadratic in t; solving that and substituting
+                     * in for x,y gives us two candidate points.
+                     */
+                    double eqs[2][4];  /* a,b,c,d : ax+by+cr=d */
+                    double eq[3];      /* a,b,c: ax+by=c */
+                    double xt[2], yt[2], rt[2]; /* a,b: {x,y,r}=at+b */
+                    double q[3];                /* a,b,c: at^2+bt+c=0 */
+                    double disc;
+
+                    /* Find equations of the two input lines. */
+                    for (m = 0; m < 2; m++) {
+                        int x1 = edgedot1[m]->x, x2 = edgedot2[m]->x;
+                        int y1 = edgedot1[m]->y, y2 = edgedot2[m]->y;
+                        int dx = x2-x1, dy = y2-y1;
+
+                        eqs[m][0] = dy;
+                        eqs[m][1] = -dx;
+                        eqs[m][2] = -sqrt(dx*dx+dy*dy);
+                        eqs[m][3] = x1*dy - y1*dx;
+                    }
 
-    int sx = 0, sy = 0; /* sums */
-    for (i = 0; i < f->order; i++) {
-        grid_dot *d = f->dots[i];
-        sx += d->x;
-        sy += d->y;
+                    /* Derive the angle bisector by eliminating r. */
+                    eq[0] = eqs[0][0]*eqs[1][2] - eqs[1][0]*eqs[0][2];
+                    eq[1] = eqs[0][1]*eqs[1][2] - eqs[1][1]*eqs[0][2];
+                    eq[2] = eqs[0][3]*eqs[1][2] - eqs[1][3]*eqs[0][2];
+
+                    /* Parametrise x and y in terms of some t. */
+                    if (abs(eq[0]) < abs(eq[1])) {
+                        /* Parameter is x. */
+                        xt[0] = 1; xt[1] = 0;
+                        yt[0] = -eq[0]/eq[1]; yt[1] = eq[2]/eq[1];
+                    } else {
+                        /* Parameter is y. */
+                        yt[0] = 1; yt[1] = 0;
+                        xt[0] = -eq[1]/eq[0]; xt[1] = eq[2]/eq[0];
+                    }
+
+                    /* Find a linear representation of r using eqs[0]. */
+                    rt[0] = -(eqs[0][0]*xt[0] + eqs[0][1]*yt[0])/eqs[0][2];
+                    rt[1] = (eqs[0][3] - eqs[0][0]*xt[1] -
+                             eqs[0][1]*yt[1])/eqs[0][2];
+
+                    /* Construct the quadratic equation. */
+                    q[0] = -rt[0]*rt[0];
+                    q[1] = -2*rt[0]*rt[1];
+                    q[2] = -rt[1]*rt[1];
+                    q[0] += xt[0]*xt[0];
+                    q[1] += 2*xt[0]*(xt[1]-dots[0]->x);
+                    q[2] += (xt[1]-dots[0]->x)*(xt[1]-dots[0]->x);
+                    q[0] += yt[0]*yt[0];
+                    q[1] += 2*yt[0]*(yt[1]-dots[0]->y);
+                    q[2] += (yt[1]-dots[0]->y)*(yt[1]-dots[0]->y);
+
+                    /* And solve it. */
+                    disc = q[1]*q[1] - 4*q[0]*q[2];
+                    if (disc >= 0) {
+                        double t;
+
+                        disc = sqrt(disc);
+
+                        t = (-q[1] + disc) / (2*q[0]);
+                        cx[cn] = xt[0]*t + xt[1];
+                        cy[cn] = yt[0]*t + yt[1];
+                        cn++;
+
+                        t = (-q[1] - disc) / (2*q[0]);
+                        cx[cn] = xt[0]*t + xt[1];
+                        cy[cn] = yt[0]*t + yt[1];
+                        cn++;
+                    }
+                } else if (nedges == 1) {
+                    /*
+                     * Two dots and an edge. This one's another
+                     * quadratic equation.
+                     *
+                     * The point we want must lie on the perpendicular
+                     * bisector of the two dots; that much is obvious.
+                     * So we can construct a parametrisation of that
+                     * bisecting line, giving linear formulae for x,y
+                     * in terms of t. We can also express the distance
+                     * from the edge as such a linear formula.
+                     *
+                     * Then we set that equal to the radius of the
+                     * circle passing through the two points, which is
+                     * a Pythagoras exercise; that gives rise to a
+                     * quadratic in t, which we solve.
+                     */
+                    double xt[2], yt[2], rt[2]; /* a,b: {x,y,r}=at+b */
+                    double q[3];                /* a,b,c: at^2+bt+c=0 */
+                    double disc;
+                    double halfsep;
+
+                    /* Find parametric formulae for x,y. */
+                    {
+                        int x1 = dots[0]->x, x2 = dots[1]->x;
+                        int y1 = dots[0]->y, y2 = dots[1]->y;
+                        int dx = x2-x1, dy = y2-y1;
+                        double d = sqrt((double)dx*dx + (double)dy*dy);
