Generate pig spawners in mineshafts

In Minecraft Java Edition, when the map generator generates a spawner,
it can generates a pig spawner instead of the spawner it should create.
That behaviour is very rare, but has never been removed from Minecraft.

This patch changes 1 in 1000 spawners in a mineshaft to be a pig spawner
instead of a cave spider spawner.

mods/MAPGEN/tsm_railcorridors/gameconfig.lua

@@ -51,10 +51,23 @@ function tsm_railcorridors.get_default_treasure(pr)
 	-- UNUSED IN MINECLONE 2!
 end
 
--- All spawners spawn cave spiders
+-- Almost all spawners spawn cave spiders, but there is a 1 in 1000
-function tsm_railcorridors.on_construct_spawner(pos)
+-- chance to spawn a pig spawner instead, inspired by multiple bugs
+-- in Minecraft that turn spawners in Mineshafts into pig spawners.
+-- (Basically any spawner bug in Minecraft generates pig spawners.)
+--
+-- The Minecraft chance to get a pig spawner here is way lower than
+-- 1 in 1000, but because of the small size of Minetest worlds, the
+-- Minecraft chance would risk not having a pig spawner in a world.
+--
+-- The world with the seed “pigs” generates a pig spawner at 3,-8,7
+function tsm_railcorridors.on_construct_spawner(pos, pr)
+	if pr:next(1,1000) == 900 then
+		mcl_mobspawners.setup_spawner(pos, "mobs_mc:pig", 0, 7)
+	else
 		mcl_mobspawners.setup_spawner(pos, "mobs_mc:cave_spider", 0, 7)
 	end
+end
 
 -- MineClone 2's treasure function. Gets all treasures for a single chest.
 -- Based on information from Minecraft Wiki.

mods/MAPGEN/tsm_railcorridors/init.lua

@@ -114,7 +114,7 @@ if not tsm_railcorridors.nodes.corridor_woods_function then
 end
 
 -- Random Perlin noise generators
-local pr, pr_carts, pr_treasures, pr_deco, webperlin_major, webperlin_minor
+local pr, pr_carts, pr_spawner, pr_treasures, pr_deco, webperlin_major, webperlin_minor
 
 local function InitRandomizer(seed)
 	-- Mostly used for corridor gen.
@@ -123,6 +123,8 @@ local function InitRandomizer(seed)
 	pr_deco = PseudoRandom(seed+25)
 	-- Separate randomizer for carts because spawning carts is very timing-dependent
 	pr_carts = PseudoRandom(seed-654)
+	-- Spawner type randomiser
+	pr_spawner = PseudoRandom(seed+345)
 	-- Chest contents randomizer
 	pr_treasures = PseudoRandom(seed+777)
 	-- Used for cobweb generation, both noises have to reach a high value for cobwebs to appear
@@ -751,7 +753,7 @@ local function create_corridor_section(waypoint, axis, sign, up_or_down, up_or_d
 				if place_cobwebs then
 					Cube(p, size, {name=tsm_railcorridors.nodes.cobweb}, true)
 				end
-				tsm_railcorridors.on_construct_spawner(p)
+				tsm_railcorridors.on_construct_spawner(p, pr_spawner)
 				no_spawner = true
 			end
 		end