//! Responsible for playing & queueing audio.
//! This also has the code for the underlying
//! audio server which adds new tracks.

use std::{collections::VecDeque, sync::Arc};

use arc_swap::ArcSwapOption;
use reqwest::Client;
use rodio::{OutputStream, OutputStreamHandle, Sink};
use tokio::{
    select,
    sync::{
        mpsc::{self, Receiver},
        RwLock,
    },
    task,
};

use crate::tracks::{DecodedTrack, Track, TrackInfo};

pub mod ui;

/// Handles communication between the frontend & audio player.
pub enum Messages {
    /// Notifies the audio server that it should update the track.
    Next,

    /// Similar to Next, but specific to the first track.
    Init,

    /// Pauses the [Sink]. This will also unpause it if it is paused.
    Pause,
}

/// The amount of songs to buffer up.
const BUFFER_SIZE: usize = 5;

/// Main struct responsible for queuing up & playing tracks.
pub struct Player {
    /// [rodio]'s [`Sink`] which can control playback.
    pub sink: Sink,

    /// The [`TrackInfo`] of the current track.
    /// This is [`None`] when lowfi is buffering.
    pub current: ArcSwapOption<TrackInfo>,

    /// The tracks, which is a [VecDeque] that holds
    /// *undecoded* [Track]s.
    tracks: RwLock<VecDeque<Track>>,

    /// The web client, which can contain a UserAgent & some
    /// settings that help lowfi work more effectively.
    client: Client,

    /// The [OutputStreamHandle], which also can control some
    /// playback, is for now unused and is here just to keep it
    /// alive so the playback can function properly.
    _handle: OutputStreamHandle,

    /// The [OutputStream], which is just here to keep the playback
    /// alive and functioning.
    _stream: OutputStream,
}

/// SAFETY: This is necessary because [OutputStream] does not implement [Send],
/// SAFETY: even though it is perfectly possible.
unsafe impl Send for Player {}

/// SAFETY: See implementation for [Send].
unsafe impl Sync for Player {}

impl Player {
    /// Initializes the entire player, including audio devices & sink.
    pub async fn new() -> eyre::Result<Self> {
        let (_stream, handle) = OutputStream::try_default()?;
        let sink = Sink::try_new(&handle)?;

        Ok(Self {
            tracks: RwLock::new(VecDeque::with_capacity(5)),
            current: ArcSwapOption::new(None),
            client: Client::builder().build()?,
            sink,
            _handle: handle,
            _stream,
        })
    }

    /// Just a shorthand for setting `current`.
    async fn set_current(&self, info: TrackInfo) -> eyre::Result<()> {
        self.current.store(Some(Arc::new(info)));

        Ok(())
    }

    /// This will play the next track, as well as refilling the buffer in the background.
    pub async fn next(queue: Arc<Self>) -> eyre::Result<DecodedTrack> {
        let track = match queue.tracks.write().await.pop_front() {
            Some(x) => x,
            // If the queue is completely empty, then fallback to simply getting a new track.
            // This is relevant particularly at the first song.
            None => Track::random(&queue.client).await?,
        };

        let decoded = track.decode()?;
        queue.set_current(decoded.info.clone()).await?;

        Ok(decoded)
    }

    /// This is the main "audio server".
    ///
    /// `rx` is used to communicate with it, for example when to
    /// skip tracks or pause.
    pub async fn play(queue: Arc<Self>, mut rx: Receiver<Messages>) -> eyre::Result<()> {
        // This is an internal channel which serves pretty much only one purpose,
        // which is to notify the buffer refiller to get back to work.
        // This channel is useful to prevent needing to check with some infinite loop.
        let (itx, mut irx) = mpsc::channel(8);

        // This refills the queue in the background.
        task::spawn({
            let queue = Arc::clone(&queue);

            async move {
                while irx.recv().await == Some(()) {
                    while queue.tracks.read().await.len() < BUFFER_SIZE {
                        let Ok(track) = Track::random(&queue.client).await else {
                            continue;
                        };
                        queue.tracks.write().await.push_back(track);
                    }
                }
            }
        });

        // Start buffering tracks immediately.
        itx.send(()).await?;

        loop {
            let clone = Arc::clone(&queue);
            let msg = select! {
                Some(x) = rx.recv() => x,

                // This future will finish only at the end of the current track.
                Ok(_) = task::spawn_blocking(move || clone.sink.sleep_until_end()) => Messages::Next,
            };

            match msg {
                Messages::Next | Messages::Init => {
                    // Serves as an indicator that the queue is "loading".
                    // This is also set by Player::next.
                    queue.current.store(None);

                    // Notify the background downloader that there's an empty spot
                    // in the buffer.
                    itx.send(()).await?;

                    queue.sink.stop();
                    let track = Self::next(Arc::clone(&queue)).await?;
                    queue.sink.append(track.data);
                }
                Messages::Pause => {
                    if queue.sink.is_paused() {
                        queue.sink.play();
                    } else {
                        queue.sink.pause();
                    }
                }
            }
        }
    }
}