+
+                        xt[1] = (x1+x2)/2.0;
+                        yt[1] = (y1+y2)/2.0;
+                        /* It's convenient if we have t at standard scale. */
+                        xt[0] = -dy/d;
+                        yt[0] = dx/d;
+
+                        /* Also note down half the separation between
+                         * the dots, for use in computing the circle radius. */
+                        halfsep = 0.5*d;
+                    }
+
+                    /* Find a parametric formula for r. */
+                    {
+                        int x1 = edgedot1[0]->x, x2 = edgedot2[0]->x;
+                        int y1 = edgedot1[0]->y, y2 = edgedot2[0]->y;
+                        int dx = x2-x1, dy = y2-y1;
+                        double d = sqrt((double)dx*dx + (double)dy*dy);
+                        rt[0] = (xt[0]*dy - yt[0]*dx) / d;
+                        rt[1] = ((xt[1]-x1)*dy - (yt[1]-y1)*dx) / d;
+                    }
+
+                    /* Construct the quadratic equation. */
+                    q[0] = rt[0]*rt[0];
+                    q[1] = 2*rt[0]*rt[1];
+                    q[2] = rt[1]*rt[1];
+                    q[0] -= 1;
+                    q[2] -= halfsep*halfsep;
+
+                    /* And solve it. */
+                    disc = q[1]*q[1] - 4*q[0]*q[2];
+                    if (disc >= 0) {
+                        double t;
+
+                        disc = sqrt(disc);
+
+                        t = (-q[1] + disc) / (2*q[0]);
+                        cx[cn] = xt[0]*t + xt[1];
+                        cy[cn] = yt[0]*t + yt[1];
+                        cn++;
+
+                        t = (-q[1] - disc) / (2*q[0]);
+                        cx[cn] = xt[0]*t + xt[1];
+                        cy[cn] = yt[0]*t + yt[1];
+                        cn++;
+                    }
+                } else if (nedges == 0) {
+                    /*
+                     * Three dots. This is another linear matrix
+                     * equation, this time with each row of the matrix
+                     * representing the perpendicular bisector between
+                     * two of the points. Of course we only need two
+                     * such lines to find their intersection, so we
+                     * need only solve a 2x2 matrix equation.
+                     */
+
+                    double matrix[4], vector[2], vector2[2];
+                    int m;
+
+                    for (m = 0; m < 2; m++) {
+                        int x1 = dots[m]->x, x2 = dots[m+1]->x;
+                        int y1 = dots[m]->y, y2 = dots[m+1]->y;
+                        int dx = x2-x1, dy = y2-y1;
+
+                        /*
+                         * ((x,y) - (x1,y1)) . (dx,dy) = 1/2 |(dx,dy)|^2
+                         *
+                         * => 2x dx + 2y dy = dx^2+dy^2 + (2 x1 dx + 2 y1 dy)
+                         */
+                        matrix[2*m+0] = 2*dx;
+                        matrix[2*m+1] = 2*dy;
+                        vector[m] = ((double)dx*dx + (double)dy*dy +
+                                     2.0*x1*dx + 2.0*y1*dy);
+                    }
+
+                    if (solve_2x2_matrix(matrix, vector, vector2)) {
+                        cx[cn] = vector2[0];
+                        cy[cn] = vector2[1];
+                        cn++;
+                    }
+                }
+
+                /*
+                 * Now go through our candidate points and see if any
+                 * of them are better than what we've got so far.
+                 */
+                for (m = 0; m < cn; m++) {
+                    double x = cx[m], y = cy[m];
+
+                    /*
+                     * First, disqualify the point if it's not inside
+                     * the polygon, which we work out by counting the
+                     * edges to the right of the point. (For
+                     * tiebreaking purposes when edges start or end on
+                     * our y-coordinate or go right through it, we
+                     * consider our point to be offset by a small
+                     * _positive_ epsilon in both the x- and
+                     * y-direction.)
+                     */
+                    int e, in = 0;
+                    for (e = 0; e < f->order; e++) {
+                        int xs = f->edges[e]->dot1->x;
+                        int xe = f->edges[e]->dot2->x;
+                        int ys = f->edges[e]->dot1->y;
+                        int ye = f->edges[e]->dot2->y;
+                        if ((y >= ys && y < ye) || (y >= ye && y < ys)) {
+                            /*
+                             * The line goes past our y-position. Now we need
