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Add a difficulty exception to prevent Pearl spinning forever when
asked to generate a 5x5 Tricky puzzle. (Debian bug #667963)

[originally from svn r9454]

--- a/pearl.c

+++ b/pearl.c

@@ -1161,9 +1161,17 @@

 static int new_clues(game_params *params, random_state *rs,

                      char *clues, char *grid)

 {

-    int w = params->w, h = params->h;

+    int w = params->w, h = params->h, diff = params->difficulty;

     int ngen = 0, x, y, d, ret, i;

+

+    /*

+     * Difficulty exception: 5x5 Tricky is not generable (the

+     * generator will spin forever trying) and so we fudge it to Easy.

+     */

+    if (w == 5 && h == 5 && diff > DIFF_EASY)

+        diff = DIFF_EASY;

+

     while (1) {

         ngen++;

 	pearl_loopgen(w, h, grid, rs);

@@ -1245,7 +1253,7 @@

             /*

              * See if we can solve the puzzle just like this.

              */

-            ret = pearl_solve(w, h, clues, grid, params->difficulty, FALSE);

+            ret = pearl_solve(w, h, clues, grid, diff, FALSE);

             assert(ret > 0);	       /* shouldn't be inconsistent! */

             if (ret != 1)

                 continue;		       /* go round and try again */

@@ -1253,8 +1261,8 @@

             /*

              * Check this puzzle isn't too easy.

              */

-            if (params->difficulty > DIFF_EASY) {

-                ret = pearl_solve(w, h, clues, grid, params->difficulty-1, FALSE);

+            if (diff > DIFF_EASY) {

+                ret = pearl_solve(w, h, clues, grid, diff-1, FALSE);

                 assert(ret > 0);

                 if (ret == 1)

                     continue; /* too easy: try again */

@@ -1321,7 +1329,7 @@

                 clue = clues[y*w+x];

                 clues[y*w+x] = 0;	       /* try removing this clue */

-                ret = pearl_solve(w, h, clues, grid, params->difficulty, FALSE);

+                ret = pearl_solve(w, h, clues, grid, diff, FALSE);

                 assert(ret > 0);

                 if (ret != 1)

                     clues[y*w+x] = clue;   /* oops, put it back again */