// Copyright (c) The partial-io Contributors
// SPDX-License-Identifier: MIT

//! This module contains an `AsyncRead` wrapper that breaks its inputs up
//! according to a provided iterator.
//!
//! This is separate from `PartialWrite` because on `WouldBlock` errors, it
//! causes `futures` to try writing or flushing again.

use crate::{futures_util::FuturesOps, PartialOp};
use futures::prelude::*;
use pin_project::pin_project;
use std::{
    fmt, io,
    pin::Pin,
    task::{Context, Poll},
};

/// A wrapper that breaks inner `AsyncRead` instances up according to the
/// provided iterator.
///
/// Available with the `futures03` feature for `futures` traits, and with the `tokio1` feature for
/// `tokio` traits.
///
/// # Examples
///
/// This example uses `tokio`.
///
/// ```rust
/// # #[cfg(feature = "tokio1")]
/// use partial_io::{PartialAsyncRead, PartialOp};
/// # #[cfg(feature = "tokio1")]
/// use std::io::{self, Cursor};
/// # #[cfg(feature = "tokio1")]
/// use tokio::io::AsyncReadExt;
///
/// # #[cfg(feature = "tokio1")]
/// #[tokio::main]
/// async fn main() -> io::Result<()> {
///     let reader = Cursor::new(vec![1, 2, 3, 4]);
///     // Sequential calls to `poll_read()` and the other `poll_` methods simulate the following behavior:
///     let iter = vec![
///         PartialOp::Err(io::ErrorKind::WouldBlock),   // A not-ready state.
///         PartialOp::Limited(2),                       // Only allow 2 bytes to be read.
///         PartialOp::Err(io::ErrorKind::InvalidData),  // Error from the underlying stream.
///         PartialOp::Unlimited,                        // Allow as many bytes to be read as possible.
///     ];
///     let mut partial_reader = PartialAsyncRead::new(reader, iter);
///     let mut out = vec![0; 256];
///
///     // This causes poll_read to be called twice, yielding after the first call (WouldBlock).
///     assert_eq!(partial_reader.read(&mut out).await?, 2, "first read with Limited(2)");
///     assert_eq!(&out[..4], &[1, 2, 0, 0]);
///
///     // This next call returns an error.
///     assert_eq!(
///         partial_reader.read(&mut out[2..]).await.unwrap_err().kind(),
///         io::ErrorKind::InvalidData,
///     );
///
///     // And this one causes the last two bytes to be written.
///     assert_eq!(partial_reader.read(&mut out[2..]).await?, 2, "second read with Unlimited");
///     assert_eq!(&out[..4], &[1, 2, 3, 4]);
///
///     Ok(())
/// }
///
/// # #[cfg(not(feature = "tokio1"))]
/// # fn main() {
/// #     assert!(true, "dummy test");
/// # }
/// ```
#[pin_project]
pub struct PartialAsyncRead<R> {
    #[pin]
    inner: R,
    ops: FuturesOps,
}

impl<R> PartialAsyncRead<R> {
    /// Creates a new `PartialAsyncRead` wrapper over the reader with the specified `PartialOp`s.
    pub fn new<I>(inner: R, iter: I) -> Self
    where
        I: IntoIterator<Item = PartialOp> + 'static,
        I::IntoIter: Send,
    {
        PartialAsyncRead {
            inner,
            ops: FuturesOps::new(iter),
        }
    }

    /// Sets the `PartialOp`s for this reader.
    pub fn set_ops<I>(&mut self, iter: I) -> &mut Self
    where
        I: IntoIterator<Item = PartialOp> + 'static,
        I::IntoIter: Send,
    {
        self.ops.replace(iter);
        self
    }

    /// Sets the `PartialOp`s for this reader in a pinned context.
    pub fn pin_set_ops<I>(self: Pin<&mut Self>, iter: I) -> Pin<&mut Self>
    where
        I: IntoIterator<Item = PartialOp> + 'static,
        I::IntoIter: Send,
    {
        let mut this = self;
        this.as_mut().project().ops.replace(iter);
        this
    }

    /// Returns a shared reference to the underlying reader.
    pub fn get_ref(&self) -> &R {
        &self.inner
    }

    /// Returns a mutable reference to the underlying reader.
    pub fn get_mut(&mut self) -> &mut R {
        &mut self.inner
    }

    /// Returns a pinned mutable reference to the underlying reader.
    pub fn pin_get_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
        self.project().inner
    }

    /// Consumes this wrapper, returning the underlying reader.
    pub fn into_inner(self) -> R {
        self.inner
    }
}

