bytes.rs

use {IntoBuf, Buf, BufMut};
use buf::Iter;
use debug;

use std::{cmp, fmt, mem, hash, ops, slice, ptr, usize};
use std::borrow::{Borrow, BorrowMut};
use std::io::Cursor;
use std::sync::atomic::{self, AtomicUsize, AtomicPtr};
use std::sync::atomic::Ordering::{Relaxed, Acquire, Release, AcqRel};
use std::iter::{FromIterator, Iterator};

/// A reference counted contiguous slice of memory.
///
/// `Bytes` is an efficient container for storing and operating on contiguous
/// slices of memory. It is intended for use primarily in networking code, but
/// could have applications elsewhere as well.
///
/// `Bytes` values facilitate zero-copy network programming by allowing multiple
/// `Bytes` objects to point to the same underlying memory. This is managed by
/// using a reference count to track when the memory is no longer needed and can
/// be freed.
///
/// ```
/// use bytes::Bytes;
///
/// let mut mem = Bytes::from(&b"Hello world"[..]);
/// let a = mem.slice(0, 5);
///
/// assert_eq!(&a[..], b"Hello");
///
/// let b = mem.split_to(6);
///
/// assert_eq!(&mem[..], b"world");
/// assert_eq!(&b[..], b"Hello ");
/// ```
///
/// # Memory layout
///
/// The `Bytes` struct itself is fairly small, limited to a pointer to the
/// memory and 4 `usize` fields used to track information about which segment of
/// the underlying memory the `Bytes` handle has access to.
///
/// The memory layout looks like this:
///
/// ```text
/// +-------+
/// | Bytes |
/// +-------+
///  /      \_____
/// |              \
/// v               v
/// +-----+------------------------------------+
/// | Arc |         |      Data     |          |
/// +-----+------------------------------------+
/// ```
///
/// `Bytes` keeps both a pointer to the shared `Arc` containing the full memory
/// slice and a pointer to the start of the region visible by the handle.
/// `Bytes` also tracks the length of its view into the memory.
///
/// # Sharing
///
/// The memory itself is reference counted, and multiple `Bytes` objects may
/// point to the same region. Each `Bytes` handle point to different sections within
/// the memory region, and `Bytes` handle may or may not have overlapping views
/// into the memory.
///
///
/// ```text
///
///    Arc ptrs                   +---------+
///    ________________________ / | Bytes 2 |
///   /                           +---------+
///  /          +-----------+     |         |
/// |_________/ |  Bytes 1  |     |         |
/// |           +-----------+     |         |
/// |           |           | ___/ data     | tail
/// |      data |      tail |/              |
/// v           v           v               v
/// +-----+---------------------------------+-----+
/// | Arc |     |           |               |     |
/// +-----+---------------------------------+-----+
/// ```
///
/// # Mutating
///
/// While `Bytes` handles may potentially represent overlapping views of the
/// underlying memory slice and may not be mutated, `BytesMut` handles are
/// guaranteed to be the only handle able to view that slice of memory. As such,
/// `BytesMut` handles are able to mutate the underlying memory. Note that
/// holding a unique view to a region of memory does not mean that there are no
/// other `Bytes` and `BytesMut` handles with disjoint views of the underlying
/// memory.
///
/// # Inline bytes
///
/// As an optimization, when the slice referenced by a `Bytes` or `BytesMut`
/// handle is small enough [^1], `with_capacity` will avoid the allocation by
/// inlining the slice directly in the handle. In this case, a clone is no
/// longer "shallow" and the data will be copied.  Converting from a `Vec` will
/// never use inlining.
///
/// [^1]: Small enough: 31 bytes on 64 bit systems, 15 on 32 bit systems.
///
pub struct Bytes {
    inner: Inner,
}

/// A unique reference to a contiguous slice of memory.
///
/// `BytesMut` represents a unique view into a potentially shared memory region.
/// Given the uniqueness guarantee, owners of `BytesMut` handles are able to
/// mutate the memory. It is similar to a `Vec<u8>` but with less copies and
/// allocations.
///
/// For more detail, see [Bytes](struct.Bytes.html).
///
/// # Growth
///
/// One key difference from `Vec<u8>` is that most operations **do not
/// implicitly grow the buffer**. This means that calling `my_bytes.put("hello
/// world");` could panic if `my_bytes` does not have enough capacity. Before
/// writing to the buffer, ensure that there is enough remaining capacity by
/// calling `my_bytes.remaining_mut()`. In general, avoiding calls to `reserve`
/// is preferable.
///
/// The only exception is `extend` which implicitly reserves required capacity.
///
/// # Examples
///
/// ```
/// use bytes::{BytesMut, BufMut};
///
/// let mut buf = BytesMut::with_capacity(64);
///
/// buf.put(b'h');
/// buf.put(b'e');
/// buf.put("llo");
///
/// assert_eq!(&buf[..], b"hello");
///
/// // Freeze the buffer so that it can be shared
/// let a = buf.freeze();
///
/// // This does not allocate, instead `b` points to the same memory.
/// let b = a.clone();
///
/// assert_eq!(&a[..], b"hello");
/// assert_eq!(&b[..], b"hello");
/// ```
pub struct BytesMut {
    inner: Inner,
}

