Error highlighting bug in Bridges: when counting the number of extra

bridges that could go to an island, the game was not correctly
accounting for existing bridges in all circumstances.

(E.g. in 7x7m2:2a6a2a1g4a6c4i1a1h23c2b, connect the top right 1 to
the 2 left of it, and the 6 left of that correctly lights up red.
But now connect the 6 to the 6 below it, and it wrongly unlights
again.)

[originally from svn r8905]

diff --git a/bridges.c b/bridges.c
index 5bb73f468d63c9d37d46f88e8d2d15025af608b1..4219df4c0d07a22d083d2de2ce2170c3a35dee56 100644 (file)
--- a/bridges.c
+++ b/bridges.c
@@ -542,9 +542,24 @@ static int island_impossible(struct island *is, int strict)
         assert(is_orth);
 
         ifree = is_orth->count - island_countbridges(is_orth);
-        if (ifree > 0)
-            nsurrspc += min(ifree, MAXIMUM(is->state, dx,
-                                           is->adj.points[i].x, is->adj.points[i].y));
+        if (ifree > 0) {
+           /*
+            * ifree is the number of bridges unfilled in the other
+            * island, which is clearly an upper bound on the number
+            * of extra bridges this island may run to it.
+            *
+            * Another upper bound is the number of bridges unfilled
+            * on the specific line between here and there. We must
+            * take the minimum of both.
+            */
+           int bmax = MAXIMUM(is->state, dx,
+                              is->adj.points[i].x, is->adj.points[i].y);
+           int bcurr = GRIDCOUNT(is->state,
+                                 is->adj.points[i].x, is->adj.points[i].y,
+                                 dx ? G_LINEH : G_LINEV);
+           assert(bcurr <= bmax);
+            nsurrspc += min(ifree, bmax - bcurr);
+       }
     }
     if (nsurrspc < nspc) {
         debug(("island at (%d,%d) impossible: surr. islands %d spc, need %d.\n",