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Lambros points out that trying to generate a 3x3 Cairo Easy grid
spins forever, but observes that raising the limit to 4x4 across all
grid types is not good for the complex grids like great-hexagonal.
Introduce per-grid minimum sizes using mad macro trickery.

(In fact, for each grid type I've put in a minimum size which _both_
dimensions must equal or exceed, plus another minimum size which _at
least one_ must equal or exceed; that permits both 3x4 and 4x3 Cairo
while disallowing 3x3.)

[originally from svn r8191]

--- a/loopy.c

+++ b/loopy.c

@@ -228,22 +228,30 @@

 /* ------- List of grid generators ------- */

 #define GRIDLIST(A) \

-    A(Squares,grid_new_square) \

-    A(Triangular,grid_new_triangular) \

-    A(Honeycomb,grid_new_honeycomb) \

-    A(Snub-Square,grid_new_snubsquare) \

-    A(Cairo,grid_new_cairo) \

-    A(Great-Hexagonal,grid_new_greathexagonal) \

-    A(Octagonal,grid_new_octagonal) \

-    A(Kites,grid_new_kites)

+    A(Squares,grid_new_square,3,3) \

+    A(Triangular,grid_new_triangular,3,3) \

+    A(Honeycomb,grid_new_honeycomb,3,3) \

+    A(Snub-Square,grid_new_snubsquare,3,3) \

+    A(Cairo,grid_new_cairo,3,4) \

+    A(Great-Hexagonal,grid_new_greathexagonal,3,3) \

+    A(Octagonal,grid_new_octagonal,3,3) \

+    A(Kites,grid_new_kites,3,3)

-#define GRID_NAME(title,fn) #title,

-#define GRID_CONFIG(title,fn) ":" #title

-#define GRID_FN(title,fn) &fn,

+#define GRID_NAME(title,fn,amin,omin) #title,

+#define GRID_CONFIG(title,fn,amin,omin) ":" #title

+#define GRID_FN(title,fn,amin,omin) &fn,

+#define GRID_SIZES(title,fn,amin,omin) \

+    {amin, omin, \

+     "Width and height for this grid type must both be at least " #amin, \

+     "At least one of width and height for this grid type must be at least " #omin,},

 static char const *const gridnames[] = { GRIDLIST(GRID_NAME) };

 #define GRID_CONFIGS GRIDLIST(GRID_CONFIG)

 static grid * (*(grid_fns[]))(int w, int h) = { GRIDLIST(GRID_FN) };

 #define NUM_GRID_TYPES (sizeof(grid_fns) / sizeof(grid_fns[0]))

+static const struct {

+    int amin, omin;

+    char *aerr, *oerr;

+} grid_size_limits[] = { GRIDLIST(GRID_SIZES) };

 /* Generates a (dynamically allocated) new grid, according to the

  * type and size requested in params.  Does nothing if the grid is already

@@ -619,10 +627,14 @@

 static char *validate_params(game_params *params, int full)

 {

-    if (params->w < 3 || params->h < 3)

-        return "Width and height must both be at least 3";

     if (params->type < 0 || params->type >= NUM_GRID_TYPES)

         return "Illegal grid type";

+    if (params->w < grid_size_limits[params->type].amin ||

+	params->h < grid_size_limits[params->type].amin)

+        return grid_size_limits[params->type].aerr;

+    if (params->w < grid_size_limits[params->type].omin &&

+	params->h < grid_size_limits[params->type].omin)

+        return grid_size_limits[params->type].oerr;

     /*

      * This shouldn't be able to happen at all, since decode_params