+                             * to know if its x-coordinate when it does so is
+                             * to our right.
+                             *
+                             * The x-coordinate in question is mathematically
+                             * (y - ys) * (xe - xs) / (ye - ys), and we want
+                             * to know whether (x - xs) >= that. Of course we
+                             * avoid the division, so we can work in integers;
+                             * to do this we must multiply both sides of the
+                             * inequality by ye - ys, which means we must
+                             * first check that's not negative.
+                             */
+                            int num = xe - xs, denom = ye - ys;
+                            if (denom < 0) {
+                                num = -num;
+                                denom = -denom;
+                            }
+                            if ((x - xs) * denom >= (y - ys) * num)
+                                in ^= 1;
+                        }
+                    }
+
+                    if (in) {
+                        double mindist = HUGE_VAL;
+                        int e, d;
+
+                        /*
+                         * This point is inside the polygon, so now we check
+                         * its minimum distance to every edge and corner.
+                         * First the corners ...
+                         */
+                        for (d = 0; d < f->order; d++) {
+                            int xp = f->dots[d]->x;
+                            int yp = f->dots[d]->y;
+                            double dx = x - xp, dy = y - yp;
+                            double dist = dx*dx + dy*dy;
+                            if (mindist > dist)
+                                mindist = dist;
+                        }
+
+                        /*
+                         * ... and now also check the perpendicular distance
+                         * to every edge, if the perpendicular lies between
+                         * the edge's endpoints.
+                         */
+                        for (e = 0; e < f->order; e++) {
+                            int xs = f->edges[e]->dot1->x;
+                            int xe = f->edges[e]->dot2->x;
+                            int ys = f->edges[e]->dot1->y;
+                            int ye = f->edges[e]->dot2->y;
+
+                            /*
+                             * If s and e are our endpoints, and p our
+                             * candidate circle centre, the foot of a
+                             * perpendicular from p to the line se lies
+                             * between s and e if and only if (p-s).(e-s) lies
+                             * strictly between 0 and (e-s).(e-s).
+                             */
+                            int edx = xe - xs, edy = ye - ys;
+                            double pdx = x - xs, pdy = y - ys;
+                            double pde = pdx * edx + pdy * edy;
+                            long ede = (long)edx * edx + (long)edy * edy;
+                            if (0 < pde && pde < ede) {
+                                /*
+                                 * Yes, the nearest point on this edge is
+                                 * closer than either endpoint, so we must
+                                 * take it into account by measuring the
+                                 * perpendicular distance to the edge and
+                                 * checking its square against mindist.
+                                 */
+
+                                double pdre = pdx * edy - pdy * edx;
+                                double sqlen = pdre * pdre / ede;
+
+                                if (mindist > sqlen)
+                                    mindist = sqlen;
+                            }
+                        }
+
+                        /*
+                         * Right. Now we know the biggest circle around this
+                         * point, so we can check it against bestdist.
+                         */
+                        if (bestdist < mindist) {
+                            bestdist = mindist;
+                            xbest = x;
+                            ybest = y;
+                        }
+                    }
+                }
+
+                if (k < f->order)
+                    nedges--;
+                else
+                    ndots--;
+            }
+            if (j < f->order)
+                nedges--;
+            else
+                ndots--;
+        }
+        if (i < f->order)
+            nedges--;
+        else
+            ndots--;
     }
-    sx /= f->order;
-    sy /= f->order;
 
