go.property("acceleration", 100)
go.property("deceleration", 200)
go.property("max_speed", 400)
go.property("wheel_base", 40)  -- pixels between left and right wheels

local UP = vmath.vector3(0, 1, 0)

function init(self)
    msg.post(".", "acquire_input_focus")

    self.left_speed = 0
    self.right_speed = 0
    self.left_input = false
    self.right_input = false
end

function update(self, dt)
    -- each wheel accelerates when its key is held, decelerates otherwise
    if self.left_input then
        self.left_speed = math.min(self.left_speed + self.acceleration * dt, self.max_speed)
    else
        self.left_speed = math.max(self.left_speed - self.deceleration * dt, 0)
    end

    if self.right_input then
        self.right_speed = math.min(self.right_speed + self.acceleration * dt, self.max_speed)
    else
        self.right_speed = math.max(self.right_speed - self.deceleration * dt, 0)
    end

    -- differential drive: speed difference creates rotation, average creates forward motion
    local linear_speed = (self.left_speed + self.right_speed) / 2
    local angular_vel = (self.right_speed - self.left_speed) / self.wheel_base

    local rot = go.get_rotation()
    rot = rot * vmath.quat_rotation_z(angular_vel * dt)
    go.set_rotation(rot)

    local p = go.get_position()
    p = p + vmath.rotate(rot, UP * linear_speed * dt)
    go.set_position(p)

    -- reset each frame (on_input re-sets if key is still held)
    self.left_input = false
    self.right_input = false
end

function on_input(self, action_id, action)
    if action_id == hash("a_key") then
        self.left_input = true
    elseif action_id == hash("d_key") then
        self.right_input = true
    end
end