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I've proved that a grid dimension of 2 is capable of irretrievably
hanging the grid generator and that there's no way to fix it.
Therefore, lower limit of 3 squares in each direction, which will
upset envelope-pushers everywhere but doesn't destroy any actually
interesting puzzles.

[originally from svn r5895]

--- a/mines.c

+++ b/mines.c

@@ -237,12 +237,22 @@

 static char *validate_params(game_params *params)

 {

-    if (params->w <= 0 && params->h <= 0)

-	return "Width and height must both be greater than zero";

-    if (params->w <= 0)

-	return "Width must be greater than zero";

-    if (params->h <= 0)

-	return "Height must be greater than zero";

+    /*

+     * Lower limit on grid size: each dimension must be at least 3.

+     * 1 is theoretically workable if rather boring, but 2 is a

+     * real problem: there is often _no_ way to generate a uniquely

+     * solvable 2xn Mines grid. You either run into two mines

+     * blocking the way and no idea what's behind them, or one mine

+     * and no way to know which of the two rows it's in. If the

+     * mine count is even you can create a soluble grid by packing

+     * all the mines at one end (so what when you hit a two-mine

+     * wall there are only as many covered squares left as there

+     * are mines); but if it's odd, you are doomed, because you

+     * _have_ to have a gap somewhere which you can't determine the

+     * position of.

+     */

+    if (params->w <= 2 || params->h <= 2)

+	return "Width and height must both be greater than two";

     if (params->n > params->w * params->h - 9)

 	return "Too many mines for grid size";