Fix liquid flowing down ledges

README.md

@@ -1,5 +1,2 @@
 For a description of this Minetest mod, see
 https://forum.minetest.net/viewtopic.php?f=9&t=10271.
-
-TODO:
-* Support more fluids which push items, e.g. river water

init.lua

@@ -2,68 +2,85 @@
 -- The setting may change in-game but for simplicity we don't support this.
 local movement_gravity = tonumber(core.settings:get("movement_gravity")) or 9.81
 
--- get_flow_target_raw determines position to which the water flows, or returns
--- nothing if no target position was found
+local function is_flowing_liquid(nodename)
+	local def = minetest.registered_nodes[nodename]
+	return def and def.liquidtype == "flowing"
+end
+
+-- get_flow_raw determines the fluid flow vector, or returns nothing if
+-- the flow is zero
 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 function get_flow_target_raw(pos)
+local function get_flow_raw(pos)
+	-- FIXME: Do we need to treat nodes with special liquid_range differently?
 	local param2 = minetest.get_node(pos).param2
-	if param2 > 7 then
-		-- The liquid flows downwards
+	if param2 == 15 then
+		-- The liquid has full height and flows downwards
 		return
 	end
 	local neighbours = {}
 	for n = 1, 4 do
 		local p = vector.add(pos, neighbour_offsets[n])
-		neighbours[n] = p
-		neighbours[n + 4] = minetest.get_node(p)
+		neighbours[n] = minetest.get_node(p)
 	end
+	if param2 < 8 then
+		-- The liquid does not flow downwards
 		for i = 1, 4 do
-		-- If a neighbour has a lower height, flow to it
-		local node = neighbours[i + 4]
-		if node.name == "default:water_flowing"
-		and node.param2 < param2 then
-			return neighbours[i]
-		end
-	end
-	for i = 1, 4 do
-		-- 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
-			return neighbours[i]
-		end
-	end
-	for i = 1, 4 do
-		-- Flow to a neighbouring unsolid node
-		local node = neighbours[i + 4]
-		if node.name ~= "default:water_flowing" then
+			-- Flow to a neighbouring free space if possible
+			local node = neighbours[i]
 			local def = minetest.registered_nodes[node.name]
-			if def and not def.walkable then
-				return neighbours[i]
+			if def and not def.walkable and def.liquidtype == "none" then
+				return neighbour_offsets[i]
 			end
 		end
+		-- Find a liquid neighbour with lowest height
+		local min_height_neig = param2
+		local min_neighbour
+		for i = 1, 4 do
+			local node = neighbours[i]
+			local height_neigh = node.param2 % 8
+			if height_neigh < min_height_neig
+			and is_flowing_liquid(node.name) then
+				min_height_neig = height_neigh
+				min_neighbour = neighbour_offsets[i]
 			end
-	-- No neighbour found
+		end
+		if min_neighbour then
+			return min_neighbour
+		end
+		-- No free neighbour and no flowing liquid neighbour
+	end
+	-- The flowing liquid ends here in the XZ plane, so determine the
+	-- flow from higher neighbour nodes
+	local flow = {x = 0, y = 0, z = 0}
+	for i = 1, 4 do
+		local node = neighbours[i]
+		local height_neigh = node.param2 % 8
+		if is_flowing_liquid(node.name) then
+			-- A higher liquid is coming from there
+			flow = vector.subtract(flow, neighbour_offsets[i])
+		end
+	end
+	return not vector.equals(flow, {x = 0, y = 0, z = 0}) and flow or nil
 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)
+-- get_flow caches the results from get_flow_raw for 10 s
+local flow_cache = {}
+setmetatable(flow_cache, {__mode = "kv"})
+local function get_flow(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]
+	if flow_cache[vi]
+	and t - flow_cache[vi][1] < 10 * 1000000 then
+		return flow_cache[vi][2]
 	end
-	local flow_target = get_flow_target_raw(pos)
-	flow_target_cache[vi] = {t, flow_target}
-	return flow_target
+	local flow = get_flow_raw(pos)
+	flow_cache[vi] = {t, flow}
+	return flow
 end
 
 local item_entity = minetest.registered_entities["__builtin:item"]
@@ -112,17 +129,17 @@ item_entity.on_step = function(self, dtime, ...)
 	if not def or not def.liquidtype or def.liquidtype ~= "flowing" then
 		return
 	end
-	local pos_next = get_flow_target(pos)
-	if not pos_next then
+	local flow = get_flow(pos)
+	if not flow then
 		return
 	end
 	local vel_current = vel_desired or self.object:get_velocity()
-	local acc = vector.multiply(vector.subtract(pos_next, pos), 2.0)
-	if math.abs(vel_current.x) > 1.0 then
-		acc.x = 0
-	end
-	if math.abs(vel_current.z) > 1.0 then
-		acc.z = 0
+	local acc
+	if vector.dot(vel_current, flow) < 1.0 then
+		acc = vector.multiply(flow, 2.0)
+	else
+		-- The item is already accelerated by the fluids
+		acc = {x = 0, y = 0, z = 0}
 	end
 	acc.y = -movement_gravity
 	self.object:set_acceleration(acc)