use std::{
    fmt, io,
    pin::Pin,
    task::{Context, Poll},
};

use bitflags::bitflags;
use bytes::{Buf, BytesMut};
use futures_core::{ready, Stream};
use futures_sink::Sink;
use pin_project_lite::pin_project;

use crate::{AsyncRead, AsyncWrite, Decoder, Encoder};

/// Low-water mark
const LW: usize = 1024;
/// High-water mark
const HW: usize = 8 * 1024;

bitflags! {
    struct Flags: u8 {
        const EOF = 0b0001;
        const READABLE = 0b0010;
    }
}

pin_project! {
    /// A unified `Stream` and `Sink` interface to an underlying I/O object, using the `Encoder` and
    /// `Decoder` traits to encode and decode frames.
    ///
    /// Raw I/O objects work with byte sequences, but higher-level code usually wants to batch these
    /// into meaningful chunks, called "frames". This method layers framing on top of an I/O object,
    /// by using the `Encoder`/`Decoder` traits to handle encoding and decoding of message frames.
    /// Note that the incoming and outgoing frame types may be distinct.
    pub struct Framed<T, U> {
        #[pin]
        io: T,
        codec: U,
        flags: Flags,
        read_buf: BytesMut,
        write_buf: BytesMut,
    }
}

impl<T, U> Framed<T, U>
where
    T: AsyncRead + AsyncWrite,
    U: Decoder,
{
    /// This function returns a *single* object that is both `Stream` and `Sink`; grouping this into
    /// a single object is often useful for layering things like gzip or TLS, which require both
    /// read and write access to the underlying object.
    pub fn new(io: T, codec: U) -> Framed<T, U> {
        Framed {
            io,
            codec,
            flags: Flags::empty(),
            read_buf: BytesMut::with_capacity(HW),
            write_buf: BytesMut::with_capacity(HW),
        }
    }
}

impl<T, U> Framed<T, U> {
    /// Returns a reference to the underlying codec.
    pub fn codec_ref(&self) -> &U {
        &self.codec
    }

    /// Returns a mutable reference to the underlying codec.
    pub fn codec_mut(&mut self) -> &mut U {
        &mut self.codec
    }

    /// Returns a reference to the underlying I/O stream wrapped by `Frame`.
    ///
    /// Note that care should be taken to not tamper with the underlying stream of data coming in as
    /// it may corrupt the stream of frames otherwise being worked with.
    pub fn io_ref(&self) -> &T {
        &self.io
    }

    /// Returns a mutable reference to the underlying I/O stream.
    ///
    /// Note that care should be taken to not tamper with the underlying stream of data coming in as
    /// it may corrupt the stream of frames otherwise being worked with.
    pub fn io_mut(&mut self) -> &mut T {
        &mut self.io
    }

    /// Returns a `Pin` of a mutable reference to the underlying I/O stream.
    pub fn io_pin(self: Pin<&mut Self>) -> Pin<&mut T> {
        self.project().io
    }

    /// Check if read buffer is empty.
    pub fn is_read_buf_empty(&self) -> bool {
        self.read_buf.is_empty()
    }

    /// Check if write buffer is empty.
    pub fn is_write_buf_empty(&self) -> bool {
        self.write_buf.is_empty()
    }

    /// Check if write buffer is full.
    pub fn is_write_buf_full(&self) -> bool {
        self.write_buf.len() >= HW
    }

    /// Check if framed is able to write more data.
    ///
    /// `Framed` object considers ready if there is free space in write buffer.
    pub fn is_write_ready(&self) -> bool {
        self.write_buf.len() < HW
    }

    /// Consume the `Frame`, returning `Frame` with different codec.
    pub fn replace_codec<U2>(self, codec: U2) -> Framed<T, U2> {
        Framed {
            codec,
            io: self.io,
            flags: self.flags,
            read_buf: self.read_buf,
            write_buf: self.write_buf,
        }
    }

