--- a/map.c	Sun Jan 09 16:02:06 2005 +0000
+++ b/map.c	Sun Jan 09 17:55:11 2005 +0000
@@ -18,6 +18,42 @@
 uint16 _map2                [MAP_SIZE];
 byte   _map_extra_bits      [MAP_SIZE / 4];
 
+
+#ifdef _DEBUG
+TileIndex TileAdd(TileIndex tile, TileIndexDiff add,
+	const char *exp, const char *file, int line)
+{
+	int dx;
+	int dy;
+	uint x;
+	uint y;
+
+	dx = add & MapMaxX();
+	if (dx >= MapSizeX() / 2) dx -= MapSizeX();
+	dy = (add - dx) / (int)MapSizeX();
+
+	x = TileX(tile) + dx;
+	y = TileY(tile) + dy;
+
+	if (x >= MapSizeX() || y >= MapSizeY()) {
+		char buf[512];
+
+		sprintf(buf, "TILE_ADD(%s) when adding 0x%.4X and 0x%.4X failed",
+			exp, tile, add);
+#if !defined(_MSC_VER)
+		fprintf(stderr, "%s:%d %s\n", file, line, buf);
+#else
+		_assert(buf, (char*)file, line);
+#endif
+	}
+
+	assert(TILE_XY(x,y) == TILE_MASK(tile + add));
+
+	return TILE_XY(x,y);
+}
+#endif
+
+
 const TileIndexDiffC _tileoffs_by_dir[] = {
 	{-1,  0},
 	{ 0,  1},