// ---
// Futures impls
// ---

impl<R> AsyncRead for PartialAsyncRead<R>
where
    R: AsyncRead,
{
    #[inline]
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        let this = self.project();
        let inner = this.inner;
        let len = buf.len();

        this.ops.poll_impl(
            cx,
            |cx, len| match len {
                Some(len) => inner.poll_read(cx, &mut buf[..len]),
                None => inner.poll_read(cx, buf),
            },
            len,
            "error during poll_read, generated by partial-io",
        )
    }

    // TODO: do we need to implement poll_read_vectored? It's a bit tricky to do.
}

impl<R> AsyncBufRead for PartialAsyncRead<R>
where
    R: AsyncBufRead,
{
    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<&[u8]>> {
        let this = self.project();
        let inner = this.inner;

        this.ops.poll_impl_no_limit(
            cx,
            |cx| inner.poll_fill_buf(cx),
            "error during poll_read, generated by partial-io",
        )
    }

    #[inline]
    fn consume(self: Pin<&mut Self>, amt: usize) {
        self.project().inner.consume(amt)
    }
}

/// This is a forwarding impl to support duplex structs.
impl<R> AsyncWrite for PartialAsyncRead<R>
where
    R: AsyncWrite,
{
    fn poll_write(self: Pin<&mut Self>, cx: &mut Context, buf: &[u8]) -> Poll<io::Result<usize>> {
        self.project().inner.poll_write(cx, buf)
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
        self.project().inner.poll_flush(cx)
    }

    fn poll_close(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
        self.project().inner.poll_close(cx)
    }
}

/// This is a forwarding impl to support duplex structs.
impl<R> AsyncSeek for PartialAsyncRead<R>
where
    R: AsyncSeek,
{
    #[inline]
    fn poll_seek(
        self: Pin<&mut Self>,
        cx: &mut Context,
        pos: io::SeekFrom,
    ) -> Poll<io::Result<u64>> {
        self.project().inner.poll_seek(cx, pos)
    }
}

// ---
// Tokio impls
// ---

#[cfg(feature = "tokio1")]
pub(crate) mod tokio_impl {
    use super::PartialAsyncRead;
    use std::{
        io::{self, SeekFrom},
        pin::Pin,
        task::{Context, Poll},
    };
    use tokio::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};

    impl<R> AsyncRead for PartialAsyncRead<R>
    where
        R: AsyncRead,
    {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            let this = self.project();
            let inner = this.inner;
            let capacity = buf.capacity();

            this.ops.poll_impl(
                cx,
                |cx, len| match len {
                    Some(len) => {
                        buf.with_limited(len, |limited_buf| inner.poll_read(cx, limited_buf))
                    }
                    None => inner.poll_read(cx, buf),
                },
                capacity,
                "error during poll_read, generated by partial-io",
            )
        }
    }

    /// Extensions to `tokio`'s `ReadBuf`.
    ///
    /// Requires the `tokio1` feature to be enabled.
    pub trait ReadBufExt {
        /// Convert this `ReadBuf` into a limited one backed by the same storage, then
        /// call the callback with this limited instance..
        ///
        /// Any changes to the `ReadBuf` made by the callback are reflected in the original
        /// `ReadBuf`.
        fn with_limited<F, T>(&mut self, limit: usize, callback: F) -> T
        where
            F: FnOnce(&mut ReadBuf<'_>) -> T;
    }

    impl<'a> ReadBufExt for ReadBuf<'a> {
        fn with_limited<F, T>(&mut self, limit: usize, callback: F) -> T
        where
            F: FnOnce(&mut ReadBuf<'_>) -> T,
        {
            // Use limit to set upper limits on the capacity and both cursors.
            let capacity_limit = self.capacity().min(limit);
            let old_initialized_len = self.initialized().len().min(limit);
            let old_filled_len = self.filled().len().min(limit);

            // SAFETY: We assume that the input buf's initialized length is trustworthy.
            let mut limited_buf = unsafe {
                let inner_mut = &mut self.inner_mut()[..capacity_limit];
                let mut limited_buf = ReadBuf::uninit(inner_mut);
                // Note: assume_init adds the passed-in value to self.filled, but for a freshly created
                // uninitialized buffer, self.filled is 0. The value of filled is updated below
                // with the set_filled() call.
                limited_buf.assume_init(old_initialized_len);
                limited_buf
            };
            limited_buf.set_filled(old_filled_len);

            // Call the callback.
            let ret = callback(&mut limited_buf);

