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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]

--- a/bridges.c

+++ b/bridges.c

@@ -542,9 +542,24 @@

         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",