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    use crate::fmt;
    use crate::io::{
        self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
    };

    /// Wraps a writer and buffers its output.
    ///
    /// It can be excessively inefficient to work directly with something that
    /// implements [`Write`]. For example, every call to
    /// [`write`][`TcpStream::write`] on [`TcpStream`] results in a system call. A
    /// `BufWriter<W>` keeps an in-memory buffer of data and writes it to an underlying
    /// writer in large, infrequent batches.
    ///
    /// `BufWriter<W>` can improve the speed of programs that make *small* and
    /// *repeated* write calls to the same file or network socket. It does not
    /// help when writing very large amounts at once, or writing just one or a few
    /// times. It also provides no advantage when writing to a destination that is
    /// in memory, like a [`Vec`]`<u8>`.
    ///
    /// It is critical to call [`flush`] before `BufWriter<W>` is dropped. Though
    /// dropping will attempt to flush the contents of the buffer, any errors
    /// that happen in the process of dropping will be ignored. Calling [`flush`]
    /// ensures that the buffer is empty and thus dropping will not even attempt
    /// file operations.
    ///
    /// # Examples
    ///
    /// Let's write the numbers one through ten to a [`TcpStream`]:
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::net::TcpStream;
    ///
    /// let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap();
    ///
    /// for i in 0..10 {
    ///     stream.write(&[i+1]).unwrap();
    /// }
    /// ```
    ///
    /// Because we're not buffering, we write each one in turn, incurring the
    /// overhead of a system call per byte written. We can fix this with a
    /// `BufWriter<W>`:
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::io::BufWriter;
    /// use std::net::TcpStream;
    ///
    /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
    ///
    /// for i in 0..10 {
    ///     stream.write(&[i+1]).unwrap();
    /// }
    /// stream.flush().unwrap();
    /// ```
    ///
    /// By wrapping the stream with a `BufWriter<W>`, these ten writes are all grouped
    /// together by the buffer and will all be written out in one system call when
    /// the `stream` is flushed.
    ///
    // HACK(#78696): can't use `crate` for associated items
    /// [`TcpStream::write`]: super::super::super::net::TcpStream::write
    /// [`TcpStream`]: crate::net::TcpStream
    /// [`flush`]: BufWriter::flush
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BufWriter<W: Write> {
        inner: Option<W>,
        buf: Vec<u8>,
        // #30888: If the inner writer panics in a call to write, we don't want to
        // write the buffered data a second time in BufWriter's destructor. This
        // flag tells the Drop impl if it should skip the flush.
        panicked: bool,
    }

    impl<W: Write> BufWriter<W> {
        /// Creates a new `BufWriter<W>` with a default buffer capacity. The default is currently 8 KB,
        /// but may change in the future.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: W) -> BufWriter<W> {
            BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner)
        }

        /// Creates a new `BufWriter<W>` with the specified buffer capacity.
        ///
        /// # Examples
        ///
        /// Creating a buffer with a buffer of a hundred bytes.
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let stream = TcpStream::connect("127.0.0.1:34254").unwrap();
        /// let mut buffer = BufWriter::with_capacity(100, stream);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> {
            BufWriter { inner: Some(inner), buf: Vec::with_capacity(capacity), panicked: false }
        }

        /// Send data in our local buffer into the inner writer, looping as
        /// necessary until either it's all been sent or an error occurs.
        ///
        /// Because all the data in the buffer has been reported to our owner as
        /// "successfully written" (by returning nonzero success values from
        /// `write`), any 0-length writes from `inner` must be reported as i/o
        /// errors from this method.
        pub(super) fn flush_buf(&mut self) -> io::Result<()> {
            /// Helper struct to ensure the buffer is updated after all the writes
            /// are complete. It tracks the number of written bytes and drains them
            /// all from the front of the buffer when dropped.
            struct BufGuard<'a> {
                buffer: &'a mut Vec<u8>,
                written: usize,
            }

            impl<'a> BufGuard<'a> {
                fn new(buffer: &'a mut Vec<u8>) -> Self {
                    Self { buffer, written: 0 }
                }

                /// The unwritten part of the buffer
                fn remaining(&self) -> &[u8] {
                    &self.buffer[self.written..]
                }

                /// Flag some bytes as removed from the front of the buffer
                fn consume(&mut self, amt: usize) {
                    self.written += amt;
                }

                /// true if all of the bytes have been written
                fn done(&self) -> bool {
                    self.written >= self.buffer.len()
                }
            }

            impl Drop for BufGuard<'_> {
                fn drop(&mut self) {
                    if self.written > 0 {
                        self.buffer.drain(..self.written);
                    }
                }
            }

            let mut guard = BufGuard::new(&mut self.buf);
            let inner = self.inner.as_mut().unwrap();
            while !guard.done() {
                self.panicked = true;
                let r = inner.write(guard.remaining());
                self.panicked = false;

                match r {
                    Ok(0) => {
                        return Err(Error::new(
                            ErrorKind::WriteZero,
                            "failed to write the buffered data",
                        ));
                    }
                    Ok(n) => guard.consume(n),
                    Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
                    Err(e) => return Err(e),
                }
            }
            Ok(())
        }

