Optimized Collision Detection

function collisions.check_rectangles_overlap( a, b )
   local overlap = false
   local shift_b_x, shift_b_y = 0, 0
   local dist_x = b.position_x - a.position_x
   local dist_y = b.position_y - a.position_y
   if math.abs( dist_x ) <= a.halfwidth + b.halfwidth and
      math.abs( dist_y ) <= a.halfheight + b.halfheight then
	 overlap = true
	 shift_b_x = dist_x / math.abs(dist_x) * (
	    ( a.halfwidth + b.halfwidth ) - math.abs(dist_x) )
	 shift_b_y = dist_y / math.abs(dist_y) * (
	    ( a.halfheight + b.halfheight ) - math.abs(dist_y) )
   end
   return overlap, shift_b_x, shift_b_y
end

This allows to avoid creating intermediate tables:

function collisions.ball_platform_collision( ball, platform )
   local overlap, shift_ball_x, shift_ball_y
   overlap, shift_ball_x, shift_ball_y =
      collisions.check_rectangles_overlap( platform, ball )   
   if overlap then
      ball.rebound( shift_ball_x, shift_ball_y )
   end      
end

function collisions.ball_walls_collision( ball, walls )
   local overlap, shift_ball_x, shift_ball_y
   for _, wall in pairs( walls.current_level_walls ) do
      overlap, shift_ball_x, shift_ball_y =
      	 collisions.check_rectangles_overlap( wall, ball )
      if overlap then
	 ball.rebound( shift_ball_x, shift_ball_y )
      end
   end
end

and so on.