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use crate::error::{Error, ErrorKind, Result};
use crate::traits::{BufRead, Read, Seek, SeekFrom, Write};
use core::{cmp, fmt, mem};

// =============================================================================
// Forwarding implementations

impl<R: Read + ?Sized> Read for &mut R {
    #[inline]
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        (**self).read(buf)
    }

    #[inline]
    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
        (**self).read_exact(buf)
    }
}

impl<W: Write + ?Sized> Write for &mut W {
    #[inline]
    fn write(&mut self, buf: &[u8]) -> Result<usize> {
        (**self).write(buf)
    }

    #[inline]
    fn flush(&mut self) -> Result<()> {
        (**self).flush()
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> Result<()> {
        (**self).write_all(buf)
    }

    #[inline]
    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
        (**self).write_fmt(fmt)
    }
}

impl<S: Seek + ?Sized> Seek for &mut S {
    #[inline]
    fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
        (**self).seek(pos)
    }
}

impl<B: BufRead + ?Sized> BufRead for &mut B {
    #[inline]
    fn fill_buf(&mut self) -> Result<&[u8]> {
        (**self).fill_buf()
    }

    #[inline]
    fn consume(&mut self, amt: usize) {
        (**self).consume(amt)
    }
}

// =============================================================================
// In-memory buffer implementations

/// Read is implemented for `&[u8]` by copying from the slice.
///
/// Note that reading updates the slice to point to the yet unread part.
/// The slice will be empty when EOF is reached.
impl Read for &[u8] {
    #[inline]
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        let amt = cmp::min(buf.len(), self.len());
        let (a, b) = self.split_at(amt);

        // First check if the amount of bytes we want to read is small:
        // `copy_from_slice` will generally expand to a call to `memcpy`, and
        // for a single byte the overhead is significant.
        if amt == 1 {
            buf[0] = a[0];
        } else {
            buf[..amt].copy_from_slice(a);
        }

        *self = b;
        Ok(amt)
    }

    #[inline]
    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
        if buf.len() > self.len() {
            return Err(Error::new(
                ErrorKind::UnexpectedEof,
                "failed to fill whole buffer",
            ));
        }
        let (a, b) = self.split_at(buf.len());

        // First check if the amount of bytes we want to read is small:
        // `copy_from_slice` will generally expand to a call to `memcpy`, and
        // for a single byte the overhead is significant.
        if buf.len() == 1 {
            buf[0] = a[0];
        } else {
            buf.copy_from_slice(a);
        }

        *self = b;
        Ok(())
    }
}

impl BufRead for &[u8] {
    #[inline]
    fn fill_buf(&mut self) -> Result<&[u8]> {
        Ok(*self)
    }

    #[inline]
    fn consume(&mut self, amt: usize) {
        *self = &self[amt..];
    }
}

/// Write is implemented for `&mut [u8]` by copying into the slice, overwriting
/// its data.
///
/// Note that writing updates the slice to point to the yet unwritten part.
/// The slice will be empty when it has been completely overwritten.
impl Write for &mut [u8] {
    #[inline]
    fn write(&mut self, data: &[u8]) -> Result<usize> {
        let amt = cmp::min(data.len(), self.len());
        let (a, b) = mem::replace(self, &mut []).split_at_mut(amt);
        a.copy_from_slice(&data[..amt]);
        *self = b;
        Ok(amt)
    }

    #[inline]
    fn write_all(&mut self, data: &[u8]) -> Result<()> {
        if self.write(data)? == data.len() {
            Ok(())
        } else {
            Err(Error::new(
                ErrorKind::WriteZero,
                "failed to write whole buffer",
            ))
        }
    }

    #[inline]
    fn flush(&mut self) -> Result<()> {
        Ok(())
    }
}

/// Write is implemented for `Vec<u8>` by appending to the vector.
/// The vector will grow as needed.
#[cfg(feature = "alloc")]
impl Write for alloc::vec::Vec<u8> {
    #[inline]
    fn write(&mut self, buf: &[u8]) -> Result<usize> {
        self.extend_from_slice(buf);
        Ok(buf.len())
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> Result<()> {
        self.extend_from_slice(buf);
        Ok(())
    }

    #[inline]
    fn flush(&mut self) -> Result<()> {
        Ok(())
    }
}