function init(self)
    msg.post(".", "acquire_input_focus") -- <1>
    self.vel = vmath.vector3() -- <2>    
end

function update(self, dt)
    local pos = go.get_position() -- <3>
    pos = pos + self.vel * dt -- <4>
    go.set_position(pos) -- <5>
    
    self.vel.x = 0 -- <6>
    self.vel.y = 0
end

function on_input(self, action_id, action)
    if action_id == hash("up") then
        self.vel.y = 150 -- <7>
    elseif action_id == hash("down") then
        self.vel.y = -150
    elseif action_id == hash("left") then
        self.vel.x = -150 -- <8>
    elseif action_id == hash("right") then
        self.vel.x = 150
    end
end

--[[
1. Tell the engine that the current game object ("." is 
   shorthand for that) should receive user input to the function
   `on_input()` in its script components.
2. Construct a vector to indicate velocity. It will initially be
   zero.
3. Each frame, get the current position and store in `pos`.
4. Add the velocity, scaled to the current frame length. Velocity
   is therefore expressed in pixels per second.
5. Set the game object's position to the newly calculated position.
6. Zero out the velocity. If no input is given, there should be
   no movement.
7. If the user presses "up", set the y component of the velocity to 150.
   If the user presses "down", set the y component to -150.
8. Similarly, if the user presses "left", set the x component of the velocity to -150.
   And finally, if the user presses "right", set the x component to 150.
--]]