use std::{
    io::{self, BufRead, Read, Seek, SeekFrom},
    mem,
    num::NonZeroUsize,
    thread::{self, JoinHandle},
};

use crossbeam_channel::{Receiver, Sender};

use crate::{gzi, Block, VirtualPosition};

type BufferedTx = Sender<io::Result<Buffer>>;
type BufferedRx = Receiver<io::Result<Buffer>>;
type InflateTx = Sender<(Buffer, BufferedTx)>;
type InflateRx = Receiver<(Buffer, BufferedTx)>;
type ReadTx = Sender<BufferedRx>;
type ReadRx = Receiver<BufferedRx>;
type RecycleTx = Sender<Buffer>;
type RecycleRx = Receiver<Buffer>;

enum State<R> {
    Paused(R),
    Running {
        reader_handle: JoinHandle<Result<R, ReadError<R>>>,
        inflater_handles: Vec<JoinHandle<()>>,
        read_rx: ReadRx,
        recycle_tx: RecycleTx,
    },
    Done,
}

#[derive(Debug, Default)]
struct Buffer {
    buf: Vec<u8>,
    block: Block,
}

/// A multithreaded BGZF reader.
///
/// This is a multithreaded BGZF reader that uses a thread pool to decompress block data. It places
/// the inner reader on its own thread to read raw frames asynchronously.
pub struct MultithreadedReader<R> {
    state: State<R>,
    worker_count: NonZeroUsize,
    position: u64,
    buffer: Buffer,
}

impl<R> MultithreadedReader<R> {
    /// Returns the current position of the stream.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use noodles_bgzf as bgzf;
    /// let reader = bgzf::MultithreadedReader::new(io::empty());
    /// assert_eq!(reader.position(), 0);
    /// ```
    pub fn position(&self) -> u64 {
        self.position
    }

    /// Returns the current virtual position of the stream.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use noodles_bgzf as bgzf;
    /// let reader = bgzf::MultithreadedReader::new(io::empty());
    /// assert_eq!(reader.virtual_position(), bgzf::VirtualPosition::MIN);
    /// ```
    pub fn virtual_position(&self) -> VirtualPosition {
        self.buffer.block.virtual_position()
    }

    /// Shuts down the reader.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use noodles_bgzf as bgzf;
    /// let mut reader = bgzf::MultithreadedReader::new(io::empty());
    /// reader.finish()?;
    /// # Ok::<_, io::Error>(())
    /// ```
    pub fn finish(&mut self) -> io::Result<R> {
        let state = mem::replace(&mut self.state, State::Done);

        match state {
            State::Paused(inner) => Ok(inner),
            State::Running {
                reader_handle,
                mut inflater_handles,
                recycle_tx,
                ..
            } => {
                drop(recycle_tx);

                for handle in inflater_handles.drain(..) {
                    handle.join().unwrap();
                }

                reader_handle.join().unwrap().map_err(|e| e.1)
            }
            State::Done => panic!("invalid state"),
        }
    }
}

impl<R> MultithreadedReader<R>
where
    R: Read + Send + 'static,
{
    /// Creates a multithreaded BGZF reader with a worker count of 1.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use noodles_bgzf as bgzf;
    /// let reader = bgzf::MultithreadedReader::new(io::empty());
    /// ```
    pub fn new(inner: R) -> Self {
        Self::with_worker_count(NonZeroUsize::MIN, inner)
    }

    /// Creates a multithreaded BGZF reader with a worker count.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use std::num::NonZeroUsize;
    /// use noodles_bgzf as bgzf;
    /// let reader = bgzf::MultithreadedReader::with_worker_count(NonZeroUsize::MIN, io::empty());
    /// ```
    pub fn with_worker_count(worker_count: NonZeroUsize, inner: R) -> Self {
        Self {
            state: State::Paused(inner),
            worker_count,
            position: 0,
            buffer: Buffer::default(),
        }
    }

