Initial work on leaf composting.

- Leaves left in the sun decay to nothing.
- Leaves deprived of air decay into dirt.

TODO:
- Decay out of stack nodes, half-life qty?
- Dig particle effects for node destruction; create API and
  apply to other places?

mods/nc_tree/compost.lua

@@ -0,0 +1,50 @@
+-- LUALOCALS < ---------------------------------------------------------
+local minetest, nodecore, pairs
+    = minetest, nodecore, pairs
+-- LUALOCALS > ---------------------------------------------------------
+
+local modname = minetest.get_current_modname()
+
+local airnames = {}
+minetest.after(0, function()
+		for k, v in pairs(minetest.registered_nodes) do
+			if v.air_equivalent then
+				airnames[#airnames + 1] = k
+			end
+		end
+	end)
+
+local function addto(meta, key, max, func)
+	local qty = (meta:get_float(key) or 0) + 1
+	if qty >= max then return func() end
+	return meta:set_float(key, qty)
+end
+
+nodecore.register_limited_abm({
+		label = "Fallen Leaf Composting",
+		interval = 10,
+		chance = 2,
+		nodenames = {modname .. ":leaves_loose"},
+		neighbors = {"group:soil"},
+		action = function(pos)
+			local meta = minetest.get_meta(pos)
+			local min = {x = pos.x - 1, y = pos.y - 1, z = pos.z - 1}
+			local max = {x = pos.x + 1, y = pos.y + 1, z = pos.z + 1}
+			local any
+			for _, p in pairs(minetest.find_nodes_in_area(min, max, airnames)) do
+				local nl = minetest.get_node_light(p, 0.5) or 0
+				if nl >= 15 then
+					return addto(meta, "air", 5, function()
+							nodecore.node_sound(pos, "dig")
+							return minetest.remove_node(pos)
+						end)
+				end
+				any = true
+			end
+			if any then return end
+			return addto(meta, "compost", 25, function()
+					minetest.set_node(pos, {name = "nc_terrain:dirt_loose"})
+					return nodecore.node_sound(pos, "place")
+				end)
+		end
+	})

mods/nc_tree/init.lua

@@ -6,6 +6,7 @@ local include
 include("api")
 include("node")
 include("leafdecay")
+include("compost")
 
 include("stick")