Lua Cave Mapgen: Support random chance of flipping the map

This has a similar effect to the flipx_chance and flipy_chance in the old
cave generator, except only one flip will be applied.

data/campaigns/Heir_To_The_Throne/scenarios/17_Scepter_of_Fire.cfg

@@ -12,7 +12,6 @@
         
         map_width=50
         map_height=70
-        flipx_chance=50
         village_density=20
         terrain_wall = "Xu"
         terrain_clear = "Uu"
@@ -20,6 +19,9 @@
         terrain_castle = "Cud"
         terrain_village = "Uu^Vud"
 
+        transforms=flip_x
+        transform_chance=50
+
         #
         # The chamber with the player. Somewhere in the far south
         #

data/campaigns/Sceptre_of_Fire/scenarios/4_Gathering_Materials.cfg

@@ -23,7 +23,6 @@
 
         map_width=45
         map_height=45
-        flipx_chance=0
         village_density=60
 
         terrain_wall = "Xu"

data/lua/cave_map_generator.lua

@@ -142,6 +142,22 @@ function callbacks.generate_map(params)
 
 	end
 
+	if type(params.transform) == "string" then
+		local chance = params.transform_chance or 100
+		if random(100) <= chance then
+			local transforms = {}
+			for t in params.transform:gmatch("[^%s,][^,]*") do
+				-- TODO: Possibly support other transformations, such as transpose, rotate_cw, rotate_ccw, etc
+				if t == 'flip_x' or t == 'flip_y' then
+					table.insert(transforms, t)
+				else
+					error("Unknown transformation '" .. t .. "'")
+				end
+			end
+			map[transforms[random(#transforms)]](map)
+		end
+	end
+
 	return tostring(map)
 end
 

data/lua/mapgen_helper.lua

@@ -45,6 +45,26 @@ function map_mt.__index.add_location(map, x, y, name)
 	end
 end
 
+function map_mt.__index.flip_x(map)
+	for y = 0, map.h - 1 do
+		for x = 0, map.w - 1 do
+			local i = loc_to_index(map, x, y)
+			local j = loc_to_index(map, map.w - x, y)
+			map[i], map[j] = map[j], map[i]
+		end
+	end
+end
+
+function map_mt.__index.flip_y(map)
+	for x = 0, map.w - 1 do
+		for y = 0, map.h - 1 do
+			local i = loc_to_index(map, x, y)
+			local j = loc_to_index(map, x, map.h - y)
+			map[i], map[j] = map[j], map[i]
+		end
+	end
+end
+
 function map_mt.__tostring(map)
 	local map_builder = {}
 	-- The coordinates are 0-based to match the in-game coordinates