`Fifteen' was getting the parity wrong on any size of board where

[sgt/puzzles] / fifteen.c

diff --git a/fifteen.c b/fifteen.c
index f7bcc68..04a8f75 100644 (file)
--- a/fifteen.c
+++ b/fifteen.c
@@ -80,19 +80,19 @@ config_item *game_configure(game_params *params)
     ret = snewn(3, config_item);
 
     ret[0].name = "Width";
-    ret[0].type = STRING;
+    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 = STRING;
+    ret[1].type = C_STRING;
     sprintf(buf, "%d", params->h);
     ret[1].sval = dupstr(buf);
     ret[1].ival = 0;
 
     ret[2].name = NULL;
-    ret[2].type = ENDCFG;
+    ret[2].type = C_END;
     ret[2].sval = NULL;
     ret[2].ival = 0;
 
@@ -199,15 +199,16 @@ char *new_game_seed(game_params *params)
      * Determine the required parity of the overall permutation.
      * This is the XOR of:
      * 
-     *  - The chessboard parity ((x^y)&1) of the gap square. The
-     *    bottom right, and therefore also the top left, count as
-     *    even.
+     *         - The chessboard parity ((x^y)&1) of the gap square. The
+     *           bottom right counts as even.
      * 
      *  - The parity of n. (The target permutation is 1,...,n-1,0
      *    rather than 0,...,n-1; this is a cyclic permutation of
      *    the starting point and hence is odd iff n is even.)
      */
-    parity = (X(params, gap) ^ Y(params, gap) ^ (n+1)) & 1;
+    parity = ((X(params, gap) - (params->w-1)) ^
+             (Y(params, gap) - (params->h-1)) ^
+             (n+1)) & 1;
 
     /*
      * Try the last two tiles one way round. If that fails, swap