-    /* convert to screen coordinates */
-    grid_to_screen(ds, g, sx, sy, x, y);
+    assert(bestdist > 0);
+
+    /* convert to screen coordinates. Round doubles to nearest. */
+    grid_to_screen(ds, g, xbest+0.5, ybest+0.5,
+                   &ds->textx[faceindex], &ds->texty[faceindex]);
+
+    *xret = ds->textx[faceindex];
+    *yret = ds->texty[faceindex];
 }
 
-static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
-                        game_state *state, int dir, game_ui *ui,
-                        float animtime, float flashtime)
+static void face_text_bbox(game_drawstate *ds, grid *g, grid_face *f,
+                           int *x, int *y, int *w, int *h)
 {
-    grid *g = state->game_grid;
-    int border = BORDER(ds->tilesize);
-    int i, n;
-    char c[2];
-    int line_colour, flash_changed;
-    int clue_mistake;
-    int clue_satisfied;
+    int xx, yy;
+    face_text_pos(ds, g, f, &xx, &yy);
 
-    if (!ds->started) {
-        /*
-         * The initial contents of the window are not guaranteed and
-         * can vary with front ends. To be on the safe side, all games
-         * should start by drawing a big background-colour rectangle
-         * covering the whole window.
-         */
-        int grid_width = g->highest_x - g->lowest_x;
-        int grid_height = g->highest_y - g->lowest_y;
-        int w = grid_width * ds->tilesize / g->tilesize;
-        int h = grid_height * ds->tilesize / g->tilesize;
-        draw_rect(dr, 0, 0, w + 2 * border + 1, h + 2 * border + 1,
-                  COL_BACKGROUND);
+    /* There seems to be a certain amount of trial-and-error involved
+     * in working out the correct bounding-box for the text. */
 
-        /* Draw clues */
-        for (i = 0; i < g->num_faces; i++) {
-           grid_face *f;
-            int x, y;
+    *x = xx - ds->tilesize/4 - 1;
+    *y = yy - ds->tilesize/4 - 3;
+    *w = ds->tilesize/2 + 2;
+    *h = ds->tilesize/2 + 5;
+}
 
-            c[0] = CLUE2CHAR(state->clues[i]);
-            c[1] = '\0';
-            f = g->faces + i;
-            face_text_pos(ds, g, f, &x, &y);
-            draw_text(dr, x, y, FONT_VARIABLE, ds->tilesize/2,
-                      ALIGN_VCENTRE | ALIGN_HCENTRE, COL_FOREGROUND, c);
-        }
-        draw_update(dr, 0, 0, w + 2 * border, h + 2 * border);
+static void game_redraw_clue(drawing *dr, game_drawstate *ds,
+                            game_state *state, int i)
+{
+    grid *g = state->game_grid;
+    grid_face *f = g->faces + i;
+    int x, y;
+    char c[3];
+
+    if (state->clues[i] < 10) {
+        c[0] = CLUE2CHAR(state->clues[i]);
+        c[1] = '\0';
+    } else {
+        sprintf(c, "%d", state->clues[i]);
     }
 
-    if (flashtime > 0 &&
-        (flashtime <= FLASH_TIME/3 ||
-         flashtime >= FLASH_TIME*2/3)) {
-        flash_changed = !ds->flashing;
-        ds->flashing = TRUE;
+    face_text_pos(ds, g, f, &x, &y);
+    draw_text(dr, x, y,
+             FONT_VARIABLE, ds->tilesize/2,
+             ALIGN_VCENTRE | ALIGN_HCENTRE,
+             ds->clue_error[i] ? COL_MISTAKE :
+             ds->clue_satisfied[i] ? COL_SATISFIED : COL_FOREGROUND, c);
+}
+
+static void edge_bbox(game_drawstate *ds, grid *g, grid_edge *e,
+                      int *x, int *y, int *w, int *h)
+{
+    int x1 = e->dot1->x;
+    int y1 = e->dot1->y;
+    int x2 = e->dot2->x;
+    int y2 = e->dot2->y;
+    int xmin, xmax, ymin, ymax;
+
+    grid_to_screen(ds, g, x1, y1, &x1, &y1);
+    grid_to_screen(ds, g, x2, y2, &x2, &y2);
+    /* Allow extra margin for dots, and thickness of lines */
+    xmin = min(x1, x2) - 2;
+    xmax = max(x1, x2) + 2;
+    ymin = min(y1, y2) - 2;
+    ymax = max(y1, y2) + 2;
+
+    *x = xmin;
+    *y = ymin;
+    *w = xmax - xmin + 1;
+    *h = ymax - ymin + 1;
+}
+
+static void dot_bbox(game_drawstate *ds, grid *g, grid_dot *d,
+                     int *x, int *y, int *w, int *h)
+{
+    int x1, y1;
+
+    grid_to_screen(ds, g, d->x, d->y, &x1, &y1);
+
+    *x = x1 - 2;
+    *y = y1 - 2;
+    *w = 5;
+    *h = 5;
+}
+
+static const int loopy_line_redraw_phases[] = {
+    COL_FAINT, COL_LINEUNKNOWN, COL_FOREGROUND, COL_HIGHLIGHT, COL_MISTAKE
+};
+#define NPHASES lenof(loopy_line_redraw_phases)
+
+static void game_redraw_line(drawing *dr, game_drawstate *ds,
+                            game_state *state, int i, int phase)
+{
+    grid *g = state->game_grid;
+    grid_edge *e = g->edges + i;
+    int x1, x2, y1, y2;
+    int xmin, ymin, xmax, ymax;
+    int line_colour;
+
+    if (state->line_errors[i])
+       line_colour = COL_MISTAKE;
+    else if (state->lines[i] == LINE_UNKNOWN)
+       line_colour = COL_LINEUNKNOWN;
+    else if (state->lines[i] == LINE_NO)
+       line_colour = COL_FAINT;
+    else if (ds->flashing)
+       line_colour = COL_HIGHLIGHT;
+    else
+       line_colour = COL_FOREGROUND;
+    if (line_colour != loopy_line_redraw_phases[phase])
+        return;
+
+    /* Convert from grid to screen coordinates */
+    grid_to_screen(ds, g, e->dot1->x, e->dot1->y, &x1, &y1);
+    grid_to_screen(ds, g, e->dot2->x, e->dot2->y, &x2, &y2);
+
+    xmin = min(x1, x2);
+    xmax = max(x1, x2);
+    ymin = min(y1, y2);
+    ymax = max(y1, y2);
+
+    if (line_colour == COL_FAINT) {
+       static int draw_faint_lines = -1;
+       if (draw_faint_lines < 0) {
+           char *env = getenv("LOOPY_FAINT_LINES");
+           draw_faint_lines = (!env || (env[0] == 'y' ||
+                                        env[0] == 'Y'));
+       }
+       if (draw_faint_lines)
+           draw_line(dr, x1, y1, x2, y2, line_colour);
     } else {
-        flash_changed = ds->flashing;
-        ds->flashing = FALSE;
+       draw_thick_line(dr, 3.0,
+                       x1 + 0.5, y1 + 0.5,
+                       x2 + 0.5, y2 + 0.5,
+                       line_colour);
     }
+}
 
