Filling: remove directional bias in grid generation.

The method of generating a solved Filling grid (before winnowing
clues) is to loop over every square of the board, and for each one, if
it has a neighbour which is part of a different region of the same
size (i.e. the board is not currently legal), fix it by merging with
one of its neighbours. We pick a neighbour to merge with based on the
size of its region - but we always loop over the four possible
neighbours in the same order, which introduces a directional bias into
the breaking of ties in that comparison.

Now we iterate over the four directions in random order.

1 files changed, 7 insertions, 2 deletions

diff --git a/filling.c b/filling.c
index a62161a..ffa3ab1 100644
--- a/filling.c
+++ b/filling.c
@@ -414,10 +414,15 @@ retry:
             int merge = SENTINEL, min = maxsize - size + 1;
 	    bool error = false;
             int neighbour, neighbour_size, j;
+	    int directions[4];
+
+            for (j = 0; j < 4; ++j)
+		directions[j] = j;
+	    shuffle(directions, 4, sizeof(int), rs);
 
             for (j = 0; j < 4; ++j) {
-                const int x = (board[i] % w) + dx[j];
-                const int y = (board[i] / w) + dy[j];
+                const int x = (board[i] % w) + dx[directions[j]];
+                const int y = (board[i] / w) + dy[directions[j]];
                 if (x < 0 || x >= w || y < 0 || y >= h) continue;
 
                 neighbour = dsf_canonify(dsf, w*y + x);