Convert furnace ABM to node timer

Node timers are higher precision and a better guarantee
of happening at regular intervals, whereas ABM's may be
postponed, cancelled or missed if a player is too far.

The largest benefit is that once the furnace is done
cooking, no more ABM's are fired - the timer is stopped
instead and no more events are created until items
are put in the furnace.

This patch is larger due to the migration of the timer
function and indentation change as a result of the somewhat
reduced complexity. I've tested with several furnaces and
this works correctly and behavior is not affected, although
people may find that their furnaces now work more
regularly.

If you place several furnaces next to eachother, you will
still find all furnace timers firing exactly at the same
time. This is a bug in core that should not coalesce node
timers at second intervals.

mods/default/furnace.lua

@@ -90,66 +90,6 @@ local function allow_metadata_inventory_take(pos, listname, index, stack, player
 	return stack:get_count()
 end
 
---
--- Node definitions
---
-
-minetest.register_node("default:furnace", {
-	description = "Furnace",
-	tiles = {
-		"default_furnace_top.png", "default_furnace_bottom.png",
-		"default_furnace_side.png", "default_furnace_side.png",
-		"default_furnace_side.png", "default_furnace_front.png"
-	},
-	paramtype2 = "facedir",
-	groups = {cracky=2},
-	legacy_facedir_simple = true,
-	is_ground_content = false,
-	sounds = default.node_sound_stone_defaults(),
-	
-	can_dig = can_dig,
-	
-	allow_metadata_inventory_put = allow_metadata_inventory_put,
-	allow_metadata_inventory_move = allow_metadata_inventory_move,
-	allow_metadata_inventory_take = allow_metadata_inventory_take,
-})
-
-minetest.register_node("default:furnace_active", {
-	description = "Furnace",
-	tiles = {
-		"default_furnace_top.png", "default_furnace_bottom.png",
-		"default_furnace_side.png", "default_furnace_side.png",
-		"default_furnace_side.png",
-		{
-			image = "default_furnace_front_active.png",
-			backface_culling = false,
-			animation = {
-				type = "vertical_frames",
-				aspect_w = 16,
-				aspect_h = 16,
-				length = 1.5
-			},
-		}
-	},
-	paramtype2 = "facedir",
-	light_source = 8,
-	drop = "default:furnace",
-	groups = {cracky=2, not_in_creative_inventory=1},
-	legacy_facedir_simple = true,
-	is_ground_content = false,
-	sounds = default.node_sound_stone_defaults(),
-	
-	can_dig = can_dig,
-	
-	allow_metadata_inventory_put = allow_metadata_inventory_put,
-	allow_metadata_inventory_move = allow_metadata_inventory_move,
-	allow_metadata_inventory_take = allow_metadata_inventory_take,
-})
-
---
--- ABM
---
-
 local function swap_node(pos, name)
 	local node = minetest.get_node(pos)
 	if node.name == name then
@@ -159,11 +99,7 @@ local function swap_node(pos, name)
 	minetest.swap_node(pos, node)
 end
 
-minetest.register_abm({
+local function furnace_node_timer(pos, elapsed)
-	nodenames = {"default:furnace", "default:furnace_active"},
-	interval = 1.0,
-	chance = 1,
-	action = function(pos, node, active_object_count, active_object_count_wider)
 	--
 	-- Inizialize metadata
 	--
@@ -172,19 +108,7 @@ minetest.register_abm({
 	local src_time = meta:get_float("src_time") or 0
 	local fuel_totaltime = meta:get_float("fuel_totaltime") or 0
 
