blockgame/mods/bg_tree/grow_sapling.lua

local modname = minetest.get_current_modname()

-- NODE NAMES

local sapling = modname .. ":sapling"
local log_alive = modname .. ":log_alive"
local leaves_growing = modname .. ":leaves_growing"



local function supports_sapling (name)
	if name == log_alive or name == root_alive then return true end
	return minetest.get_item_group(name, "dirty") > 0
end

-- this commented-out function will be used when roots start affecting tree growth.
--[[
local function find_first_root (pos, max_height = 8)
	local pos = vector.copy(pos)
	local height = 0
	while height < max_height do
		height = height + 1

		local nodename = minetest.get_node(pos).name

		if nodename == modname .. ":root_alive" then return pos end

		if not blockgame.item_matches(nodename, {
			modname .. ":sapling",
			modname .. ":log_alive",
		}) then return end

		pos = pos + blockgame.vector.dirs.down
	end
end
]]--

blockgame.register_increasing_abm({
	id = modname .. ":grow_sapling",
	label = "grow sapling",
	nodenames = {sapling},
	neighbors = {"group:dirty", log_alive},
	interval = 15,
	chance = 4,
	rate = function (pos, node, data)
		-- TODO: find connected living logs and leaves, and nearby light levels, and soil richness near roots, to calculate growth rate.
		return data.value + math.random(1, 99)
	end,
	check = function (pos, node, data)
		if data.value < 200 then return false end

		local below = pos + blockgame.vector.dirs.down
		local below_name = minetest.get_node(below).name
		if not supports_sapling(below_name) then return false end

		local above = pos + blockgame.vector.dirs.up
		if minetest.get_node(above).name ~= "air" then return false end

		return true
	end,
	action = function (pos, node, data)
		-- TODO: decrease energy (`data.value`) when growing, but don't completely reset it to 0.
		-- (this will require the api adding support for increasing_abm actions modifying their data.)
		local above = pos + blockgame.vector.dirs.up

		-- grow up, with a chance to stop growing.
		if not blockgame.chance(4) then
			minetest.set_node(above, {name = sapling})
		end

		local below = pos + blockgame.vector.dirs.down
		-- if this tree have already grown at least once, create a log...
		if blockgame.item_matches(minetest.get_node(below).name, {
			modname .. ":root_alive",
			modname .. ":log_alive",
		}) then
			minetest.set_node(pos, {name = log_alive})
		else
			-- ... otherwise, create a root
			minetest.set_node(pos, {name = modname .. ":root_alive"})
		end

		-- grow out leaves to the sides.
		local sides = blockgame.vector.get_sides_of(pos)
		for _, side in pairs(sides) do
			blockgame.attempt_place(side, {name = leaves_growing})
		end
	end,
})