Update carts from boost_cart

Better pathfinder algorithm, allows tuning the lag spike compensation.
Smoother movement (when it's laggy).
Set the player animation to stand on attach.
Remove driver when they leave.
Only update velocity when it's necessary.

mods/carts/cart_entity.lua

@@ -27,6 +27,10 @@ function cart_entity:on_rightclick(clicker)
 	elseif not self.driver then
 		self.driver = player_name
 		carts:manage_attachment(clicker, self.object)
+
+		-- player_api does not update the animation
+		-- when the player is attached, reset to default animation
+		player_api.set_animation(clicker, "stand")
 	end
 end
 
@@ -36,7 +40,7 @@ function cart_entity:on_activate(staticdata, dtime_s)
 		return
 	end
 	local data = minetest.deserialize(staticdata)
-	if not data or type(data) ~= "table" then
+	if type(data) ~= "table" then
 		return
 	end
 	self.railtype = data.railtype
@@ -52,6 +56,13 @@ function cart_entity:get_staticdata()
 	})
 end
 
+-- 0.5.x and later: When the driver leaves
+function cart_entity:on_detach_child(child)
+	if child and child:get_player_name() == self.driver then
+		self.driver = nil
+	end
+end
+
 function cart_entity:on_punch(puncher, time_from_last_punch, tool_capabilities, direction)
 	local pos = self.object:get_pos()
 	local vel = self.object:get_velocity()
@@ -82,7 +93,7 @@ function cart_entity:on_punch(puncher, time_from_last_punch, tool_capabilities,
 			local player = minetest.get_player_by_name(self.driver)
 			carts:manage_attachment(player, nil)
 		end
-		for _,obj_ in ipairs(self.attached_items) do
+		for _, obj_ in ipairs(self.attached_items) do
 			if obj_ then
 				obj_:set_detach()
 			end
@@ -165,6 +176,7 @@ local function get_railparams(pos)
 	return carts.railparams[node.name] or {}
 end
 
+local v3_len = vector.length
 local function rail_on_step(self, dtime)
 	local vel = self.object:get_velocity()
 	if self.punched then
@@ -201,17 +213,23 @@ local function rail_on_step(self, dtime)
 
 	local stop_wiggle = false
 	if self.old_pos and same_dir then
-		-- Detection for "skipping" nodes
-		local found_path = carts:pathfinder(
-			pos, self.old_pos, self.old_dir, ctrl, self.old_switch, self.railtype
+		-- Detection for "skipping" nodes (perhaps use average dtime?)
+		-- It's sophisticated enough to take the acceleration in account
+		local acc = self.object:get_acceleration()
+		local distance = dtime * (v3_len(vel) + 0.5 * dtime * v3_len(acc))
+
+		local new_pos, new_dir = carts:pathfinder(
+			pos, self.old_pos, self.old_dir, distance, ctrl,
+			self.old_switch, self.railtype
 		)
 
-		if not found_path then
-			-- No rail found: reset back to the expected position
-			pos = vector.new(self.old_pos)
+		if new_pos then
+			-- No rail found: set to the expected position
+			pos = new_pos
 			update.pos = true
+			cart_dir = new_dir
 		end
-	elseif self.old_pos and cart_dir.y ~= -1 and not self.punched then
+	elseif self.old_pos and self.old_dir.y ~= 1 and not self.punched then
 		-- Stop wiggle
 		stop_wiggle = true
 	end
@@ -223,12 +241,14 @@ local function rail_on_step(self, dtime)
 	local dir, switch_keys = carts:get_rail_direction(
 		pos, cart_dir, ctrl, self.old_switch, self.railtype
 	)
+	local dir_changed = not vector.equals(dir, self.old_dir)
 
 	local new_acc = {x=0, y=0, z=0}
 	if stop_wiggle or vector.equals(dir, {x=0, y=0, z=0}) then
 		vel = {x = 0, y = 0, z = 0}
 		local pos_r = vector.round(pos)
-		if not carts:is_rail(pos_r, self.railtype) then
+		if not carts:is_rail(pos_r, self.railtype)
+				and self.old_pos then
 			pos = self.old_pos
 		elseif not stop_wiggle then
 			pos = pos_r
@@ -239,7 +259,7 @@ local function rail_on_step(self, dtime)
 		update.vel = true
 	else
 		-- Direction change detected
-		if not vector.equals(dir, self.old_dir) then
+		if dir_changed then
 			vel = vector.multiply(dir, math.abs(vel.x + vel.z))
 			update.vel = true
 			if dir.y ~= self.old_dir.y then
@@ -291,7 +311,7 @@ local function rail_on_step(self, dtime)
 	end
 
 	self.object:set_acceleration(new_acc)
-	self.old_pos = vector.new(pos)
+	self.old_pos = vector.round(pos)
 	if not vector.equals(dir, {x=0, y=0, z=0}) and not stop_wiggle then
 		self.old_dir = vector.new(dir)
 	end
@@ -338,9 +358,15 @@ local function rail_on_step(self, dtime)
 	end
 	self.object:set_animation(anim, 1, 0)
 
-	self.object:set_velocity(vel)
+	if update.vel then
+		self.object:set_velocity(vel)
+	end
 	if update.pos then
-		self.object:set_pos(pos)
+		if dir_changed then
+			self.object:set_pos(pos)
+		else
+			self.object:move_to(pos)
+		end
 	end
 
 	-- call event handler