# chain.rs - source
Source of the Rust file `src/buf/chain.rs`.

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    use crate::buf::{IntoIter, UninitSlice};
    use crate::{Buf, BufMut};

    #[cfg(feature = "std")]
    use std::io::IoSlice;

    /// A `Chain` sequences two buffers.
    ///
    /// `Chain` is an adapter that links two underlying buffers and provides a
    /// continuous view across both buffers. It is able to sequence either immutable
    /// buffers ([`Buf`] values) or mutable buffers ([`BufMut`] values).
    ///
    /// This struct is generally created by calling [`Buf::chain`]. Please see that
    /// function's documentation for more detail.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::{Bytes, Buf};
    ///
    /// let mut buf = (&b"hello "[..])
    ///     .chain(&b"world"[..]);
    ///
    /// let full: Bytes = buf.copy_to_bytes(11);
    /// assert_eq!(full[..], b"hello world"[..]);
    /// ```
    ///
    /// [`Buf::chain`]: Buf::chain
    #[derive(Debug)]
    pub struct Chain<T, U> {
        a: T,
        b: U,
    }

    impl<T, U> Chain<T, U> {
        /// Creates a new `Chain` sequencing the provided values.
        pub(crate) fn new(a: T, b: U) -> Chain<T, U> {
            Chain { a, b }
        }

        /// Gets a reference to the first underlying `Buf`.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::Buf;
        ///
        /// let buf = (&b"hello"[..])
        ///     .chain(&b"world"[..]);
        ///
        /// assert_eq!(buf.first_ref()[..], b"hello"[..]);
        /// ```
        pub fn first_ref(&self) -> &T {
            &self.a
        }

        /// Gets a mutable reference to the first underlying `Buf`.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::Buf;
        ///
        /// let mut buf = (&b"hello"[..])
        ///     .chain(&b"world"[..]);
        ///
        /// buf.first_mut().advance(1);
        ///
        /// let full = buf.copy_to_bytes(9);
        /// assert_eq!(full, b"elloworld"[..]);
        /// ```
        pub fn first_mut(&mut self) -> &mut T {
            &mut self.a
        }

        /// Gets a reference to the last underlying `Buf`.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::Buf;
        ///
        /// let buf = (&b"hello"[..])
        ///     .chain(&b"world"[..]);
        ///
        /// assert_eq!(buf.last_ref()[..], b"world"[..]);
        /// ```
        pub fn last_ref(&self) -> &U {
            &self.b
        }

        /// Gets a mutable reference to the last underlying `Buf`.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::Buf;
        ///
        /// let mut buf = (&b"hello "[..])
        ///     .chain(&b"world"[..]);
        ///
        /// buf.last_mut().advance(1);
        ///
        /// let full = buf.copy_to_bytes(10);
        /// assert_eq!(full, b"hello orld"[..]);
        /// ```
        pub fn last_mut(&mut self) -> &mut U {
            &mut self.b
        }

        /// Consumes this `Chain`, returning the underlying values.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::Buf;
        ///
        /// let chain = (&b"hello"[..])
        ///     .chain(&b"world"[..]);
        ///
        /// let (first, last) = chain.into_inner();
        /// assert_eq!(first[..], b"hello"[..]);
        /// assert_eq!(last[..], b"world"[..]);
        /// ```
        pub fn into_inner(self) -> (T, U) {
            (self.a, self.b)
        }
    }

    impl<T, U> Buf for Chain<T, U>
    where
        T: Buf,
        U: Buf,
    {
        fn remaining(&self) -> usize {
            self.a.remaining().saturating_add(self.b.remaining())
        }

        fn chunk(&self) -> &[u8] {
            if self.a.has_remaining() {
                self.a.chunk()
            } else {
                self.b.chunk()
            }
        }

        fn advance(&mut self, mut cnt: usize) {
            let a_rem = self.a.remaining();

            if a_rem != 0 {
                if a_rem >= cnt {
                    self.a.advance(cnt);
                    return;
                }

                // Consume what is left of a
                self.a.advance(a_rem);

                cnt -= a_rem;
            }

            self.b.advance(cnt);
        }

        #[cfg(feature = "std")]
        fn chunks_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
            let mut n = self.a.chunks_vectored(dst);
            n += self.b.chunks_vectored(&mut dst[n..]);
            n
        }

        fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
            let a_rem = self.a.remaining();
            if a_rem >= len {
                self.a.copy_to_bytes(len)
            } else if a_rem == 0 {
                self.b.copy_to_bytes(len)
            } else {
                assert!(
                    len - a_rem <= self.b.remaining(),
                    "`len` greater than remaining"
                );
                let mut ret = crate::BytesMut::with_capacity(len);
                ret.put(&mut self.a);
                ret.put((&mut self.b).take(len - a_rem));
                ret.freeze()
            }
        }
    }

    unsafe impl<T, U> BufMut for Chain<T, U>
    where
        T: BufMut,
        U: BufMut,
    {
        fn remaining_mut(&self) -> usize {
            self.a
                .remaining_mut()
                .saturating_add(self.b.remaining_mut())
        }

        fn chunk_mut(&mut self) -> &mut UninitSlice {
            if self.a.has_remaining_mut() {
                self.a.chunk_mut()
            } else {
                self.b.chunk_mut()
            }
        }

        unsafe fn advance_mut(&mut self, mut cnt: usize) {
            let a_rem = self.a.remaining_mut();

            if a_rem != 0 {
                if a_rem >= cnt {
                    self.a.advance_mut(cnt);
                    return;
                }

                // Consume what is left of a
                self.a.advance_mut(a_rem);

                cnt -= a_rem;
            }

            self.b.advance_mut(cnt);
        }
    }

    impl<T, U> IntoIterator for Chain<T, U>
    where
        T: Buf,
        U: Buf,
    {
        type Item = u8;
        type IntoIter = IntoIter<Chain<T, U>>;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter::new(self)
        }
    }