    /// Consume the `Frame`, returning `Frame` with different io.
    pub fn into_map_io<F, T2>(self, f: F) -> Framed<T2, U>
    where
        F: Fn(T) -> T2,
    {
        Framed {
            io: f(self.io),
            codec: self.codec,
            flags: self.flags,
            read_buf: self.read_buf,
            write_buf: self.write_buf,
        }
    }

    /// Consume the `Frame`, returning `Frame` with different codec.
    pub fn into_map_codec<F, U2>(self, f: F) -> Framed<T, U2>
    where
        F: Fn(U) -> U2,
    {
        Framed {
            io: self.io,
            codec: f(self.codec),
            flags: self.flags,
            read_buf: self.read_buf,
            write_buf: self.write_buf,
        }
    }
}

impl<T, U> Framed<T, U> {
    /// Serialize item and write to the inner buffer
    pub fn write<I>(mut self: Pin<&mut Self>, item: I) -> Result<(), <U as Encoder<I>>::Error>
    where
        T: AsyncWrite,
        U: Encoder<I>,
    {
        let this = self.as_mut().project();
        let remaining = this.write_buf.capacity() - this.write_buf.len();
        if remaining < LW {
            this.write_buf.reserve(HW - remaining);
        }

        this.codec.encode(item, this.write_buf)?;
        Ok(())
    }

    /// Try to read underlying I/O stream and decode item.
    pub fn next_item(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<<U as Decoder>::Item, U::Error>>>
    where
        T: AsyncRead,
        U: Decoder,
    {
        loop {
            let this = self.as_mut().project();
            // Repeatedly call `decode` or `decode_eof` as long as it is "readable". Readable is
            // defined as not having returned `None`. If the upstream has returned EOF, and the
            // decoder is no longer readable, it can be assumed that the decoder will never become
            // readable again, at which point the stream is terminated.

            if this.flags.contains(Flags::READABLE) {
                if this.flags.contains(Flags::EOF) {
                    match this.codec.decode_eof(this.read_buf) {
                        Ok(Some(frame)) => return Poll::Ready(Some(Ok(frame))),
                        Ok(None) => return Poll::Ready(None),
                        Err(err) => return Poll::Ready(Some(Err(err))),
                    }
                }

                log::trace!("attempting to decode a frame");

                match this.codec.decode(this.read_buf) {
                    Ok(Some(frame)) => {
                        log::trace!("frame decoded from buffer");
                        return Poll::Ready(Some(Ok(frame)));
                    }
                    Err(err) => return Poll::Ready(Some(Err(err))),
                    _ => (), // Need more data
                }

                this.flags.remove(Flags::READABLE);
            }

            debug_assert!(!this.flags.contains(Flags::EOF));

            // Otherwise, try to read more data and try again. Make sure we've got room.
            let remaining = this.read_buf.capacity() - this.read_buf.len();
            if remaining < LW {
                this.read_buf.reserve(HW - remaining)
            }

            let cnt = match tokio_util::io::poll_read_buf(this.io, cx, this.read_buf) {
                Poll::Pending => return Poll::Pending,
                Poll::Ready(Err(err)) => return Poll::Ready(Some(Err(err.into()))),
                Poll::Ready(Ok(cnt)) => cnt,
            };

            if cnt == 0 {
                this.flags.insert(Flags::EOF);
            }
            this.flags.insert(Flags::READABLE);
        }
    }

    /// Flush write buffer to underlying I/O stream.
    pub fn flush<I>(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), U::Error>>
    where
        T: AsyncWrite,
        U: Encoder<I>,
    {
        let mut this = self.as_mut().project();
        log::trace!("flushing framed transport");

        while !this.write_buf.is_empty() {
            log::trace!("writing; remaining={}", this.write_buf.len());

            let n = ready!(this.io.as_mut().poll_write(cx, this.write_buf))?;

            if n == 0 {
                return Poll::Ready(Err(io::Error::new(
                    io::ErrorKind::WriteZero,
                    "failed to write frame to transport",
                )
                .into()));
            }

