Simplify the code and change behaviour a bit

Example: accelerate instead of setting velocities

README.md

@@ -2,4 +2,4 @@ For a description of this Minetest mod, see
 https://forum.minetest.net/viewtopic.php?f=9&t=10271.
 
 TODO:
-* Support mods liquids
+* Support more fluids which push items, e.g. river water

init.lua

@@ -1,31 +1,20 @@
--- Use the movement gravity for the downwards direction. Get the setting rarely
+-- Use the movement gravity for the downwards acceleration.
-local cached_gravity
+-- The setting may change in-game but for simplicity we don't support this.
-local function get_gravity()
+local movement_gravity = tonumber(core.settings:get("movement_gravity")) or 9.81
-	if cached_gravity then
-		return cached_gravity
-	end
-	cached_gravity = tonumber(core.settings:get("movement_gravity")) or 9.81
-	core.after(50, function()
-		cached_gravity = nil
-	end)
-	return cached_gravity
-end
 
+-- get_flow_target_raw determines position to which the water flows, or returns
+-- nothing if no target position was found
 local neighbour_offsets = {
 	{x=-1, y=0, z=0},
 	{x=1, y=0, z=0},
 	{x=0, y=0, z=-1},
 	{x=0, y=0, z=1}
 }
-local neighbours_cache = {}
+local function get_flow_target_raw(pos)
-setmetatable(neighbours_cache, {__mode = "kv"})
+	local param2 = minetest.get_node(pos).param2
-local function get_neighbour_nodes(pos)
+	if param2 > 7 then
-	-- Use previously found neighbours if they are not too old
+		-- The liquid flows downwards
-	local vi = minetest.hash_node_position(pos)
+		return
-	local t = minetest.get_us_time()
-	if neighbours_cache[vi]
-	and t - neighbours_cache[vi][1] < 10 * 1000000 then
-		return neighbours_cache[vi][2]
 	end
 	local neighbours = {}
 	for n = 1, 4 do
@@ -33,17 +22,6 @@ local function get_neighbour_nodes(pos)
 		neighbours[n] = p
 		neighbours[n + 4] = minetest.get_node(p)
 	end
-	neighbours_cache[vi] = {t, neighbours}
-	return neighbours
-end
-
--- This function determines position to which the water flows
-local function get_flow_target(pos)
-	local neighbours = get_neighbour_nodes(pos)
-	local param2 = minetest.get_node(pos).param2
-	if param2 > 7 then
-		return
-	end
 	for i = 1, 4 do
 		-- If a neighbour has a lower height, flow to it
 		local node = neighbours[i + 4]
@@ -53,7 +31,7 @@ local function get_flow_target(pos)
 		end
 	end
 	for i = 1, 4 do
-		-- TODO
+		-- Flow to a downwards-flowing neighbour if its height is not too small
 		local node = neighbours[i + 4]
 		if node.name == "default:water_flowing"
 		and node.param2 >= 11 then
@@ -61,7 +39,7 @@ local function get_flow_target(pos)
 		end
 	end
 	for i = 1, 4 do
-		-- TODO
+		-- Flow to a neighbouring unsolid node
 		local node = neighbours[i + 4]
 		if node.name ~= "default:water_flowing" then
 			local def = minetest.registered_nodes[node.name]
@@ -70,53 +48,64 @@ local function get_flow_target(pos)
 			end
 		end
 	end
+	-- No neighbour found
+end
+
+-- get_flow_target caches the results from get_flow_target_raw for 10 s
+local flow_target_cache = {}
+setmetatable(flow_target_cache, {__mode = "kv"})
+local function get_flow_target(pos)
+	local vi = minetest.hash_node_position(pos)
+	local t = minetest.get_us_time()
+	if flow_target_cache[vi]
+	and t - flow_target_cache[vi][1] < 10 * 1000000 then
+		return flow_target_cache[vi][2]
+	end
+	local flow_target = get_flow_target_raw(pos)
+	flow_target_cache[vi] = {t, flow_target}
+	return flow_target
 end
 
 local item_entity = minetest.registered_entities["__builtin:item"]
-local old_on_step = item_entity.on_step or function()end
+local old_on_step = item_entity.on_step
 
 item_entity.makes_footstep_sound = true
+-- The "bt_" prefix shows that the value comes from builtin_item
 item_entity.bt_timer = 0
 item_entity.on_step = function(self, dtime, ...)
+	-- Remember the velocity before an original on_step can change it
+	local vel_desired
+	if self.bt_reset_velocity then
+		vel_desired = self.object:get_velocity()
+	end
+
 	old_on_step(self, dtime, ...)
 
