local UPDATE_INTERVAL =
	tonumber(minetest.settings:get("large_slugs_update_interval")) or 5
local UPDATE_CHANCE =
	tonumber(minetest.settings:get("large_slugs_update_chance")) or 5
local BIRTH_CHANCE =
	tonumber(minetest.settings:get("large_slugs_birth_chance")) or 5
local BIRTH_SPACE =
	tonumber(minetest.settings:get("large_slugs_birth_space")) or 11

-- With the given vectors, computes a = b + c.
local function set_add(a, b, c)
	a.x = b.x + c.x
	a.y = b.y + c.y
	a.z = b.z + c.z
end

-- A map of wallmounted directions to their perpendicular directions.
local WALLMOUNT_TO_PERP_WALLMOUNTS = {}
WALLMOUNT_TO_PERP_WALLMOUNTS[0] = {4, 2, 5, 3}
WALLMOUNT_TO_PERP_WALLMOUNTS[2] = {4, 0, 5, 1}
WALLMOUNT_TO_PERP_WALLMOUNTS[4] = {2, 0, 3, 1}
WALLMOUNT_TO_PERP_WALLMOUNTS[1] = WALLMOUNT_TO_PERP_WALLMOUNTS[0]
WALLMOUNT_TO_PERP_WALLMOUNTS[3] = WALLMOUNT_TO_PERP_WALLMOUNTS[2]
WALLMOUNT_TO_PERP_WALLMOUNTS[5] = WALLMOUNT_TO_PERP_WALLMOUNTS[4]

-- A map of wallmounted directions to their opposites.
local WALLMOUNT_TO_OPP_WALLMOUNT = {
	[0] = 1, [1] = 0, [2] = 3, [3] = 2, [4] = 5, [5] = 4,
}

-- The numbers added to movement added wallmounted values to identify the type
-- of movement:
-- 1. Shifting to an adjacent node without rotation.
-- 2. Rotating to attach to a different node around the same position.
-- 3. Rotating around the node to which the slug is attached.
local SHIFT = 32
local ROTATE = 64
local SHIFT_ROTATE = 96

-- These are kept around to save a few allocations, probably negligible.
local opts = {}
local check_pos = {}

-- Update a slug, randomly trying to either move it or have it give birth.
local function update_slug(pos, node)
	local def = large_slugs.registered_slugs[node.name]
	if not def then return end

	local old_wallmount = node.param2
	local old_dir = minetest.wallmounted_to_dir(old_wallmount)
	if not old_dir then return end

	-- Check that the slug can move on its current surface:
	set_add(check_pos, pos, old_dir)
	local node_under = minetest.get_node(check_pos)
	if not def.ground[node_under.name] then return end

	-- Determine whether this is a move or a birth (births require an empty
	-- area around the parent):
	local move = math.random(BIRTH_CHANCE) < BIRTH_CHANCE or
		minetest.find_node_near(pos, BIRTH_SPACE, node.name) ~= nil

	-- Scan for options, looking in directions perpendicular to the current
	-- wallmounted direction of the node.
	local n_opts = 0
	local perp_wallmounts = WALLMOUNT_TO_PERP_WALLMOUNTS[old_wallmount]
	for _, perp_wallmount in ipairs(perp_wallmounts) do
		set_add(check_pos,
			pos, minetest.wallmounted_to_dir(perp_wallmount))
		local check_node = minetest.get_node(check_pos)
		if move and def.ground[check_node.name] then
			n_opts = n_opts + 1
			opts[n_opts] = perp_wallmount + ROTATE
		elseif check_node.name == "air" then
			set_add(check_pos, check_pos, old_dir)
			check_node = minetest.get_node(check_pos)
			if def.ground[check_node.name] then
				n_opts = n_opts + 1
				opts[n_opts] = perp_wallmount + SHIFT
			elseif check_node.name == "air" then
				n_opts = n_opts + 1
				opts[n_opts] = perp_wallmount + SHIFT_ROTATE
			end
		end
	end

	if n_opts < 1 then return end

	-- Choose one of the options and move in the way it specifies.
	local chosen = opts[math.random(n_opts)]
	local new_wallmount = chosen % 8
	local chosen_type = chosen - new_wallmount
	if chosen_type == SHIFT then
		if move then minetest.remove_node(pos) end
		set_add(pos, pos, minetest.wallmounted_to_dir(new_wallmount))
		minetest.set_node(pos, node)
	elseif chosen_type == ROTATE then
		node.param2 = new_wallmount
		minetest.swap_node(pos, node)
	else -- SHIFT_ROTATE
		if move then minetest.remove_node(pos) end
		set_add(pos, pos, minetest.wallmounted_to_dir(new_wallmount))
		set_add(pos, pos, old_dir)
		node.param2 = WALLMOUNT_TO_OPP_WALLMOUNT[new_wallmount]
		minetest.set_node(pos, node)
	end
end

minetest.register_abm({
	label = "Slugs updating",
	nodenames = "group:large_slug",
	interval = UPDATE_INTERVAL,
	chance = UPDATE_CHANCE,
	catch_up = false,
	action = update_slug,
})