            // The callback may have modified the cursors in `limited_buf` -- if so, port them back to
            // the original.
            let new_initialized_len = limited_buf.initialized().len();
            let new_filled_len = limited_buf.filled().len();

            if new_initialized_len > old_initialized_len {
                // SAFETY: We assume that if new_initialized_len > old_initialized_len, that
                // the extra bytes were initialized by the callback.
                unsafe {
                    // Note: assume_init adds the passed-in value to buf.filled.len().
                    self.assume_init(new_initialized_len - self.filled().len());
                }
            }

            if new_filled_len != old_filled_len {
                // This can happen if either:
                // * old_filled_len < limit, and the callback filled some more bytes into buf ->
                //   reflect that in the original buffer.
                // * old_filled_len <= limit, and the callback *shortened* the filled bytes -> reflect
                //   that in the original buffer as well.
                //
                // (Note if old_filled_len == limit, then new_filled_len cannot be greater than
                // old_filled_len since it's at the limit already.)
                self.set_filled(new_filled_len);
            }

            ret
        }
    }

    impl<R> AsyncBufRead for PartialAsyncRead<R>
    where
        R: AsyncBufRead,
    {
        fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
            let this = self.project();
            let inner = this.inner;

            this.ops.poll_impl_no_limit(
                cx,
                |cx| inner.poll_fill_buf(cx),
                "error during poll_fill_buf, generated by partial-io",
            )
        }

        fn consume(self: Pin<&mut Self>, amt: usize) {
            self.project().inner.consume(amt)
        }
    }

    /// This is a forwarding impl to support duplex structs.
    impl<R> AsyncWrite for PartialAsyncRead<R>
    where
        R: AsyncWrite,
    {
        #[inline]
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context,
            buf: &[u8],
        ) -> Poll<io::Result<usize>> {
            self.project().inner.poll_write(cx, buf)
        }

        #[inline]
        fn poll_flush(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
            self.project().inner.poll_flush(cx)
        }

        #[inline]
        fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context) -> Poll<io::Result<()>> {
            self.project().inner.poll_shutdown(cx)
        }
    }

    /// This is a forwarding impl to support duplex structs.
    impl<R> AsyncSeek for PartialAsyncRead<R>
    where
        R: AsyncSeek,
    {
        #[inline]
        fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> io::Result<()> {
            self.project().inner.start_seek(position)
        }

        #[inline]
        fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
            self.project().inner.poll_complete(cx)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use itertools::Itertools;
        use std::mem::MaybeUninit;

        // with_limited is pretty complex: test that it works properly.
        #[test]
        fn test_with_limited() {
            const CAPACITY: usize = 256;

            let inputs = vec![
                // Columns are (filled, initialized). The capacity is always 256.

                // Fully filled, fully initialized buffer.
                (256, 256),
                // Partly filled, fully initialized buffer.
                (64, 256),
                // Unfilled, fully initialized buffer.
                (0, 256),
                // Fully filled, partly initialized buffer.
                (128, 128),
                // Partly filled, partly initialized buffer.
                (64, 128),
                // Unfilled, partly initialized buffer.
                (0, 128),
                // Unfilled, uninitialized buffer.
                (0, 0),
            ];
            // Test a series of limits for every possible case.
            let limits = vec![0, 32, 64, 128, 192, 256, 384];

            for ((filled, initialized), limit) in inputs.into_iter().cartesian_product(limits) {
                // Create an uninitialized array of `MaybeUninit` for storage. The `assume_init` is
                // safe because the type we are claiming to have initialized here is a
                // bunch of `MaybeUninit`s, which do not require initialization.
                let mut storage: [MaybeUninit<u8>; CAPACITY] =
                    unsafe { MaybeUninit::uninit().assume_init() };
                let mut buf = ReadBuf::uninit(&mut storage);
                buf.initialize_unfilled_to(initialized);
                buf.set_filled(filled);

                println!("*** limit = {}, original buf = {:?}", limit, buf);

                // ---
                // Test that making no changes to the limited buffer causes no changes to the
                // original buffer.
                // ---
                buf.with_limited(limit, |limited_buf| {
                    println!("  * do-nothing: limited buf = {:?}", limited_buf);
                    assert!(
                        limited_buf.capacity() <= limit,
                        "limit is applied to capacity"
                    );
                    assert!(
                        limited_buf.initialized().len() <= limit,
                        "limit is applied to initialized len"
                    );
                    assert!(
                        limited_buf.filled().len() <= limit,
                        "limit is applied to filled len"
                    );
                });

                assert_eq!(
                    buf.filled().len(),
                    filled,
                    "do-nothing -> filled is the same as before"
                );
                assert_eq!(
                    buf.initialized().len(),
                    initialized,
                    "do-nothing -> initialized is the same as before"
                );

