SMBX deceleration logic

foo.lua

if player.speedX ~= 0 then
	local direction = (player.speedX > 0 and 1 or -1)
	local amt = 0
	
	local fwd, back = player.rightKeyPressing, player.leftKeyPressing
	if direction < 0 then
		fwd, back = back, fwd
	end
	local run = player.runKeyPressing
	local gnd = player:isGroundTouching()
	local fast = (math.abs(player.speedX) > Defines.player_walkspeed + 0.00001)
	
	if gnd and player.downKeyPressing then
		fwd, back = false, false
	end
	
	if gnd and not fwd then
		amt = amt + 0.07
	end
	if fast and not run then
		amt = amt + 0.1
	end
	if back then
		amt = amt + 0.21
		if not gnd then
			amt = amt + 0.07
		end
	end
	
	player.speedX = player.speedX + direction * amt
end