improve leaves decomposition.

leaves will now decompose faster depending on their surroundings.
touching dirty nodes or other decomposing leaves will make them decay
quicker.

mods/bg_tree/decompose.lua

@@ -27,20 +27,57 @@ blockgame.register_increasing_abm({
 	end,
 })
 
+local decompose_node_scores = {
+	[leaves_decomposing] = 30,
+}
+local decompose_group_scores = {
+	dirty = 50,
+}
+
+local decompose_cost = 2000
+
 blockgame.register_increasing_abm({
 	id = modname .. ":decompose",
 	label = "decompose leaves",
 	nodenames = {leaves_decomposing},
-	neighbors = {"group:dirty"},
-	interval = 20,
-	chance = 8,
+	neighbors = {"group:dirty", leaves_decomposing},
+	interval = 30,
+	chance = 10,
 	rate = function (pos, node, data)
-		-- TODO: decompose faster depending on surrounding dirty nodes & other decomposing leaves.
-		-- can probably do that with a flood fill that counts up a "score" depending on the nodes it encounters.
-		return data.value + math.random(1, 9)
+		local score = 100
+
+		local source_pos = pos
+		blockgame.flood_fill(pos, function (pos, distance)
+			if pos == source_pos then return true end
+
+			local name = minetest.get_node(pos).name
+
+			local gain = 0
+			for group, value in pairs(decompose_group_scores) do
+				if minetest.get_item_group(name, group) > 0 then
+					gain = math.max(gain, value)
+				end
+			end
+
+			gain = math.floor(gain / distance)
+
+			if gain > 0 then
+				score = score + gain
+				return true
+			end
+
+			if decompose_node_scores[name] then
+				score = score + math.floor(decompose_node_scores[name] / distance)
+				return true
+			end
+
+			return false
+		end, 4)
+
+		return data.value + score
 	end,
 	check = function (pos, node, data)
-		return data.value >= 25
+		return data.value >= decompose_cost
 	end,
 	action = function (pos, node, data)
 		if blockgame.chance(2) then