-	--~ if not self.physical_state then
+	-- Ignore the item if it should not interact with physics
-		--~ return
+	if not self.physical_state then
-	--~ end
+		return
-
-	-- Force-adjust an acceleration and/or velocity if needed
-	if self.bt_acc
-	and not vector.equals(self.object:get_acceleration(), self.bt_acc) then
-		self.object:set_acceleration(self.bt_acc)
-	end
-	if self.bt_vel
-	and not vector.equals(self.object:get_velocity(), self.bt_vel) then
-		self.object:set_velocity(self.bt_vel)
 	end
 
-	-- TODO: was ist pyhsical state?
+	-- Reset the velocity if needed
-	if self.bt_phys ~= nil
+	if vel_desired
-	and self.physical_state ~= self.bt_phys then
+	and not vector.equals(self.object:get_velocity(), vel_desired) then
-		self.physical_state = self.bt_phys
+		self.object:set_velocity(vel_desired)
-		self.object:set_properties({
-			physical = self.bt_phys
-		})
 	end
 
-	-- For performance reasons, skip the remaining code except every second
+	-- For performance reasons, skip the remaining code in frequent steps
 	self.bt_timer = self.bt_timer + dtime
-	if self.bt_timer < 1 then
+	if self.bt_timer < 0.1 then
 		return
 	end
 	self.bt_timer = 0
 
 	local p = self.object:get_pos()
 	local pos = vector.round(p)
+	local nodename = minetest.get_node(pos).name
 
-	local name = minetest.get_node(pos).name
+	-- Burn the item if it is in lava
-	if name == "default:lava_flowing"
+	if nodename == "default:lava_flowing"
-	or name == "default:lava_source" then
+	or nodename == "default:lava_source" then
-		-- TODO: more generic burn cases
 		minetest.sound_play("builtin_item_lava", {pos=p})
 		minetest.add_particlespawner({
 			amount = 3,
@@ -144,38 +133,30 @@ item_entity.on_step = function(self, dtime, ...)
 		return
 	end
 
-	local def = minetest.registered_nodes[name]
+	if self.bt_reset_velocity then
-	if not def then
+		-- Set the item acceleration to its default (changed again below)
+		self.object:set_acceleration({x=0, y=-movement_gravity, z=0})
+		self.bt_reset_velocity = nil
+	end
+	local def = minetest.registered_nodes[nodename]
+	if not def or not def.liquidtype or def.liquidtype ~= "flowing" then
 		return
 	end
-	-- Adjust the acceleration in liquid nodes
+	local pos_next = get_flow_target(pos)
-	self.bt_acc = nil
+	if not pos_next then
-	self.bt_vel = nil
-	if def.liquidtype then
-		-- Set the strongest acceleration when we are in the middle of the node
-		local acc_strength = 1.0 - ((p.y - 0.5) % 1.0) * 0.9
-		local acc = {x = 0, y = -acc_strength * get_gravity(), z = 0}
-		self.object:set_acceleration(acc)
-		self.bt_acc = acc
-	end
-	if def.liquidtype ~= "flowing" then
 		return
 	end
-	local vec = get_flow_target(pos)
+	local vel_current = vel_desired or self.object:get_velocity()
-	if not vec then
+	local acc = vector.multiply(vector.subtract(pos_next, pos), 2.0)
-		return
+	if math.abs(vel_current.x) > 1.0 then
+		acc.x = 0
 	end
-	local v = vector.add(
+	if math.abs(vel_current.z) > 1.0 then
-		self.object:get_velocity(),
+		acc.z = 0
-		vector.multiply(vector.subtract(vec, pos),.5)
+	end
-	)
+	acc.y = -movement_gravity
-	self.bt_vel = v
+	self.object:set_acceleration(acc)
-	self.object:set_velocity(v)
+	self.bt_reset_velocity = true
-	self.physical_state = true
-	self.bt_phys = true
-	self.object:set_properties({
-		physical = true
-	})
 end
 
 minetest.register_entity(":__builtin:item", item_entity)