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    use crate::cmp;
    use crate::fmt;
    use crate::io::{self, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, DEFAULT_BUF_SIZE};

    /// The `BufReader<R>` struct adds buffering to any reader.
    ///
    /// It can be excessively inefficient to work directly with a [`Read`] instance.
    /// For example, every call to [`read`][`TcpStream::read`] on [`TcpStream`]
    /// results in a system call. A `BufReader<R>` performs large, infrequent reads on
    /// the underlying [`Read`] and maintains an in-memory buffer of the results.
    ///
    /// `BufReader<R>` can improve the speed of programs that make *small* and
    /// *repeated* read calls to the same file or network socket. It does not
    /// help when reading very large amounts at once, or reading just one or a few
    /// times. It also provides no advantage when reading from a source that is
    /// already in memory, like a [`Vec`]`<u8>`.
    ///
    /// When the `BufReader<R>` is dropped, the contents of its buffer will be
    /// discarded. Creating multiple instances of a `BufReader<R>` on the same
    /// stream can cause data loss. Reading from the underlying reader after
    /// unwrapping the `BufReader<R>` with [`BufReader::into_inner`] can also cause
    /// data loss.
    ///
    // HACK(#78696): can't use `crate` for associated items
    /// [`TcpStream::read`]: super::super::super::net::TcpStream::read
    /// [`TcpStream`]: crate::net::TcpStream
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::io::BufReader;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let f = File::open("log.txt")?;
    ///     let mut reader = BufReader::new(f);
    ///
    ///     let mut line = String::new();
    ///     let len = reader.read_line(&mut line)?;
    ///     println!("First line is {} bytes long", len);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BufReader<R> {
        inner: R,
        buf: Box<[u8]>,
        pos: usize,
        cap: usize,
    }

    impl<R: Read> BufReader<R> {
        /// Creates a new `BufReader<R>` with a default buffer capacity. The default is currently 8 KB,
        /// but may change in the future.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let reader = BufReader::new(f);
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: R) -> BufReader<R> {
            BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
        }

        /// Creates a new `BufReader<R>` with the specified buffer capacity.
        ///
        /// # Examples
        ///
        /// Creating a buffer with ten bytes of capacity:
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let reader = BufReader::with_capacity(10, f);
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
            unsafe {
                let mut buffer = Vec::with_capacity(capacity);
                buffer.set_len(capacity);
                inner.initializer().initialize(&mut buffer);
                BufReader { inner, buf: buffer.into_boxed_slice(), pos: 0, cap: 0 }
            }
        }
    }

    impl<R> BufReader<R> {
        /// Gets a reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f1 = File::open("log.txt")?;
        ///     let reader = BufReader::new(f1);
        ///
        ///     let f2 = reader.get_ref();
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &R {
            &self.inner
        }

        /// Gets a mutable reference to the underlying reader.
        ///
        /// It is inadvisable to directly read from the underlying reader.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f1 = File::open("log.txt")?;
        ///     let mut reader = BufReader::new(f1);
        ///
        ///     let f2 = reader.get_mut();
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut R {
            &mut self.inner
        }

        /// Returns a reference to the internally buffered data.
        ///
        /// Unlike [`fill_buf`], this will not attempt to fill the buffer if it is empty.
        ///
        /// [`fill_buf`]: BufRead::fill_buf
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::{BufReader, BufRead};
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let mut reader = BufReader::new(f);
        ///     assert!(reader.buffer().is_empty());
        ///
        ///     if reader.fill_buf()?.len() > 0 {
        ///         assert!(!reader.buffer().is_empty());
        ///     }
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "bufreader_buffer", since = "1.37.0")]
        pub fn buffer(&self) -> &[u8] {
            &self.buf[self.pos..self.cap]
        }

        /// Returns the number of bytes the internal buffer can hold at once.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::{BufReader, BufRead};
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f = File::open("log.txt")?;
        ///     let mut reader = BufReader::new(f);
        ///
        ///     let capacity = reader.capacity();
        ///     let buffer = reader.fill_buf()?;
        ///     assert!(buffer.len() <= capacity);
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
        pub fn capacity(&self) -> usize {
            self.buf.len()
        }

        /// Unwraps this `BufReader<R>`, returning the underlying reader.
        ///
        /// Note that any leftover data in the internal buffer is lost. Therefore,
        /// a following read from the underlying reader may lead to data loss.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// fn main() -> std::io::Result<()> {
        ///     let f1 = File::open("log.txt")?;
        ///     let reader = BufReader::new(f1);
        ///
        ///     let f2 = reader.into_inner();
        ///     Ok(())
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> R {
            self.inner
        }

        /// Invalidates all data in the internal buffer.
        #[inline]
        fn discard_buffer(&mut self) {
            self.pos = 0;
            self.cap = 0;
        }
    }

    impl<R: Seek> BufReader<R> {
        /// Seeks relative to the current position. If the new position lies within the buffer,
        /// the buffer will not be flushed, allowing for more efficient seeks.
        /// This method does not return the location of the underlying reader, so the caller
        /// must track this information themselves if it is required.
        #[unstable(feature = "bufreader_seek_relative", issue = "31100")]
        pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
            let pos = self.pos as u64;
            if offset < 0 {
                if let Some(new_pos) = pos.checked_sub((-offset) as u64) {
                    self.pos = new_pos as usize;
                    return Ok(());
                }
            } else {
                if let Some(new_pos) = pos.checked_add(offset as u64) {
                    if new_pos <= self.cap as u64 {
                        self.pos = new_pos as usize;
                        return Ok(());
                    }
                }
            }
            self.seek(SeekFrom::Current(offset)).map(drop)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> Read for BufReader<R> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            // If we don't have any buffered data and we're doing a massive read
            // (larger than our internal buffer), bypass our internal buffer
            // entirely.
            if self.pos == self.cap && buf.len() >= self.buf.len() {
                self.discard_buffer();
                return self.inner.read(buf);
            }
            let nread = {
                let mut rem = self.fill_buf()?;
                rem.read(buf)?
            };
            self.consume(nread);
            Ok(nread)
        }

        fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
            let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
            if self.pos == self.cap && total_len >= self.buf.len() {
                self.discard_buffer();
                return self.inner.read_vectored(bufs);
            }
            let nread = {
                let mut rem = self.fill_buf()?;
                rem.read_vectored(bufs)?
            };
            self.consume(nread);
            Ok(nread)
        }

        fn is_read_vectored(&self) -> bool {
            self.inner.is_read_vectored()
        }

        // we can't skip unconditionally because of the large buffer case in read.
        unsafe fn initializer(&self) -> Initializer {
            self.inner.initializer()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> BufRead for BufReader<R> {
        fn fill_buf(&mut self) -> io::Result<&[u8]> {
            // If we've reached the end of our internal buffer then we need to fetch
            // some more data from the underlying reader.
            // Branch using `>=` instead of the more correct `==`
            // to tell the compiler that the pos..cap slice is always valid.
            if self.pos >= self.cap {
                debug_assert!(self.pos == self.cap);
                self.cap = self.inner.read(&mut self.buf)?;
                self.pos = 0;
            }
            Ok(&self.buf[self.pos..self.cap])
        }

        fn consume(&mut self, amt: usize) {
            self.pos = cmp::min(self.pos + amt, self.cap);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R> fmt::Debug for BufReader<R>
    where
        R: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("BufReader")
                .field("reader", &self.inner)
                .field("buffer", &format_args!("{}/{}", self.cap - self.pos, self.buf.len()))
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Seek> Seek for BufReader<R> {
        /// Seek to an offset, in bytes, in the underlying reader.
        ///
        /// The position used for seeking with [`SeekFrom::Current`]`(_)` is the
        /// position the underlying reader would be at if the `BufReader<R>` had no
        /// internal buffer.
        ///
        /// Seeking always discards the internal buffer, even if the seek position
        /// would otherwise fall within it. This guarantees that calling
        /// [`BufReader::into_inner()`] immediately after a seek yields the underlying reader
        /// at the same position.
        ///
        /// To seek without discarding the internal buffer, use [`BufReader::seek_relative`].
        ///
        /// See [`std::io::Seek`] for more details.
        ///
        /// Note: In the edge case where you're seeking with [`SeekFrom::Current`]`(n)`
        /// where `n` minus the internal buffer length overflows an `i64`, two
        /// seeks will be performed instead of one. If the second seek returns
        /// [`Err`], the underlying reader will be left at the same position it would
        /// have if you called `seek` with [`SeekFrom::Current`]`(0)`.
        ///
        /// [`std::io::Seek`]: Seek
        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
            let result: u64;
            if let SeekFrom::Current(n) = pos {
                let remainder = (self.cap - self.pos) as i64;
                // it should be safe to assume that remainder fits within an i64 as the alternative
                // means we managed to allocate 8 exbibytes and that's absurd.
                // But it's not out of the realm of possibility for some weird underlying reader to
                // support seeking by i64::MIN so we need to handle underflow when subtracting
                // remainder.
                if let Some(offset) = n.checked_sub(remainder) {
                    result = self.inner.seek(SeekFrom::Current(offset))?;
                } else {
                    // seek backwards by our remainder, and then by the offset
                    self.inner.seek(SeekFrom::Current(-remainder))?;
                    self.discard_buffer();
                    result = self.inner.seek(SeekFrom::Current(n))?;
                }
            } else {
                // Seeking with Start/End doesn't care about our buffer length.
                result = self.inner.seek(pos)?;
            }
            self.discard_buffer();
            Ok(result)
        }

        /// Returns the current seek position from the start of the stream.
        ///
        /// The value returned is equivalent to `self.seek(SeekFrom::Current(0))`
        /// but does not flush the internal buffer. Due to this optimization the
        /// function does not guarantee that calling `.into_inner()` immediately
        /// afterwards will yield the underlying reader at the same position. Use
        /// [`BufReader::seek`] instead if you require that guarantee.
        ///
        /// # Panics
        ///
        /// This function will panic if the position of the inner reader is smaller
        /// than the amount of buffered data. That can happen if the inner reader
        /// has an incorrect implementation of [`Seek::stream_position`], or if the
        /// position has gone out of sync due to calling [`Seek::seek`] directly on
        /// the underlying reader.
        ///
        /// # Example
        ///
        /// ```no_run
        /// #![feature(seek_convenience)]
        /// use std::{
        ///     io::{self, BufRead, BufReader, Seek},
        ///     fs::File,
        /// };
        ///
        /// fn main() -> io::Result<()> {
        ///     let mut f = BufReader::new(File::open("foo.txt")?);
        ///
        ///     let before = f.stream_position()?;
        ///     f.read_line(&mut String::new())?;
        ///     let after = f.stream_position()?;
        ///
        ///     println!("The first line was {} bytes long", after - before);
        ///     Ok(())
        /// }
        /// ```
        fn stream_position(&mut self) -> io::Result<u64> {
            let remainder = (self.cap - self.pos) as u64;
            self.inner.stream_position().map(|pos| {
                pos.checked_sub(remainder).expect(
                    "overflow when subtracting remaining buffer size from inner stream position",
                )
            })
        }
    }