//! A Piston event loop for games and interactive applications

#![deny(missing_docs)]
#![deny(missing_copy_implementations)]

extern crate window;
extern crate input;
extern crate viewport;

use std::thread::sleep;
use std::time::{Duration, Instant};
use std::cmp;
use window::Window;
use input::{Input, AfterRenderArgs, IdleArgs, RenderArgs, UpdateArgs};

/// Tells whether last emitted event was idle or not.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum Idle {
    No,
    Yes,
}

#[derive(Copy, Clone, Debug)]
enum State {
    Render,
    SwapBuffers,
    UpdateLoop(Idle),
    HandleEvents,
    Update,
}

/// Stores event loop settings.
#[derive(Copy, Clone, Debug)]
pub struct EventSettings {
    /// The maximum number of frames per second
    ///
    /// The frame rate can be lower because the
    /// next frame is always scheduled from the previous frame.
    /// This causes the frames to "slip" over time.
    pub max_fps: u64,
    /// The number of updates per second
    ///
    /// This is the fixed update rate on average over time.
    /// If the event loop lags, it will try to catch up.
    /// When set to `0`, update events are disabled.
    pub ups: u64,
    /// The number of delayed updates before skipping them to catch up.
    /// When set to `0`, it will always try to catch up.
    pub ups_reset: u64,
    /// Enable or disable automatic swapping of buffers.
    pub swap_buffers: bool,
    /// Enable or disable benchmark mode.
    /// When enabled, it will render and update without sleep and ignore input.
    /// Used to test performance by playing through as fast as possible.
    /// Requires `lazy` to be set to `false`.
    pub bench_mode: bool,
    /// Enable or disable rendering only when receiving input.
    /// When enabled, update events are disabled.
    /// Idle events are emitted while receiving input.
    pub lazy: bool,
}

impl EventSettings {
    /// Creates new with default settings.
    pub fn new() -> EventSettings {
        EventSettings {
            max_fps: DEFAULT_MAX_FPS,
            ups: DEFAULT_UPS,
            swap_buffers: true,
            bench_mode: false,
            lazy: false,
            ups_reset: DEFAULT_UPS_RESET,
        }
    }
}

impl Default for EventSettings {
    fn default() -> EventSettings {
        EventSettings::new()
    }
}

/// An event loop iterator
///
/// *Warning: Because the iterator polls events from the window back-end,
/// it must be used on the same thread as the window back-end (usually main thread),
/// unless the window back-end supports multi-thread event polling.*
#[derive(Copy, Clone)]
pub struct Events {
    state: State,
    last_update: Instant,
    last_frame: Instant,
    dt_update_in_ns: u64,
    dt_frame_in_ns: u64,
    dt: f64,
    settings: EventSettings,
    first_frame: bool,
}

static BILLION: u64 = 1_000_000_000;

fn ns_to_duration(ns: u64) -> Duration {
    let secs = ns / BILLION;
    let nanos = (ns % BILLION) as u32;
    Duration::new(secs, nanos)
}

fn duration_to_secs(dur: Duration) -> f64 {
    dur.as_secs() as f64 + dur.subsec_nanos() as f64 / 1_000_000_000.0
}

/// The default updates per second.
pub const DEFAULT_UPS: u64 = 120;
/// The default delayed updates reset.
pub const DEFAULT_UPS_RESET: u64 = 2;
/// The default maximum frames per second.
pub const DEFAULT_MAX_FPS: u64 = 60;

impl Events {
    /// Creates a new event iterator with default UPS and FPS settings.
    pub fn new(settings: EventSettings) -> Events {
        let start = Instant::now();
        Events {
            state: State::Render,
            last_update: start,
            last_frame: start,
            dt_update_in_ns: if settings.ups == 0 {
                0
            } else {
                BILLION / settings.ups
            },
            dt_frame_in_ns: BILLION / settings.max_fps,
            dt: if settings.ups == 0 {
                0.0
            } else {
                1.0 / settings.ups as f64
            },
            settings: settings,
            first_frame: true,
        }
    }