-    /* Some platforms may perform anti-aliasing, which may prevent clean
-     * repainting of lines when the colour is changed.
-     * If a line needs to be over-drawn in a different colour, erase a
-     * bounding-box around the line, then flag all nearby objects for redraw.
+static void game_redraw_dot(drawing *dr, game_drawstate *ds,
+                           game_state *state, int i)
+{
+    grid *g = state->game_grid;
+    grid_dot *d = g->dots + i;
+    int x, y;
+
+    grid_to_screen(ds, g, d->x, d->y, &x, &y);
+    draw_circle(dr, x, y, 2, COL_FOREGROUND, COL_FOREGROUND);
+}
+
+static int boxes_intersect(int x0, int y0, int w0, int h0,
+                           int x1, int y1, int w1, int h1)
+{
+    /*
+     * Two intervals intersect iff neither is wholly on one side of
+     * the other. Two boxes intersect iff their horizontal and
+     * vertical intervals both intersect.
      */
-    if (ds->started) {
-        const char redraw_flag = (char)(1<<7);
+    return (x0 < x1+w1 && x1 < x0+w0 && y0 < y1+h1 && y1 < y0+h0);
+}
+
+static void game_redraw_in_rect(drawing *dr, game_drawstate *ds,
+                                game_state *state, int x, int y, int w, int h)
+{
+    grid *g = state->game_grid;
+    int i, phase;
+    int bx, by, bw, bh;
+
+    clip(dr, x, y, w, h);
+    draw_rect(dr, x, y, w, h, COL_BACKGROUND);
+
+    for (i = 0; i < g->num_faces; i++) {
+        if (state->clues[i] >= 0) {
+            face_text_bbox(ds, g, &g->faces[i], &bx, &by, &bw, &bh);
+            if (boxes_intersect(x, y, w, h, bx, by, bw, bh))
+                game_redraw_clue(dr, ds, state, i);
+        }
+    }
+    for (phase = 0; phase < NPHASES; phase++) {
         for (i = 0; i < g->num_edges; i++) {
-            char prev_ds = (ds->lines[i] & ~redraw_flag);
-            char new_ds = state->lines[i];
-            if (state->line_errors[i])
-                new_ds = DS_LINE_ERROR;
-
-            /* If we're changing state, AND
-             * the previous state was a coloured line */
-            if ((prev_ds != new_ds) && (prev_ds != LINE_NO)) {
-                grid_edge *e = g->edges + i;
-                int x1 = e->dot1->x;
-                int y1 = e->dot1->y;
-                int x2 = e->dot2->x;
-                int y2 = e->dot2->y;
-                int xmin, xmax, ymin, ymax;
-                int j;
-                grid_to_screen(ds, g, x1, y1, &x1, &y1);
-                grid_to_screen(ds, g, x2, y2, &x2, &y2);
-                /* Allow extra margin for dots, and thickness of lines */
-                xmin = min(x1, x2) - 2;
-                xmax = max(x1, x2) + 2;
-                ymin = min(y1, y2) - 2;
-                ymax = max(y1, y2) + 2;
-                /* For testing, I find it helpful to change COL_BACKGROUND
-                 * to COL_SATISFIED here. */
-                draw_rect(dr, xmin, ymin, xmax - xmin + 1, ymax - ymin + 1,
-                          COL_BACKGROUND);
-                draw_update(dr, xmin, ymin, xmax - xmin + 1, ymax - ymin + 1);
-
-                /* Mark nearby lines for redraw */
-                for (j = 0; j < e->dot1->order; j++)
-                    ds->lines[e->dot1->edges[j] - g->edges] |= redraw_flag;
-                for (j = 0; j < e->dot2->order; j++)
-                    ds->lines[e->dot2->edges[j] - g->edges] |= redraw_flag;
-                /* Mark nearby clues for redraw.  Use a value that is
-                 * neither TRUE nor FALSE for this. */
-                if (e->face1)
-                    ds->clue_error[e->face1 - g->faces] = 2;
-                if (e->face2)
-                    ds->clue_error[e->face2 - g->faces] = 2;
-            }
+            edge_bbox(ds, g, &g->edges[i], &bx, &by, &bw, &bh);
+            if (boxes_intersect(x, y, w, h, bx, by, bw, bh))
+                game_redraw_line(dr, ds, state, i, phase);
         }
     }
+    for (i = 0; i < g->num_dots; i++) {
+        dot_bbox(ds, g, &g->dots[i], &bx, &by, &bw, &bh);
+        if (boxes_intersect(x, y, w, h, bx, by, bw, bh))
+            game_redraw_dot(dr, ds, state, i);
+    }
 
