Fix grid generation crashes at Penrose 3x3 sizes. What seemed to be

happening was that at the point of calling grid_make_consistent, the
grid had no faces or vertices, probably because grid_trim_vigorously
had removed them all, causing grid_make_consistent to try to allocate
a negative amount of memory and die in snewn.

Fixed by detecting this case in new_desc_penrose and retrying until
generation is successful. (It wasn't happening 100% of the time, just
_most_ of the time.) The same verification step is also used in
validate_desc_penrose in case a user manages to manually construct a
set of parameters leading to this failure mode.

[originally from svn r9840]

grid.c

@@ -2631,6 +2631,8 @@ static void grid_size_penrose(int width, int height,
     *yextent = l * height;
 }
 
+static grid *grid_new_penrose(int width, int height, int which, const char *desc); /* forward reference */
+
 static char *grid_new_desc_penrose(grid_type type, int width, int height, random_state *rs)
 {
     int tilesize = PENROSE_TILESIZE, startsz, depth, xoff, yoff, aoff;
@@ -2638,10 +2640,13 @@ static char *grid_new_desc_penrose(grid_type type, int width, int height, random
     int inner_radius;
     char gd[255];
     int which = (type == GRID_PENROSE_P2 ? PENROSE_P2 : PENROSE_P3);
+    grid *g;
 
-    /* We want to produce a random bit of penrose tiling, so we calculate
+    while (1) {
-     * a random offset (within the patch that penrose.c calculates for us)
+        /* We want to produce a random bit of penrose tiling, so we
-     * and an angle (multiple of 36) to rotate the patch. */
+         * calculate a random offset (within the patch that penrose.c
+         * calculates for us) and an angle (multiple of 36) to rotate
+         * the patch. */
 
         penrose_calculate_size(which, tilesize, width, height,
                                &outer_radius, &startsz, &depth);
@@ -2650,8 +2655,8 @@ static char *grid_new_desc_penrose(grid_type type, int width, int height, random
          * radius calculated. */
         inner_radius = (int)(outer_radius - sqrt(width*width + height*height));
 
-    /* Pick a random offset (the easy way: choose within outer square,
+        /* Pick a random offset (the easy way: choose within outer
-     * discarding while it's outside the circle) */
+         * square, discarding while it's outside the circle) */
         do {
             xoff = random_upto(rs, 2*inner_radius) - inner_radius;
             yoff = random_upto(rs, 2*inner_radius) - inner_radius;
@@ -2665,6 +2670,19 @@ static char *grid_new_desc_penrose(grid_type type, int width, int height, random
 
         sprintf(gd, "G%d,%d,%d", xoff, yoff, aoff);
 
+        /*
+         * Now test-generate our grid, to make sure it actually
+         * produces something.
+         */
+        g = grid_new_penrose(width, height, which, gd);
+        if (g) {
+            grid_free(g);
+            break;
+        }
+        /* If not, go back to the top of this while loop and try again
+         * with a different random offset. */
+    }
+
     return dupstr(gd);
 }
 
@@ -2674,6 +2692,7 @@ static char *grid_validate_desc_penrose(grid_type type, int width, int height,
     int tilesize = PENROSE_TILESIZE, startsz, depth, xoff, yoff, aoff, inner_radius;
     double outer_radius;
     int which = (type == GRID_PENROSE_P2 ? PENROSE_P2 : PENROSE_P3);
+    grid *g;
 
     if (!desc)
         return "Missing grid description string.";
@@ -2690,6 +2709,15 @@ static char *grid_validate_desc_penrose(grid_type type, int width, int height,
     if ((aoff % 36) != 0 || aoff < 0 || aoff >= 360)
         return "Angle offset out of bounds.";
 
+    /*
+     * Test-generate to ensure these parameters don't end us up with
+     * no grid at all.
+     */
+    g = grid_new_penrose(width, height, which, desc);
+    if (!g)
+        return "Patch coordinates do not identify a usable grid fragment";
+    grid_free(g);
+
     return NULL;
 }
 
@@ -2763,7 +2791,22 @@ static grid *grid_new_penrose(int width, int height, int which, const char *desc
     debug(("penrose: %d faces total (equivalent to %d wide by %d high)",
            g->num_faces, g->num_faces/height, g->num_faces/width));
 
+    /*
+     * Return NULL if we ended up with an empty grid, either because
+     * the initial generation was over too small a rectangle to
+     * encompass any face or because grid_trim_vigorously ended up
+     * removing absolutely everything.
+     */
+    if (g->num_faces == 0 || g->num_dots == 0) {
+        grid_free(g);
+        return NULL;
+    }
     grid_trim_vigorously(g);
+    if (g->num_faces == 0 || g->num_dots == 0) {
+        grid_free(g);
+        return NULL;
+    }
+
     grid_make_consistent(g);
 
     /*