    /// Returns the next game event.
    pub fn next<W>(&mut self, window: &mut W) -> Option<Input>
        where W: Window
    {
        if self.settings.lazy || self.settings.ups == 0 {
            // This mode does not emit update events.
            // More commonly used in UI applications.
            if window.should_close() {
                return None;
            }
            match self.state {
                State::SwapBuffers => {
                    if self.settings.swap_buffers {
                        window.swap_buffers();
                    }
                    // This mode needs no `Render` state.
                    self.state = State::UpdateLoop(Idle::Yes);
                    return Some(Input::AfterRender(AfterRenderArgs));
                }
                _ => {}
            }
            loop {
                let current_time = Instant::now();
                let next_frame = self.last_frame + ns_to_duration(self.dt_frame_in_ns);

                if !self.first_frame && next_frame > current_time {
                    if let State::UpdateLoop(Idle::Yes) = self.state {
                        // Emit idle event with time until next frame,
                        // in case the application wants to do some background work.
                        self.state = State::UpdateLoop(Idle::No);
                        let seconds = duration_to_secs(next_frame - current_time);
                        return Some(Input::Idle(IdleArgs { dt: seconds }));
                    }
                    self.state = State::UpdateLoop(Idle::Yes);
                    if self.settings.lazy {
                        let ev = window.wait_event();
                        return Some(ev);
                    } else {
                        match window.wait_event_timeout(next_frame - current_time) {
                            None => {}
                            x => return x,
                        }
                    }
                }

                self.first_frame = false;

                // In normal mode, let the FPS slip if late.
                self.last_frame = Instant::now();

                let size = window.size();
                let draw_size = window.draw_size();
                if size.width != 0 && size.height != 0 {
                    // Swap buffers next time.
                    self.state = State::SwapBuffers;
                    return Some(Input::Render(RenderArgs {
                        // Extrapolate time forward to allow smooth motion.
                        ext_dt: 0.0,
                        width: size.width,
                        height: size.height,
                        draw_width: draw_size.width,
                        draw_height: draw_size.height,
                    }));
                }
            }
        }

        loop {
            if window.should_close() {
                return None;
            }
            self.state = match self.state {
                State::Render => {
                    // Handle input events before rendering,
                    // because window might be closed and destroy
                    // the graphics context.
                    if let Some(e) = window.poll_event() {
                        if self.settings.bench_mode {
                            // Ignore input events in benchmark mode.
                            // This is to avoid the input events affecting
                            // the application state when benchmarking.
                            continue;
                        } else {
                            return Some(e);
                        }
                    }
                    if window.should_close() {
                        return None;
                    }

                    if self.settings.bench_mode {
                        // In benchmark mode, pretend FPS is perfect.
                        self.last_frame += ns_to_duration(self.dt_frame_in_ns);
                    } else {
                        // In normal mode, let the FPS slip if late.
                        self.last_frame = Instant::now();
                    }

                    let size = window.size();
                    let draw_size = window.draw_size();
                    if size.width != 0 && size.height != 0 {
                        // Swap buffers next time.
                        self.state = State::SwapBuffers;
                        return Some(Input::Render(RenderArgs {
                            // Extrapolate time forward to allow smooth motion.
                            ext_dt: duration_to_secs(self.last_frame
                                .duration_since(self.last_update)),
                            width: size.width,
                            height: size.height,
                            draw_width: draw_size.width,
                            draw_height: draw_size.height,
                        }));
                    }

                    State::UpdateLoop(Idle::No)
                }
                State::SwapBuffers => {
                    if self.settings.swap_buffers {
                        window.swap_buffers();
                    }
                    self.state = State::UpdateLoop(Idle::No);
                    return Some(Input::AfterRender(AfterRenderArgs));
                }
                State::UpdateLoop(ref mut idle) => {
                    if self.settings.bench_mode {
                        // In benchmark mode, pick the next event without sleep.
                        // Idle and input events are ignored.
                        // This is to avoid the input events affecting
                        // the application state when benchmarking.
                        let next_frame = self.last_frame + ns_to_duration(self.dt_frame_in_ns);
                        let next_update = self.last_update + ns_to_duration(self.dt_update_in_ns);
                        let next_event = cmp::min(next_frame, next_update);
                        if next_event == next_frame {
                            State::Render
                        } else {
                            State::HandleEvents
                        }
                    } else {
                        let current_time = Instant::now();
                        let next_frame = self.last_frame + ns_to_duration(self.dt_frame_in_ns);
                        let next_update = self.last_update + ns_to_duration(self.dt_update_in_ns);
                        let next_event = cmp::min(next_frame, next_update);
                        if next_event > current_time {
                            if let Some(x) = window.poll_event() {
                                *idle = Idle::No;
                                return Some(x);
                            } else if *idle == Idle::No {
                                *idle = Idle::Yes;
                                let seconds = duration_to_secs(next_event - current_time);
                                return Some(Input::Idle(IdleArgs { dt: seconds }));
                            }
                            sleep(next_event - current_time);
                            State::UpdateLoop(Idle::No)
                        } else if next_event == next_frame {
                            State::Render
                        } else {
                            State::HandleEvents
                        }
                    }
                }
                State::HandleEvents => {
                    if self.settings.bench_mode {
                        // Ignore input events.
                        // This is to avoid the input events affecting
                        // the application state when benchmarking.
                        match window.poll_event() {
                            None => State::Update,
                            Some(_) => State::HandleEvents,
                        }
                    } else {
                        // Handle all events before updating.
                        match window.poll_event() {
                            None => State::Update,
                            Some(x) => {
                                return Some(x);
                            }
                        }
                    }
                }
                State::Update => {
                    self.state = State::UpdateLoop(Idle::No);
                    if !self.settings.bench_mode && self.settings.ups_reset > 0 &&
                       Instant::now() - self.last_update >
                       ns_to_duration(self.settings.ups_reset * self.dt_update_in_ns) {
                        // Skip updates because CPU is too busy.
                        self.last_update = Instant::now();
                    } else {
                        // Use the update state stored right after sleep.
                        self.last_update += ns_to_duration(self.dt_update_in_ns);
                    }
                    return Some(Input::Update(UpdateArgs { dt: self.dt }));
                }
            };
        }
    }
}