-    /* Redraw clue colours if necessary */
-    for (i = 0; i < g->num_faces; i++) {
-        grid_face *f = g->faces + i;
-        int sides = f->order;
-        int j;
-        n = state->clues[i];
-        if (n < 0)
-            continue;
+    unclip(dr);
+    draw_update(dr, x, y, w, h);
+}
 
-        c[0] = CLUE2CHAR(n);
-        c[1] = '\0';
+static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
+                        game_state *state, int dir, game_ui *ui,
+                        float animtime, float flashtime)
+{
+#define REDRAW_OBJECTS_LIMIT 16                /* Somewhat arbitrary tradeoff */
 
-        clue_mistake = (face_order(state, i, LINE_YES) > n ||
-                        face_order(state, i, LINE_NO ) > (sides-n));
+    grid *g = state->game_grid;
+    int border = BORDER(ds->tilesize);
+    int i;
+    int flash_changed;
+    int redraw_everything = FALSE;
 
-        clue_satisfied = (face_order(state, i, LINE_YES) == n &&
-                          face_order(state, i, LINE_NO ) == (sides-n));
+    int edges[REDRAW_OBJECTS_LIMIT], nedges = 0;
+    int faces[REDRAW_OBJECTS_LIMIT], nfaces = 0;
 
-        if (clue_mistake != ds->clue_error[i]
-           || clue_satisfied != ds->clue_satisfied[i]) {
-            int x, y;
-            face_text_pos(ds, g, f, &x, &y);
-            /* There seems to be a certain amount of trial-and-error
-             * involved in working out the correct bounding-box for
-             * the text. */
-            draw_rect(dr, x - ds->tilesize/4 - 1, y - ds->tilesize/4 - 3,
-                      ds->tilesize/2 + 2, ds->tilesize/2 + 5,
-                      COL_BACKGROUND);
-            draw_text(dr, x, y,
-                      FONT_VARIABLE, ds->tilesize/2,
-                      ALIGN_VCENTRE | ALIGN_HCENTRE,
-                      clue_mistake ? COL_MISTAKE :
-                      clue_satisfied ? COL_SATISFIED : COL_FOREGROUND, c);
-            draw_update(dr, x - ds->tilesize/4 - 1, y - ds->tilesize/4 - 3,
-                        ds->tilesize/2 + 2, ds->tilesize/2 + 5);
+    /* Redrawing is somewhat involved.
+     *
+     * An update can theoretically affect an arbitrary number of edges
+     * (consider, for example, completing or breaking a cycle which doesn't
+     * satisfy all the clues -- we'll switch many edges between error and
+     * normal states).  On the other hand, redrawing the whole grid takes a
+     * while, making the game feel sluggish, and many updates are actually
+     * quite well localized.
+     *
+     * This redraw algorithm attempts to cope with both situations gracefully
+     * and correctly.  For localized changes, we set a clip rectangle, fill
+     * it with background, and then redraw (a plausible but conservative
+     * guess at) the objects which intersect the rectangle; if several
+     * objects need redrawing, we'll do them individually.  However, if lots
+     * of objects are affected, we'll just redraw everything.
+     *
+     * The reason for all of this is that it's just not safe to do the redraw
+     * piecemeal.  If you try to draw an antialiased diagonal line over
+     * itself, you get a slightly thicker antialiased diagonal line, which
+     * looks rather ugly after a while.
+     *
+     * So, we take two passes over the grid.  The first attempts to work out
+     * what needs doing, and the second actually does it.
+     */
 
-            ds->clue_error[i] = clue_mistake;
-            ds->clue_satisfied[i] = clue_satisfied;
+    if (!ds->started)
+       redraw_everything = TRUE;
