go.property("acceleration", 100)
go.property("deceleration", 200)
go.property("max_speed", 400)

function init(self)
    -- make sure the script will receive user input
    msg.post(".", "acquire_input_focus")

    -- movement input
    self.input = vmath.vector3()
    
    -- the current direction of movement
    self.direction = vmath.vector3()

    -- the current speed (pixels/second)
    self.speed = 0
end

function update(self, dt)
    -- is any key pressed?
    if self.input.x ~= 0 or self.input.y ~= 0 then
        -- set direction of travel from input
        self.direction = self.input
        -- increase speed
        self.speed = self.speed + self.acceleration * dt
        -- cap speed
        self.speed = math.min(self.speed, self.max_speed)
    else
        -- decrease speed when no key is pressed
        self.speed = self.speed - self.deceleration * dt
        self.speed = math.max(self.speed, 0)
    end

    -- move the game object
    local p = go.get_position()
    p = p + self.direction * self.speed * dt
    go.set_position(p)

    -- reset input
    self.input = vmath.vector3()
end

function on_input(self, action_id, action)
    -- update direction of movement based on currently pressed keys
    if action_id == hash("up") then
        self.input.y = 1
    elseif action_id == hash("down") then
        self.input.y = -1
    elseif action_id == hash("left") then
        self.input.x = -1
    elseif action_id == hash("right") then
        self.input.x = 1
    end
end