modules/world: add friction system to Actor2D

gamecore/modules/world/actors/actor2D.lua

@@ -61,6 +61,9 @@ function Actor2D:initPhysics(x, y, w, h, isSolid)
   self.xsp    = 0
   self.ysp    = 0
 
+  self.xfrc   = 0
+  self.yfrc   = 0
+
   self.isSolid = isSolid or false
 
   self:setFilter()
@@ -105,6 +108,13 @@ end
 
 function Actor2D:autoMove(dt)
   local newx, newy, cols, colNumber = self:move(self.x + self.xsp * dt, self.y + self.ysp * dt)
+
+  -- apply after the movement the friction, until the player stop
+  -- note: the friction is applied according to the delta time,
+  -- thus the friction should be how much speed is substracted in 1 second
+
+  self.xsp = utils.math.toZero(self.xsp, self.xfrc * dt)
+  self.ysp = utils.math.toZero(self.ysp, self.yfrc * dt)
 end
 
 function Actor2D:move(dx, dy)