+    else {
+
+       /* First, trundle through the faces. */
+       for (i = 0; i < g->num_faces; i++) {
+           grid_face *f = g->faces + i;
+           int sides = f->order;
+           int clue_mistake;
+           int clue_satisfied;
+           int n = state->clues[i];
+           if (n < 0)
+               continue;
+
+           clue_mistake = (face_order(state, i, LINE_YES) > n ||
+                           face_order(state, i, LINE_NO ) > (sides-n));
+           clue_satisfied = (face_order(state, i, LINE_YES) == n &&
+                             face_order(state, i, LINE_NO ) == (sides-n));
+
+           if (clue_mistake != ds->clue_error[i] ||
+               clue_satisfied != ds->clue_satisfied[i]) {
+               ds->clue_error[i] = clue_mistake;
+               ds->clue_satisfied[i] = clue_satisfied;
+               if (nfaces == REDRAW_OBJECTS_LIMIT)
+                   redraw_everything = TRUE;
+               else
+                   faces[nfaces++] = i;
+           }
+       }
 
-            /* Sometimes, the bounding-box encroaches into the surrounding
-             * lines (particularly if the window is resized fairly small).
-             * So redraw them. */
-            for (j = 0; j < f->order; j++)
-                ds->lines[f->edges[j] - g->edges] = -1;
-        }
+       /* Work out what the flash state needs to be. */
+       if (flashtime > 0 &&
+           (flashtime <= FLASH_TIME/3 ||
+            flashtime >= FLASH_TIME*2/3)) {
+           flash_changed = !ds->flashing;
+           ds->flashing = TRUE;
+       } else {
+           flash_changed = ds->flashing;
+           ds->flashing = FALSE;
+       }
+
+       /* Now, trundle through the edges. */
+       for (i = 0; i < g->num_edges; i++) {
+           char new_ds =
+               state->line_errors[i] ? DS_LINE_ERROR : state->lines[i];
+           if (new_ds != ds->lines[i] ||
+               (flash_changed && state->lines[i] == LINE_YES)) {
+               ds->lines[i] = new_ds;
+               if (nedges == REDRAW_OBJECTS_LIMIT)
+                   redraw_everything = TRUE;
+               else
+                   edges[nedges++] = i;
+           }
+       }
     }
 
-    /* Lines */
-    for (i = 0; i < g->num_edges; i++) {
-        grid_edge *e = g->edges + i;
-        int x1, x2, y1, y2;
-        int xmin, ymin, xmax, ymax;
-        char new_ds, need_draw;
-        new_ds = state->lines[i];
-        if (state->line_errors[i])
-            new_ds = DS_LINE_ERROR;
-        need_draw = (new_ds != ds->lines[i]) ? TRUE : FALSE;
-        if (flash_changed && (state->lines[i] == LINE_YES))
-            need_draw = TRUE;
-        if (!ds->started)
-            need_draw = TRUE; /* draw everything at the start */
-        ds->lines[i] = new_ds;
-        if (!need_draw)
-            continue;
-        if (state->line_errors[i])
-            line_colour = COL_MISTAKE;
-        else if (state->lines[i] == LINE_UNKNOWN)
-            line_colour = COL_LINEUNKNOWN;
-        else if (state->lines[i] == LINE_NO)
-            line_colour = COL_FAINT;
-        else if (ds->flashing)
-            line_colour = COL_HIGHLIGHT;
-        else
-            line_colour = COL_FOREGROUND;
+    /* Pass one is now done.  Now we do the actual drawing. */
+    if (redraw_everything) {
+        int grid_width = g->highest_x - g->lowest_x;
+        int grid_height = g->highest_y - g->lowest_y;
+        int w = grid_width * ds->tilesize / g->tilesize;
+        int h = grid_height * ds->tilesize / g->tilesize;
 
