function init(self)
    -- Animate automatic player position
    go.animate(".", "position.x", go.PLAYBACK_LOOP_PINGPONG, 620, go.EASING_LINEAR, 6.0)

    -- Create a timer to tick every 0.25 second:
    timer.delay(0.25, true, function()

        -- Create a simple bullet bullet using the factory
        local bullet_id = factory.create("#bulletfactory", go.get_position())

        -- Animate the created bullet towards top of screen, where it is deleted
        if bullet_id then
            go.animate(bullet_id, "position.y", go.PLAYBACK_ONCE_FORWARD, 600, go.EASING_LINEAR, 1, 0, function()
                go.delete(bullet_id)
            end)
        end
    end)
end

-- Define different properties of the script:
go.property("sprite", hash("ufoGreen"))
go.property("health_points", 1)
go.property("speed", vmath.vector3(100, 100, 0))
go.property("is_random", true)

function init(self)

    -- Set animation of the sprite to the one defined by its property self.sprite:
    sprite.play_flipbook("#sprite", self.sprite)

    -- Add randomness to horizontal direction - this way enemy horizontal speed may be inverted or cleared:
    -- -1 * self.speed.x - inverted direction
    --  0 * self.speed.x - cleared direction
    --  1 * self.speed.x - regular direction
    self.speed.x = math.random(-1, 1) * self.speed.x

    -- If self.is_random boolean property is true:
    if self.is_random then
        -- add a timer to randomly switch horizontal speed every second:
        timer.delay(1, true, function()
            self.speed.x = math.random(-1, 1) * self.speed.x
        end)
    end
end

function update(self, dt)
    -- Update enemy position based on its current speed:
    local pos = go.get_position()
    pos = pos + self.speed * dt
    go.set_position(pos)

    -- Bounce enemy off "walls":
    if pos.x > 600 or pos.x < 50 then
        self.speed.x = -self.speed.x
    end

    -- Remove enemy if it goes out of screen:
    if pos.y < -50 then
        go.delete()
    end
end

function on_message(self, message_id, message, sender)

    -- React to collision with bullet:
    if message_id == hash("trigger_response") and message.enter then

        -- Remove one health point
        self.health_points = self.health_points - 1

        -- Play particlefx for damage taken:
        particlefx.play("#boom")

        -- When no health points left - remove this ship
        if self.health_points <= 0 then
            go.delete()
        end
    end
end

-- Define different properties for different enemies:
local ENEMY_TYPES = {
    random = {
        sprite = hash("ufoGreen"),
        health_points = 1,
        speed = vmath.vector3(40, -100, 0),
        is_random = true
    },
    diagonal = {
        sprite = hash("enemyRed2"),
        health_points = 2,
        speed = vmath.vector3(120, -80, 0),
        is_random = false
    },
    straight = {
        sprite = hash("enemyBlue4"),
        health_points = 3,
        speed = vmath.vector3(0, -40, 0),
        is_random = false
    }
}

function init(self)
    -- Acquire input focus here, so we can handle inputs:
    msg.post(".", "acquire_input_focus")
end

-- Helper function to spawn given enemy by its type:
local function spawn_enemy(enemy_type)

    -- Select properties of the enemy by type:
    local properties = ENEMY_TYPES[enemy_type]

    -- Set initial position of the spawned ship.
    local position = go.get_position()

    -- This will make the position one out of (-180, -90, 0, 90, 180):
    position.x = position.x + math.random(-2,2) * 90

    -- Create enemy with passed properties
    factory.create("#enemyfactory", position, nil, properties)
end

function on_input(self, action_id, action)

    -- React to different key presses with spawning different enemies:
    if action_id == hash("key_1") and action.released then
        spawn_enemy("random")
    elseif action_id == hash("key_2") and action.released then
        spawn_enemy("diagonal")
    elseif action_id == hash("key_3") and action.released then
        spawn_enemy("straight")
    end
end