    /// Returns a mutable reference to the underlying reader.
    ///
    /// # Examples
    ///
    /// ```
    /// # use std::io;
    /// use noodles_bgzf as bgzf;
    /// let mut reader = bgzf::MultithreadedReader::new(io::empty());
    /// let _inner = reader.get_mut();
    /// ```
    pub fn get_mut(&mut self) -> &mut R {
        self.pause();

        match &mut self.state {
            State::Paused(inner) => inner,
            _ => panic!("invalid state"),
        }
    }

    fn resume(&mut self) {
        if matches!(self.state, State::Running { .. }) {
            return;
        }

        let state = mem::replace(&mut self.state, State::Done);

        let State::Paused(inner) = state else {
            panic!("invalid state");
        };

        let worker_count = self.worker_count.get();

        let (inflate_tx, inflate_rx) = crossbeam_channel::bounded(worker_count);
        let (read_tx, read_rx) = crossbeam_channel::bounded(worker_count);
        let (recycle_tx, recycle_rx) = crossbeam_channel::bounded(worker_count);

        for _ in 0..worker_count {
            recycle_tx.send(Buffer::default()).unwrap();
        }

        let reader_handle = spawn_reader(inner, inflate_tx, read_tx, recycle_rx);
        let inflater_handles = spawn_inflaters(self.worker_count, inflate_rx);

        self.state = State::Running {
            reader_handle,
            inflater_handles,
            read_rx,
            recycle_tx,
        };
    }

    fn pause(&mut self) {
        if matches!(self.state, State::Paused(_)) {
            return;
        }

        let state = mem::replace(&mut self.state, State::Done);

        let State::Running {
            reader_handle,
            mut inflater_handles,
            recycle_tx,
            ..
        } = state
        else {
            panic!("invalid state");
        };

        drop(recycle_tx);

        for handle in inflater_handles.drain(..) {
            handle.join().unwrap();
        }

        // Discard read errors.
        let inner = match reader_handle.join().unwrap() {
            Ok(inner) => inner,
            Err(ReadError(inner, _)) => inner,
        };

        self.state = State::Paused(inner);
    }

    fn read_block(&mut self) -> io::Result<()> {
        self.resume();

        let State::Running {
            read_rx,
            recycle_tx,
            ..
        } = &self.state
        else {
            panic!("invalid state");
        };

        while let Some(mut buffer) = recv_buffer(read_rx)? {
            buffer.block.set_position(self.position);
            self.position += buffer.block.size();

            let prev_buffer = mem::replace(&mut self.buffer, buffer);
            recycle_tx.send(prev_buffer).ok();

            if self.buffer.block.data().len() > 0 {
                break;
            }
        }

        Ok(())
    }
}

impl<R> Drop for MultithreadedReader<R> {
    fn drop(&mut self) {
        if !matches!(self.state, State::Done) {
            let _ = self.finish();
        }
    }
}

impl<R> Read for MultithreadedReader<R>
where
    R: Read + Send + 'static,
{
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let mut src = self.fill_buf()?;
        let amt = src.read(buf)?;
        self.consume(amt);
        Ok(amt)
    }

    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
        use super::reader::default_read_exact;

        if let Some(src) = self.buffer.block.data().as_ref().get(..buf.len()) {
            buf.copy_from_slice(src);
            self.consume(src.len());
            Ok(())
        } else {
            default_read_exact(self, buf)
        }
    }
}

impl<R> BufRead for MultithreadedReader<R>
where
    R: Read + Send + 'static,
{
    fn fill_buf(&mut self) -> io::Result<&[u8]> {
        if !self.buffer.block.data().has_remaining() {
            self.read_block()?;
        }

        Ok(self.buffer.block.data().as_ref())
    }

    fn consume(&mut self, amt: usize) {
        self.buffer.block.data_mut().consume(amt);
    }
}

impl<R> crate::io::Read for MultithreadedReader<R>
where
    R: Read + Send + 'static,
{
    fn virtual_position(&self) -> VirtualPosition {
        self.buffer.block.virtual_position()
    }
}

impl<R> crate::io::BufRead for MultithreadedReader<R> where R: Read + Send + 'static {}

impl<R> crate::io::Seek for MultithreadedReader<R>
where
    R: Read + Send + Seek + 'static,
{
    fn seek_to_virtual_position(&mut self, pos: VirtualPosition) -> io::Result<VirtualPosition> {
        let (cpos, upos) = pos.into();

        self.get_mut().seek(SeekFrom::Start(cpos))?;
        self.position = cpos;

        self.read_block()?;

        self.buffer.block.data_mut().set_position(usize::from(upos));

