blockgame/mods/bg_terrain/grass_grow.lua

-- TODO: move these to the general api.
local function listify_table (tab)
	local copy = {}
	for _, item in pairs(tab) do
		table.insert(copy, item)
	end
	return copy
end
local function shuffle (tab)
	tab = listify_table(tab)

	local shuffled = {}

	repeat
		local index = math.random(1, #tab)
		local value = tab[index]
		table.insert(shuffled, value)
		table.remove(tab, index)
	until #tab == 0

	return shuffled
end

blockgame.register_abm({
	nodenames = {"bg_terrain:grass"},
	neighbors = {"bg_terrain:dirt"},
	interval = 30,
	chance = 8,
	action = function (pos, node)
		local sides = listify_table(blockgame.vector.sides)
		local diagonally = {}

		for _, side in pairs(sides) do
			table.insert(diagonally, side + blockgame.vector.dirs.down)
			table.insert(diagonally, side + blockgame.vector.dirs.up)
		end
		sides = shuffle(sides)
		diagonally = shuffle(diagonally)

		for _, offset in pairs(sides) do
			if attempt_spread(pos + offset) then return true end
		end
		for _, offset in pairs(diagonally) do
			if attempt_spread(pos + offset) then return true end
		end
		return false
	end,
})

function attempt_spread (pos)
	local name = minetest.get_node(pos).name

	if not blockgame.item_matches(name, {"group:grassable"}) then return false end

	if not blockgame.air_flows_through(pos + blockgame.vector.dirs.up) then return false end

	minetest.set_node(pos, {name = "bg_terrain:grass"})
	return true
end