-        /* Convert from grid to screen coordinates */
-        grid_to_screen(ds, g, e->dot1->x, e->dot1->y, &x1, &y1);
-        grid_to_screen(ds, g, e->dot2->x, e->dot2->y, &x2, &y2);
+        game_redraw_in_rect(dr, ds, state,
+                            0, 0, w + 2*border + 1, h + 2*border + 1);
+    } else {
 
-        xmin = min(x1, x2);
-        xmax = max(x1, x2);
-        ymin = min(y1, y2);
-        ymax = max(y1, y2);
-
-       if (line_colour == COL_FAINT) {
-            static int draw_faint_lines = -1;
-            if (draw_faint_lines < 0) {
-               char *env = getenv("LOOPY_FAINT_LINES");
-               draw_faint_lines = (!env || (env[0] == 'y' ||
-                                            env[0] == 'Y'));
-            }
-           if (draw_faint_lines)
-               draw_line(dr, x1, y1, x2, y2, line_colour);
-       } else {
-            /* (dx, dy) points roughly from (x1, y1) to (x2, y2).
-             * The line is then "fattened" in a (roughly) perpendicular
-             * direction to create a thin rectangle. */
-            int dx = (x1 > x2) ? -1 : ((x1 < x2) ? 1 : 0);
-            int dy = (y1 > y2) ? -1 : ((y1 < y2) ? 1 : 0);
-            int points[8];
-           points[0] = x1 + dy;
-           points[1] = y1 - dx;
-           points[2] = x1 - dy;
-           points[3] = y1 + dx;
-           points[4] = x2 - dy;
-           points[5] = y2 + dx;
-           points[6] = x2 + dy;
-           points[7] = y2 - dx;
-            draw_polygon(dr, points, 4, line_colour, line_colour);
-        }
-        if (ds->started) {
-            /* Draw dots at ends of the line */
-            draw_circle(dr, x1, y1, 2, COL_FOREGROUND, COL_FOREGROUND);
-            draw_circle(dr, x2, y2, 2, COL_FOREGROUND, COL_FOREGROUND);
-        }
-        draw_update(dr, xmin-2, ymin-2, xmax - xmin + 4, ymax - ymin + 4);
-    }
-
-    /* Draw dots */
-    if (!ds->started) {
-        for (i = 0; i < g->num_dots; i++) {
-            grid_dot *d = g->dots + i;
-            int x, y;
-            grid_to_screen(ds, g, d->x, d->y, &x, &y);
-            draw_circle(dr, x, y, 2, COL_FOREGROUND, COL_FOREGROUND);
-        }
+       /* Right.  Now we roll up our sleeves. */
+
+       for (i = 0; i < nfaces; i++) {
+           grid_face *f = g->faces + faces[i];
+           int x, y, w, h;
+
+            face_text_bbox(ds, g, f, &x, &y, &w, &h);
+            game_redraw_in_rect(dr, ds, state, x, y, w, h);
+       }
+
+       for (i = 0; i < nedges; i++) {
+           grid_edge *e = g->edges + edges[i];
+            int x, y, w, h;
+
+            edge_bbox(ds, g, e, &x, &y, &w, &h);
+            game_redraw_in_rect(dr, ds, state, x, y, w, h);
+       }
     }
+
     ds->started = TRUE;
 }
 
@@ -3590,6 +4218,11 @@ static float game_flash_length(game_state *oldstate, game_state *newstate,
     return 0.0F;
 }
 
+static int game_is_solved(game_state *state)
+{
+    return state->solved;
+}
+
 static void game_print_size(game_params *params, float *x, float *y)
 {
     int pw, ph;
@@ -3716,6 +4349,7 @@ const struct game thegame = {
     game_redraw,
     game_anim_length,
     game_flash_length,
+    game_is_solved,
     TRUE, FALSE, game_print_size, game_print,
     FALSE /* wants_statusbar */,
     FALSE, game_timing_state,