-- step animation forward or backwards 'amount' number of frames
local function step(self, amount)
    -- frame_count is the number of frames in the current animationm 
    local frame_count = go.get("#sprite", "frame_count")
    -- cursor is the normalized (0.0 to 1.0) animation cursor
    local cursor = go.get("#sprite", "cursor")

    -- normalized length of a frame in the current animation
    local frame_length = 1 / frame_count

    -- move the cursor amount number of frames
    cursor = cursor + (frame_length * amount)

    -- wrap animation if advancing beyond the first or last frame
    if cursor < 0 then
        cursor = cursor + 1
    elseif cursor >= 1 then
        cursor = cursor - 1
    end

    -- set new sprite cursor position
    go.set("#sprite", "cursor", cursor)

    -- calculate the current animation frame and show on a label
    local current_frame = 1 + math.floor(cursor * frame_count)
    label.set_text("#frame", ("%d / %d"):format(current_frame, frame_count))
end

-- stop automatic animation playback
local function stop(self)
    -- only try to stop if there is an active timer
    if self.playback_timer then
        -- visually update the start/stop sprite to show the 'start image
        sprite.play_flipbook("controls#playstop", "start")
        -- cancel the animation timer
        timer.cancel(self.playback_timer)
        self.playback_timer = nil
    end
end

-- start automatic animation playback using a timer to advance the animation
-- one frame at a time
local function start(self)
    -- visually update the start/stop sprite to show the 'stop' image
    sprite.play_flipbook("controls#playstop", "stop")
    -- start a timer to advance the animation roughly every 0.15 seconds
    self.playback_timer = timer.delay(0.15, true, function()
        step(self, 1)
    end)
end

function init(self)
    msg.post(".", "acquire_input_focus")
    self.playback_timer = nil
end

function on_input(self, action_id, action)
    if action.pressed then
        -- key left or mouse click on left part of screen
        -- step animation one frame backwards
        if action_id == hash("key_left")
        or action_id == hash("mouse_button_left") and action.x < 240
        then
            stop(self)
            step(self, -1)
            return
        end
        
        -- key right or mouse click on right part of the screen
        -- step animation one frame forward    
        if action_id == hash("key_right")
        or action_id == hash("mouse_button_left") and action.x > 480 then
            stop(self)
            step(self, 1)
            return
        end

        -- key space or mouse click in central part of the screen
        -- start or stop animation playback
        if action_id == hash("key_space")
        or action_id == hash("mouse_button_left") and action.x > 240 and action.x < 480 then
            if self.playback_timer then
                stop(self)
            else
                start(self)
            end
        end
    end
end