Spawn smaller slime/magmacube if it doesn't fit

mods/ENTITIES/mcl_mobs/api.lua

@@ -2975,6 +2975,7 @@ minetest.register_entity(name, {
 	owner = def.owner or "",
 	order = def.order or "",
 	on_die = def.on_die,
+	spawn_small_alternative = def.spawn_small_alternative,
 	do_custom = def.do_custom,
 	jump_height = def.jump_height or 4, -- was 6
 	drawtype = def.drawtype, -- DEPRECATED, use rotate instead
@@ -3162,17 +3163,10 @@ function mobs:spawn_specific(name, nodes, neighbors, min_light, max_light,
 
 	end
 
-	minetest.register_abm({
+	local spawn_action
-
+	spawn_action = function(pos, node, active_object_count, active_object_count_wider, name)
-		label = name .. " spawning",
-		nodenames = nodes,
-		neighbors = neighbors,
-		interval = interval,
-		chance = max(1, (chance * mob_chance_multiplier)),
-		catch_up = false,
-
-		action = function(pos, node, active_object_count, active_object_count_wider)
 
+			local orig_pos = table.copy(pos)
 			-- is mob actually registered?
 			if not mobs.spawning_mobs[name]
 			or not minetest.registered_entities[name] then
@@ -3285,6 +3279,10 @@ function mobs:spawn_specific(name, nodes, neighbors, min_light, max_light,
 						if minetest.registered_nodes[node_ok(pos2).name].walkable == true then
 							-- inside block
 							minetest.log("info", "Mob spawn of "..name.." at "..minetest.pos_to_string(pos).." failed, too little space!")
+							if ent.spawn_small_alternative ~= nil and (not minetest.registered_nodes[node_ok(pos).name].walkable) then
+								minetest.log("info", "Trying to spawn smaller alternative mob: "..ent.spawn_small_alternative)
+								spawn_action(orig_pos, node, active_object_count, active_object_count_wider, ent.spawn_small_alternative)
+							end
 							return
 						end
 					end
@@ -3310,6 +3308,19 @@ function mobs:spawn_specific(name, nodes, neighbors, min_light, max_light,
 				on_spawn(ent, pos)
 			end
 	end
+
+	local function spawn_abm_action(pos, node, active_object_count, active_object_count_wider)
+		spawn_action(pos, node, active_object_count, active_object_count_wider, name)
+	end
+
+	minetest.register_abm({
+		label = name .. " spawning",
+		nodenames = nodes,
+		neighbors = neighbors,
+		interval = interval,
+		chance = max(1, (chance * mob_chance_multiplier)),
+		catch_up = false,
+		action = spawn_abm_action,
 	})
 end
 

mods/ENTITIES/mcl_mobs/api.txt

@@ -213,6 +213,8 @@ functions needed for the mob to work properly which contains the following:
       'rain_damage' damage per second if mob is standing in rain (default: 0)
       'sunlight_damage' holds the damage per second inflicted to mobs when they
                         are in direct sunlight
+      'spawn_small_alternative': name of a smaller mob to use as replacement if
+                                 spawning fails due to space requirements
 
 Node Replacement
 ----------------

mods/ENTITIES/mobs_mc/slime+magma_cube.lua

@@ -58,6 +58,7 @@ local slime_big = {
 	jump_height = 5.2,
 	jump_chance = 100,
 	fear_height = 60,
+	spawn_small_alternative = "mobs_mc:slime_small",
 	on_die = function(self, pos)
 		local angle, posadd
 		angle = math.random(0, math.pi*2)
@@ -83,6 +84,7 @@ slime_small.reach = 2.75
 slime_small.walk_velocity = 1.3
 slime_small.run_velocity = 1.3
 slime_small.jump_height = 4.3
+slime_small.spawn_small_alternative = "mobs_mc:slime_tiny"
 slime_small.on_die = function(self, pos)
 	local angle, posadd, dir
 	angle = math.random(0, math.pi*2)
@@ -114,6 +116,7 @@ slime_tiny.drops = {
 slime_tiny.walk_velocity = 0.7
 slime_tiny.run_velocity = 0.7
 slime_tiny.jump_height = 3
+slime_tiny.spawn_small_alternative = nil
 slime_tiny.on_die = nil
 
 mobs:register_mob("mobs_mc:slime_tiny", slime_tiny)
@@ -181,6 +184,7 @@ local magma_cube_big = {
 	walk_chance = 0,
 	jump_chance = 100,
 	fear_height = 100000,
+	spawn_small_alternative = "mobs_mc:magma_cube_small",
 	on_die = function(self, pos)
 		local angle, posadd
 		angle = math.random(0, math.pi*2)
@@ -211,6 +215,7 @@ magma_cube_small.jump_height = 6
 magma_cube_small.damage = 4
 magma_cube_small.reach = 2.75
 magma_cube_small.armor = 70
+magma_cube_small.spawn_small_alternative = "mobs_mc:magma_cube_tiny"
 magma_cube_small.on_die = function(self, pos)
 	local angle, posadd, dir
 	angle = math.random(0, math.pi*2)
@@ -239,6 +244,7 @@ magma_cube_tiny.damage = 3
 magma_cube_tiny.reach = 2.5
 magma_cube_tiny.armor = 85
 magma_cube_tiny.drops = {}
+magma_cube_tiny.spawn_small_alternative = nil
 magma_cube_tiny.on_die = nil
 
 mobs:register_mob("mobs_mc:magma_cube_tiny", magma_cube_tiny)