                // ---
                // Test that set_filled with a smaller value is reflected in the original buffer.
                // ---
                let new_filled = buf.with_limited(limit, |limited_buf| {
                    println!("  * halve-filled: limited buf = {:?}", limited_buf);
                    let new_filled = limited_buf.filled().len() / 2;
                    limited_buf.set_filled(new_filled);
                    println!("  * halve-filled: after = {:?}", limited_buf);
                    new_filled
                });

                match new_filled.cmp(&limit) {
                    std::cmp::Ordering::Less => {
                        assert_eq!(
                            buf.filled().len(),
                            new_filled,
                            "halve-filled, new filled < limit -> filled is updated"
                        );
                    }
                    std::cmp::Ordering::Equal => {
                        assert_eq!(limit, 0, "halve-filled, new filled == limit -> limit = 0");
                        assert_eq!(
                            buf.filled().len(),
                            filled,
                            "halve-filled, new filled == limit -> filled stays the same"
                        );
                    }
                    std::cmp::Ordering::Greater => {
                        panic!("new_filled {} must be <= limit {}", new_filled, limit);
                    }
                }

                assert_eq!(
                    buf.initialized().len(),
                    initialized,
                    "halve-filled -> initialized is same as before"
                );

                // ---
                // Test that pushing a single byte is reflected in the original buffer.
                // ---
                if filled < limit.min(CAPACITY) {
                    // Reset the ReadBuf.
                    let mut storage: [MaybeUninit<u8>; CAPACITY] =
                        unsafe { MaybeUninit::uninit().assume_init() };
                    let mut buf = ReadBuf::uninit(&mut storage);
                    buf.initialize_unfilled_to(initialized);
                    buf.set_filled(filled);

                    buf.with_limited(limit, |limited_buf| {
                        println!("  * push-one-byte: limited buf = {:?}", limited_buf);
                        limited_buf.put_slice(&[42]);
                        println!("  * push-one-byte: after = {:?}", limited_buf);
                    });

                    assert_eq!(
                        buf.filled().len(),
                        filled + 1,
                        "push-one-byte, filled incremented by 1"
                    );
                    assert_eq!(
                        buf.filled()[filled],
                        42,
                        "push-one-byte, correct byte was pushed"
                    );
                    if filled == initialized {
                        assert_eq!(
                            buf.initialized().len(),
                            initialized + 1,
                            "push-one-byte, filled == initialized -> initialized incremented by 1"
                        );
                    } else {
                        assert_eq!(
                            buf.initialized().len(),
                            initialized,
                            "push-one-byte, filled < initialized -> initialized stays the same"
                        );
                    }
                }

                // ---
                // Test that initializing unfilled bytes is reflected in the original buffer.
                // ---
                if initialized <= limit.min(CAPACITY) {
                    // Reset the ReadBuf.
                    let mut storage: [MaybeUninit<u8>; CAPACITY] =
                        unsafe { MaybeUninit::uninit().assume_init() };
                    let mut buf = ReadBuf::uninit(&mut storage);
                    buf.initialize_unfilled_to(initialized);
                    buf.set_filled(filled);

                    buf.with_limited(limit, |limited_buf| {
                        println!("  * initialize-unfilled: limited buf = {:?}", limited_buf);
                        limited_buf.initialize_unfilled();
                        println!("  * initialize-unfilled: after = {:?}", limited_buf);
                    });

                    assert_eq!(
                        buf.filled().len(),
                        filled,
                        "initialize-unfilled, filled stays the same"
                    );
                    assert_eq!(
                        buf.initialized().len(),
                        limit.min(CAPACITY),
                        "initialize-unfilled, initialized is capped at the limit"
                    );
                    // Actually access the bytes and ensure this doesn't crash.
                    assert_eq!(
                        buf.initialized(),
                        vec![0; buf.initialized().len()],
                        "initialize-unfilled, bytes are correct"
                    );
                }
            }
        }
    }
}

impl<R> fmt::Debug for PartialAsyncRead<R>
where
    R: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("PartialAsyncRead")
            .field("inner", &self.inner)
            .finish()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    use std::fs::File;

    use crate::tests::assert_send;

    #[test]
    fn test_sendable() {
        assert_send::<PartialAsyncRead<File>>();
    }
}