/// Methods implemented for changing event loop settings.
pub trait EventLoop: Sized {
    /// Returns event loop settings.
    fn get_event_settings(&self) -> EventSettings;
    /// Sets event loop settings.
    fn set_event_settings(&mut self, settings: EventSettings);

    /// The number of updates per second
    ///
    /// This is the fixed update rate on average over time.
    /// If the event loop lags, it will try to catch up.
    /// When set to `0`, update events are disabled.
    fn set_ups(&mut self, frames: u64) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { ups: frames, ..old_settings });
    }

    /// The number of updates per second
    ///
    /// This is the fixed update rate on average over time.
    /// If the event loop lags, it will try to catch up.
    /// When set to `0`, update events are disabled.
    fn ups(mut self, frames: u64) -> Self {
        self.set_ups(frames);
        self
    }

    /// The number of delayed updates before skipping them to catch up.
    /// When set to `0`, it will always try to catch up.
    fn set_ups_reset(&mut self, frames: u64) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { ups_reset: frames, ..old_settings });
    }

    /// The number of delayed updates before skipping them to catch up.
    /// When set to `0`, it will always try to catch up.
    fn ups_reset(mut self, frames: u64) -> Self {
        self.set_ups_reset(frames);
        self
    }

    /// The maximum number of frames per second
    ///
    /// The frame rate can be lower because the
    /// next frame is always scheduled from the previous frame.
    /// This causes the frames to "slip" over time.
    fn set_max_fps(&mut self, frames: u64) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { max_fps: frames, ..old_settings })
    }

    /// The maximum number of frames per second
    ///
    /// The frame rate can be lower because the
    /// next frame is always scheduled from the previous frame.
    /// This causes the frames to "slip" over time.
    fn max_fps(mut self, frames: u64) -> Self {
        self.set_max_fps(frames);
        self
    }

    /// Enable or disable automatic swapping of buffers.
    fn set_swap_buffers(&mut self, enable: bool) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { swap_buffers: enable, ..old_settings })
    }

    /// Enable or disable automatic swapping of buffers.
    fn swap_buffers(mut self, enable: bool) -> Self {
        self.set_swap_buffers(enable);
        self
    }

    /// Enable or disable benchmark mode.
    /// When enabled, it will render and update without sleep and ignore input.
    /// Used to test performance by playing through as fast as possible.
    /// Requires `lazy` to be set to `false`.
    fn set_bench_mode(&mut self, enable: bool) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { bench_mode: enable, ..old_settings })
    }

    /// Enable or disable benchmark mode.
    /// When enabled, it will render and update without sleep and ignore input.
    /// Used to test performance by playing through as fast as possible.
    /// Requires `lazy` to be set to `false`.
    fn bench_mode(mut self, enable: bool) -> Self {
        self.set_bench_mode(enable);
        self
    }

    /// Enable or disable rendering only when receiving input.
    /// When enabled, update events are disabled.
    /// Idle events are emitted while receiving input.
    fn set_lazy(&mut self, enable: bool) {
        let old_settings = self.get_event_settings();
        self.set_event_settings(EventSettings { lazy: enable, ..old_settings })
    }

    /// Enable or disable rendering only when receiving input.
    /// When enabled, update events are disabled.
    /// Idle events are emitted while receiving input.
    fn lazy(mut self, enable: bool) -> Self {
        self.set_lazy(enable);
        self
    }
}

impl EventLoop for EventSettings {
    fn get_event_settings(&self) -> Self {
        *self
    }
    fn set_event_settings(&mut self, settings: Self) {
        *self = settings;
    }
}

impl EventLoop for Events {
    fn get_event_settings(&self) -> EventSettings {
        self.settings
    }
    fn set_event_settings(&mut self, settings: EventSettings) {
        // Reset event loop to initial state.
        *self = Events::new(settings);
    }
}