        /// Buffer some data without flushing it, regardless of the size of the
        /// data. Writes as much as possible without exceeding capacity. Returns
        /// the number of bytes written.
        pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize {
            let available = self.buf.capacity() - self.buf.len();
            let amt_to_buffer = available.min(buf.len());
            self.buf.extend_from_slice(&buf[..amt_to_buffer]);
            amt_to_buffer
        }

        /// Gets a reference to the underlying writer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // we can use reference just like buffer
        /// let reference = buffer.get_ref();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &W {
            self.inner.as_ref().unwrap()
        }

        /// Gets a mutable reference to the underlying writer.
        ///
        /// It is inadvisable to directly write to the underlying writer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // we can use reference just like buffer
        /// let reference = buffer.get_mut();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut W {
            self.inner.as_mut().unwrap()
        }

        /// Returns a reference to the internally buffered data.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // See how many bytes are currently buffered
        /// let bytes_buffered = buf_writer.buffer().len();
        /// ```
        #[stable(feature = "bufreader_buffer", since = "1.37.0")]
        pub fn buffer(&self) -> &[u8] {
            &self.buf
        }

        /// Returns the number of bytes the internal buffer can hold without flushing.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // Check the capacity of the inner buffer
        /// let capacity = buf_writer.capacity();
        /// // Calculate how many bytes can be written without flushing
        /// let without_flush = capacity - buf_writer.buffer().len();
        /// ```
        #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
        pub fn capacity(&self) -> usize {
            self.buf.capacity()
        }

        /// Unwraps this `BufWriter<W>`, returning the underlying writer.
        ///
        /// The buffer is written out before returning the writer.
        ///
        /// # Errors
        ///
        /// An [`Err`] will be returned if an error occurs while flushing the buffer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // unwrap the TcpStream and flush the buffer
        /// let stream = buffer.into_inner().unwrap();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>> {
            match self.flush_buf() {
                Err(e) => Err(IntoInnerError::new(self, e)),
                Ok(()) => Ok(self.inner.take().unwrap()),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Write for BufWriter<W> {
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            if self.buf.len() + buf.len() > self.buf.capacity() {
                self.flush_buf()?;
            }
            // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
            if buf.len() >= self.buf.capacity() {
                self.panicked = true;
                let r = self.get_mut().write(buf);
                self.panicked = false;
                r
            } else {
                self.buf.extend_from_slice(buf);
                Ok(buf.len())
            }
        }

        fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
            // Normally, `write_all` just calls `write` in a loop. We can do better
            // by calling `self.get_mut().write_all()` directly, which avoids
            // round trips through the buffer in the event of a series of partial
            // writes in some circumstances.
            if self.buf.len() + buf.len() > self.buf.capacity() {
                self.flush_buf()?;
            }
            // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
            if buf.len() >= self.buf.capacity() {
                self.panicked = true;
                let r = self.get_mut().write_all(buf);
                self.panicked = false;
                r
            } else {
                self.buf.extend_from_slice(buf);
                Ok(())
            }
        }

        fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
            if self.get_ref().is_write_vectored() {
                let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
                if self.buf.len() + total_len > self.buf.capacity() {
                    self.flush_buf()?;
                }
                if total_len >= self.buf.capacity() {
                    self.panicked = true;
                    let r = self.get_mut().write_vectored(bufs);
                    self.panicked = false;
                    r
                } else {
                    bufs.iter().for_each(|b| self.buf.extend_from_slice(b));
                    Ok(total_len)
                }
            } else {
                let mut iter = bufs.iter();
                let mut total_written = if let Some(buf) = iter.by_ref().find(|&buf| !buf.is_empty()) {
                    // This is the first non-empty slice to write, so if it does
                    // not fit in the buffer, we still get to flush and proceed.
                    if self.buf.len() + buf.len() > self.buf.capacity() {
                        self.flush_buf()?;
                    }
                    if buf.len() >= self.buf.capacity() {
                        // The slice is at least as large as the buffering capacity,
                        // so it's better to write it directly, bypassing the buffer.
                        self.panicked = true;
                        let r = self.get_mut().write(buf);
                        self.panicked = false;
                        return r;
                    } else {
                        self.buf.extend_from_slice(buf);
                        buf.len()
                    }
                } else {
                    return Ok(0);
                };
                debug_assert!(total_written != 0);
                for buf in iter {
                    if self.buf.len() + buf.len() > self.buf.capacity() {
                        break;
                    } else {
                        self.buf.extend_from_slice(buf);
                        total_written += buf.len();
                    }
                }
                Ok(total_written)
            }
        }

        fn is_write_vectored(&self) -> bool {
            true
        }

        fn flush(&mut self) -> io::Result<()> {
            self.flush_buf().and_then(|()| self.get_mut().flush())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> fmt::Debug for BufWriter<W>
    where
        W: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("BufWriter")
                .field("writer", &self.inner.as_ref().unwrap())
                .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write + Seek> Seek for BufWriter<W> {
        /// Seek to the offset, in bytes, in the underlying writer.
        ///
        /// Seeking always writes out the internal buffer before seeking.
        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
            self.flush_buf()?;
            self.get_mut().seek(pos)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Drop for BufWriter<W> {
        fn drop(&mut self) {
            if self.inner.is_some() && !self.panicked {
                // dtors should not panic, so we ignore a failed flush
                let _r = self.flush_buf();
            }
        }
    }