epic/blocks/spawn_mob.lua

local mob_names = {} -- list<name>

minetest.register_on_mods_loaded(function()
	for _,item in pairs(minetest.registered_items) do
		if item.groups.spawn_egg == 1 then
			table.insert(mob_names, item.name)
		end
	end
end)

-- playername => pos
local punch_handler = {}

local update_formspec = function(meta)
	local pos = meta:get_string("pos")
	local mobname = meta:get_string("mobname")

	meta:set_string("infotext", "Spawn mob block: pos=" .. pos ..
		" mobname: '" .. mobname .. "'")


	local selected = 1
	local list = ""
	for i,name in ipairs(mob_names) do
		if name == mobname then
			selected = i
		end

		list = list .. minetest.formspec_escape(name)
		if i < #mob_names then
			-- not end of list
			list = list .. ","
		end
	end

	meta:set_string("formspec", "size[8,7;]" ..
		"textlist[0,0.1;8,5;mobname;" .. list .. ";" .. selected .. "]" ..

		"button_exit[0.1,5.5;4,1;setpos;Set position]" ..
		"button_exit[4.1,5.5;4,1;showpos;Show]" ..

		"button_exit[0.1,6.5;8,1;save;Save]" ..
		"")
end

local function do_spawn(pos, meta)
	local target_pos = epic.to_absolute_pos(pos, minetest.string_to_pos(meta:get_string("pos")))
	local mobname = meta:get_string("mobname")

	if mobname and mobname ~= "" then
		local mapblock_pos = epic.get_mapblock_pos(target_pos)
		-- start a transient forceload
		-- entities can only operate in active mapblocks
		minetest.forceload_block(mapblock_pos, true)

		-- spawn mob and free forceload in the next server step
		minetest.after(0.1, function()
			local objs = minetest.get_objects_inside_radius(target_pos, 9)
			local count = 0
			for _ in ipairs(objs) do
				count = count + 1
			end

			if count < 16 then
				-- only spawn new entities if not too much objects in that area already
				minetest.add_entity(target_pos, mobname)
			end

			-- release forceload
			minetest.forceload_free_block(mapblock_pos, true)
		end)
	end
end

minetest.register_node("epic:spawn_mob", {
	description = "Epic Spawn mob block: spawns a mob at the given position",
	tiles = epic.create_texture("action", "epic_mob.png"),
	paramtype2 = "facedir",
	groups = {cracky=3,oddly_breakable_by_hand=3,epic=1},
	on_rotate = epic.on_rotate,

	on_construct = function(pos)
    local meta = minetest.get_meta(pos)
		meta:set_string("pos", minetest.pos_to_string({x=0, y=0, z=0}))
		meta:set_string("mobname", "")
    update_formspec(meta, pos)
  end,

  on_receive_fields = function(pos, _, fields, sender)
		if not sender or minetest.is_protected(pos, sender:get_player_name()) then
			-- not allowed
			return
		end

		local meta = minetest.get_meta(pos);

		if fields.setpos then
			minetest.chat_send_player(sender:get_player_name(), "[epic] Please punch the desired target position")
			punch_handler[sender:get_player_name()] = pos
		end

		if fields.mobname then
			local parts = fields.mobname:split(":")
			if parts[1] == "CHG" then
				local selected_mob = tonumber(parts[2])
				meta:set_string("mobname", mob_names[selected_mob])
				update_formspec(meta, pos)
			end
		end

		if fields.showpos then
			local target_pos = minetest.string_to_pos(meta:get_string("pos"))
			if target_pos then
				epic.show_waypoint(sender:get_player_name(), epic.to_absolute_pos(pos, target_pos), "Target position", 2)
			end
		end
  end,

	-- allow mesecons triggering
	mesecons = {
		effector = {
	    action_on = function (pos)
				local meta = minetest.get_meta(pos)
				do_spawn(pos, meta)
			end
	  }
	},

	epic = {
    on_enter = function(pos, meta, _, ctx)
			do_spawn(pos, meta)
			ctx.next()
    end
  }
})

minetest.register_on_punchnode(function(pos, _, puncher, _)
	local playername = puncher:get_player_name()
	local cfg_pos = punch_handler[playername]
	if cfg_pos then
		if minetest.is_protected(pos, playername) and
			not minetest.check_player_privs(playername, {epic_admin=true}) then
			minetest.chat_send_player(playername, "[epic] target is protected! aborting selection.")

		else
			local meta = minetest.get_meta(cfg_pos)
			local pos_str = minetest.pos_to_string(vector.add(epic.to_relative_pos(cfg_pos, pos), {x=0, y=1.5, z=0}))
			meta:set_string("pos", pos_str)
			minetest.chat_send_player(playername, "[epic] target position successfully set to " .. pos_str)
			update_formspec(meta)

		end
		punch_handler[playername] = nil
	end
end)

minetest.register_on_leaveplayer(function(player)
	local playername = player:get_player_name()
	punch_handler[playername] = nil

end)