            // remove written data
            this.write_buf.advance(n);
        }

        // Try flushing the underlying IO
        ready!(this.io.poll_flush(cx))?;

        log::trace!("framed transport flushed");
        Poll::Ready(Ok(()))
    }

    /// Flush write buffer and shutdown underlying I/O stream.
    pub fn close<I>(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), U::Error>>
    where
        T: AsyncWrite,
        U: Encoder<I>,
    {
        let mut this = self.as_mut().project();
        ready!(this.io.as_mut().poll_flush(cx))?;
        ready!(this.io.as_mut().poll_shutdown(cx))?;
        Poll::Ready(Ok(()))
    }
}

impl<T, U> Stream for Framed<T, U>
where
    T: AsyncRead,
    U: Decoder,
{
    type Item = Result<U::Item, U::Error>;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        self.next_item(cx)
    }
}

impl<T, U, I> Sink<I> for Framed<T, U>
where
    T: AsyncWrite,
    U: Encoder<I>,
    U::Error: From<io::Error>,
{
    type Error = U::Error;

    fn poll_ready(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        if self.is_write_ready() {
            Poll::Ready(Ok(()))
        } else {
            self.flush(cx)
        }
    }

    fn start_send(self: Pin<&mut Self>, item: I) -> Result<(), Self::Error> {
        self.write(item)
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.flush(cx)
    }

    fn poll_close(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.close(cx)
    }
}

impl<T, U> fmt::Debug for Framed<T, U>
where
    T: fmt::Debug,
    U: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Framed")
            .field("io", &self.io)
            .field("codec", &self.codec)
            .finish()
    }
}

impl<T, U> Framed<T, U> {
    /// This function returns a *single* object that is both `Stream` and `Sink`; grouping this into
    /// a single object is often useful for layering things like gzip or TLS, which require both
    /// read and write access to the underlying object.
    ///
    /// These objects take a stream, a read buffer and a write buffer. These fields can be obtained
    /// from an existing `Framed` with the `into_parts` method.
    pub fn from_parts(parts: FramedParts<T, U>) -> Framed<T, U> {
        Framed {
            io: parts.io,
            codec: parts.codec,
            flags: parts.flags,
            write_buf: parts.write_buf,
            read_buf: parts.read_buf,
        }
    }

    /// Consumes the `Frame`, returning its underlying I/O stream, the buffer with unprocessed data,
    /// and the codec.
    ///
    /// Note that care should be taken to not tamper with the underlying stream of data coming in as
    /// it may corrupt the stream of frames otherwise being worked with.
    pub fn into_parts(self) -> FramedParts<T, U> {
        FramedParts {
            io: self.io,
            codec: self.codec,
            flags: self.flags,
            read_buf: self.read_buf,
            write_buf: self.write_buf,
        }
    }
}

/// `FramedParts` contains an export of the data of a Framed transport.
///
/// It can be used to construct a new `Framed` with a different codec. It contains all current
/// buffers and the inner transport.
#[derive(Debug)]
pub struct FramedParts<T, U> {
    /// The inner transport used to read bytes to and write bytes to.
    pub io: T,

    /// The codec object.
    pub codec: U,

    /// The buffer with read but unprocessed data.
    pub read_buf: BytesMut,

    /// A buffer with unprocessed data which are not written yet.
    pub write_buf: BytesMut,

    flags: Flags,
}

impl<T, U> FramedParts<T, U> {
    /// Creates a new default `FramedParts`.
    pub fn new(io: T, codec: U) -> FramedParts<T, U> {
        FramedParts {
            io,
            codec,
            flags: Flags::empty(),
            read_buf: BytesMut::new(),
            write_buf: BytesMut::new(),
        }
    }

    /// Creates a new `FramedParts` with read buffer.
    pub fn with_read_buf(io: T, codec: U, read_buf: BytesMut) -> FramedParts<T, U> {
        FramedParts {
            io,
            codec,
            read_buf,
            flags: Flags::empty(),
            write_buf: BytesMut::new(),
        }
    }
}