        Ok(pos)
    }

    fn seek_with_index(&mut self, index: &gzi::Index, pos: SeekFrom) -> io::Result<u64> {
        let SeekFrom::Start(pos) = pos else {
            unimplemented!();
        };

        assert!(!index.is_empty());

        let i = index.partition_point(|r| r.1 <= pos);
        // SAFETY: `i` is > 0.
        let record = index[i - 1];

        let cpos = record.0;
        self.get_mut().seek(SeekFrom::Start(cpos))?;
        self.position = cpos;

        self.read_block()?;

        let upos = usize::try_from(pos - record.1)
            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
        self.buffer.block.data_mut().set_position(upos);

        Ok(pos)
    }
}

fn recv_buffer(read_rx: &ReadRx) -> io::Result<Option<Buffer>> {
    if let Ok(buffered_rx) = read_rx.recv() {
        if let Ok(buffer) = buffered_rx.recv() {
            return buffer.map(Some);
        }
    }

    Ok(None)
}

struct ReadError<R>(R, io::Error);

fn spawn_reader<R>(
    mut reader: R,
    inflate_tx: InflateTx,
    read_tx: ReadTx,
    recycle_rx: RecycleRx,
) -> JoinHandle<Result<R, ReadError<R>>>
where
    R: Read + Send + 'static,
{
    use super::reader::frame::read_frame_into;

    thread::spawn(move || {
        while let Ok(mut buffer) = recycle_rx.recv() {
            match read_frame_into(&mut reader, &mut buffer.buf) {
                Ok(result) if result.is_none() => break,
                Ok(_) => {}
                Err(e) => return Err(ReadError(reader, e)),
            }

            let (buffered_tx, buffered_rx) = crossbeam_channel::bounded(1);

            inflate_tx.send((buffer, buffered_tx)).unwrap();
            read_tx.send(buffered_rx).unwrap();
        }

        Ok(reader)
    })
}

fn spawn_inflaters(worker_count: NonZeroUsize, inflate_rx: InflateRx) -> Vec<JoinHandle<()>> {
    use super::reader::frame::parse_block;

    (0..worker_count.get())
        .map(|_| {
            let inflate_rx = inflate_rx.clone();

            thread::spawn(move || {
                while let Ok((mut buffer, buffered_tx)) = inflate_rx.recv() {
                    let result = parse_block(&buffer.buf, &mut buffer.block).map(|_| buffer);
                    buffered_tx.send(result).unwrap();
                }
            })
        })
        .collect()
}

#[cfg(test)]
mod tests {
    use std::io::Cursor;

    use super::*;

    #[test]
    fn test_seek_to_virtual_position() -> Result<(), Box<dyn std::error::Error>> {
        use crate::io::Seek;

        #[rustfmt::skip]
        static DATA: &[u8] = &[
            // block 0 (b"noodles")
            0x1f, 0x8b, 0x08, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0x06, 0x00, 0x42, 0x43,
            0x02, 0x00, 0x22, 0x00, 0xcb, 0xcb, 0xcf, 0x4f, 0xc9, 0x49, 0x2d, 0x06, 0x00, 0xa1,
            0x58, 0x2a, 0x80, 0x07, 0x00, 0x00, 0x00,
            // EOF block
            0x1f, 0x8b, 0x08, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0x06, 0x00, 0x42, 0x43,
            0x02, 0x00, 0x1b, 0x00, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        ];

        const EOF_VIRTUAL_POSITION: VirtualPosition = match VirtualPosition::new(63, 0) {
            Some(pos) => pos,
            None => unreachable!(),
        };

        const VIRTUAL_POSITION: VirtualPosition = match VirtualPosition::new(0, 3) {
            Some(pos) => pos,
            None => unreachable!(),
        };

        let mut reader =
            MultithreadedReader::with_worker_count(NonZeroUsize::MIN, Cursor::new(DATA));

        let mut buf = Vec::new();
        reader.read_to_end(&mut buf)?;

        assert_eq!(reader.virtual_position(), EOF_VIRTUAL_POSITION);

        reader.seek_to_virtual_position(VIRTUAL_POSITION)?;

        buf.clear();
        reader.read_to_end(&mut buf)?;

        assert_eq!(buf, b"dles");
        assert_eq!(reader.virtual_position(), EOF_VIRTUAL_POSITION);

        Ok(())
    }
}