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Forbid game descriptions with joined islands in Bridges

A game description with islands in adjacent grid squares, like
"3x3:11g", shouldn't be allowed.  If it is, then bridges between the
islands are invisible and clicking one of them causes an assertion
failure: "Assertion `is_loop->adj.points[j].off > 1' failed."

The code to check this is really rather complex, but I think the
complexity is mostly necessary.

--- a/bridges.c

+++ b/bridges.c

@@ -2007,21 +2007,34 @@

 static const char *validate_desc(const game_params *params, const char *desc)

 {

-    int i, wh = params->w * params->h, nislands = 0;

+    int i, j, wh = params->w * params->h, nislands = 0;

+    bool *last_row = snewn(params->w, bool);

+    memset(last_row, 0, params->w * sizeof(bool));

     for (i = 0; i < wh; i++) {

-        if (*desc >= '1' && *desc <= '9')

+        if ((*desc >= '1' && *desc <= '9') || (*desc >= 'A' && *desc <= 'G')) {

             nislands++;

-        else if (*desc >= 'a' && *desc <= 'z')

+            /* Look for other islands to the left and above. */

+            if ((i % params->w > 0 && last_row[i % params->w - 1]) ||

+                last_row[i % params->w]) {

+                sfree(last_row);

+                return "Game description contains joined islands";

+            }

+            last_row[i % params->w] = true;

+        } else if (*desc >= 'a' && *desc <= 'z') {

+            for (j = 0; j < *desc - 'a' + 1; j++)

+                last_row[(i + j) % params->w] = false;

             i += *desc - 'a'; /* plus the i++ */

-        else if (*desc >= 'A' && *desc <= 'G')

-            nislands++;

-        else if (!*desc)

+        } else if (!*desc) {

+            sfree(last_row);

             return "Game description shorter than expected";

-        else

+        } else {

+            sfree(last_row);

             return "Game description contains unexpected character";

+        }

         desc++;

     }

+    sfree(last_row);

     if (*desc || i > wh)

         return "Game description longer than expected";

     if (nislands < 2)