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// Copyright 2019 Tokio Contributors.
// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License in the LICENSE file at the
// root of this repository, or online at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Portions of this file are derived from the ReadExact combinator in the Tokio
// project. The original source code was retrieved on March 1, 2019 from:
//
//     https://github.com/tokio-rs/tokio/blob/195c4b04963742ecfff202ee9d0b72cc923aee81/tokio-io/src/io/read_exact.rs
//
// The original source code is subject to the terms of the MIT license, a copy
// of which can be found in the LICENSE file at the root of this repository.

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

use futures::ready;
use tokio::io::{self, AsyncRead, ReadBuf};

/// A future which reads exactly enough bytes to fill a buffer, unless EOF is
/// reached first.
///
/// Create a `ReadExactOrEof` struct by calling the [`read_exact_or_eof`]
/// function.
#[derive(Debug)]
pub struct ReadExactOrEof<'a, A> {
    reader: &'a mut A,
    buf: &'a mut [u8],
    pos: usize,
}

/// Creates a future which will read exactly enough bytes to fill `buf`, unless
/// EOF is reached first. If a short read should be considered an error, use
/// [`tokio::io::AsyncReadExt::read_exact`] instead.
///
/// The returned future will resolve to the number of bytes read.
///
/// In the case of an error the contents of the buffer are unspecified.
pub fn read_exact_or_eof<'a, A>(reader: &'a mut A, buf: &'a mut [u8]) -> ReadExactOrEof<'a, A>
where
    A: AsyncRead,
{
    ReadExactOrEof {
        reader,
        buf,
        pos: 0,
    }
}

impl<A> Future for ReadExactOrEof<'_, A>
where
    A: AsyncRead + Unpin,
{
    type Output = io::Result<usize>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
        while self.pos < self.buf.len() {
            let me = &mut *self;
            let mut buf = ReadBuf::new(&mut me.buf[me.pos..]);
            ready!(Pin::new(&mut me.reader).poll_read(cx, &mut buf))?;
            me.pos += buf.filled().len();
            if buf.filled().len() == 0 {
                break;
            }
        }
        Poll::Ready(Ok(self.pos))
    }
}