-		--
-		-- Inizialize inventory
-		--
 	local inv = meta:get_inventory()
-		for listname, size in pairs({
-				src = 1,
-				fuel = 1,
-				dst = 4,
-		}) do
-			if inv:get_size(listname) ~= size then
-				inv:set_size(listname, size)
-			end
-		end
 	local srclist = inv:get_list("src")
 	local fuellist = inv:get_list("fuel")
 	local dstlist = inv:get_list("dst")
@@ -235,7 +159,6 @@ minetest.register_abm({
 
 				fuel_totaltime = fuel.time
 				fuel_time = 0
-					
 			end
 		else
 			-- We don't need to get new fuel since there is no cookable item
@@ -264,17 +187,24 @@ minetest.register_abm({
 
 	local fuel_state = "Empty"
 	local active = "inactive "
+	local result = false
+
 	if fuel_time <= fuel_totaltime and fuel_totaltime ~= 0 then
 		active = "active "
 		local fuel_percent = math.floor(fuel_time / fuel_totaltime * 100)
 		fuel_state = fuel_percent .. "%"
 		formspec = active_formspec(fuel_percent, item_percent)
 		swap_node(pos, "default:furnace_active")
+		-- make sure timer restarts automatically
+		result = true
 	else
 		if not fuellist[1]:is_empty() then
 			fuel_state = "0%"
 		end
 		swap_node(pos, "default:furnace")
+		-- stop timer on the inactive furnace
+		local timer = minetest.get_node_timer(pos)
+		timer:stop()
 	end
 
 	local infotext = "Furnace " .. active .. "(Item: " .. item_state .. "; Fuel: " .. fuel_state .. ")"
@@ -287,5 +217,81 @@ minetest.register_abm({
 	meta:set_float("src_time", src_time)
 	meta:set_string("formspec", formspec)
 	meta:set_string("infotext", infotext)
+
+	return result
+end
+
+--
+-- Node definitions
+--
+
+minetest.register_node("default:furnace", {
+	description = "Furnace",
+	tiles = {
+		"default_furnace_top.png", "default_furnace_bottom.png",
+		"default_furnace_side.png", "default_furnace_side.png",
+		"default_furnace_side.png", "default_furnace_front.png"
+	},
+	paramtype2 = "facedir",
+	groups = {cracky=2},
+	legacy_facedir_simple = true,
+	is_ground_content = false,
+	sounds = default.node_sound_stone_defaults(),
+
+	can_dig = can_dig,
+
+	on_timer = furnace_node_timer,
+
+	on_construct = function(pos)
+		local meta = minetest.get_meta(pos)
+		meta:set_string("formspec", inactive_formspec)
+		local inv = meta:get_inventory()
+		inv:set_size('src', 1)
+		inv:set_size('fuel', 1)
+		inv:set_size('dst', 4)
 	end,
+
+	on_metadata_inventory_put = function(pos)
+		-- start timer function, it will sort out whether furnace can burn or not.
+		local timer = minetest.get_node_timer(pos)
+		timer:start(1.0)
+	end,
+
+	allow_metadata_inventory_put = allow_metadata_inventory_put,
+	allow_metadata_inventory_move = allow_metadata_inventory_move,
+	allow_metadata_inventory_take = allow_metadata_inventory_take,
 })
+
+minetest.register_node("default:furnace_active", {
+	description = "Furnace",
+	tiles = {
+		"default_furnace_top.png", "default_furnace_bottom.png",
+		"default_furnace_side.png", "default_furnace_side.png",
+		"default_furnace_side.png",
+		{
+			image = "default_furnace_front_active.png",
+			backface_culling = false,
+			animation = {
+				type = "vertical_frames",
+				aspect_w = 16,
+				aspect_h = 16,
+				length = 1.5
+			},
+		}
+	},
+	paramtype2 = "facedir",
+	light_source = 8,
+	drop = "default:furnace",
+	groups = {cracky=2, not_in_creative_inventory=1},
+	legacy_facedir_simple = true,
+	is_ground_content = false,
+	sounds = default.node_sound_stone_defaults(),
+	on_timer = furnace_node_timer,
+
+	can_dig = can_dig,
+
+	allow_metadata_inventory_put = allow_metadata_inventory_put,
+	allow_metadata_inventory_move = allow_metadata_inventory_move,
+	allow_metadata_inventory_take = allow_metadata_inventory_take,
+})
+