* 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