// Both `Bytes` and `BytesMut` are backed by `Inner` and functions are delegated
// to `Inner` functions. The `Bytes` and `BytesMut` shims ensure that functions
// that mutate the underlying buffer are only performed when the data range
// being mutated is only available via a single `BytesMut` handle.
//
// # Data storage modes
//
// The goal of `bytes` is to be as efficient as possible across a wide range of
// potential usage patterns. As such, `bytes` needs to be able to handle buffers
// that are never shared, shared on a single thread, and shared across many
// threads. `bytes` also needs to handle both tiny buffers as well as very large
// buffers. For example, [Cassandra](http://cassandra.apache.org) values have
// been known to be in the hundreds of megabyte, and HTTP header values can be a
// few characters in size.
//
// To achieve high performance in these various situations, `Bytes` and
// `BytesMut` use different strategies for storing the buffer depending on the
// usage pattern.
//
// ## Delayed `Arc` allocation
//
// When a `Bytes` or `BytesMut` is first created, there is only one outstanding
// handle referencing the buffer. Since sharing is not yet required, an `Arc`* is
// not used and the buffer is backed by a `Vec<u8>` directly. Using an
// `Arc<Vec<u8>>` requires two allocations, so if the buffer ends up never being
// shared, that allocation is avoided.
//
// When sharing does become necessary (`clone`, `split_to`, `split_off`), that
// is when the buffer is promoted to being shareable. The `Vec<u8>` is moved
// into an `Arc` and both the original handle and the new handle use the same
// buffer via the `Arc`.
//
// * `Arc` is being used to signify an atomically reference counted cell. We
// don't use the `Arc` implementation provided by `std` and instead use our own.
// This ends up simplifying a number of the `unsafe` code snippets.
//
// ## Inlining small buffers
//
// The `Bytes` / `BytesMut` structs require 4 pointer sized fields. On 64 bit
// systems, this ends up being 32 bytes, which is actually a lot of storage for
// cases where `Bytes` is being used to represent small byte strings, such as
// HTTP header names and values.
//
// To avoid any allocation at all in these cases, `Bytes` will use the struct
// itself for storing the buffer, reserving 1 byte for meta data. This means
// that, on 64 bit systems, 31 byte buffers require no allocation at all.
//
// The byte used for metadata stores a 2 bits flag used to indicate that the
// buffer is stored inline as well as 6 bits for tracking the buffer length (the
// return value of `Bytes::len`).
//
// ## Static buffers
//
// `Bytes` can also represent a static buffer, which is created with
// `Bytes::from_static`. No copying or allocations are required for tracking
// static buffers. The pointer to the `&'static [u8]`, the length, and a flag
// tracking that the `Bytes` instance represents a static buffer is stored in
// the `Bytes` struct.
//
// # Struct layout
//
// Both `Bytes` and `BytesMut` are wrappers around `Inner`, which provides the
// data fields as well as all of the function implementations.
//
// The `Inner` struct is carefully laid out in order to support the
// functionality described above as well as being as small as possible. Size is
// important as growing the size of the `Bytes` struct from 32 bytes to 40 bytes
// added as much as 15% overhead in benchmarks using `Bytes` in an HTTP header
// map structure.
//
// The `Inner` struct contains the following fields:
//
// * `ptr: *mut u8`
// * `len: usize`
// * `cap: usize`
// * `arc: AtomicPtr<Shared>`
//
// ## `ptr: *mut u8`
//
// A pointer to start of the handle's buffer view. When backed by a `Vec<u8>`,
// this is always the `Vec`'s pointer. When backed by an `Arc<Vec<u8>>`, `ptr`
// may have been shifted to point somewhere inside the buffer.
//
// When in "inlined" mode, `ptr` is used as part of the inlined buffer.
//
// ## `len: usize`
//
// The length of the handle's buffer view. When backed by a `Vec<u8>`, this is
// always the `Vec`'s length. The slice represented by `ptr` and `len` should
// (ideally) always be initialized memory.
//
// When in "inlined" mode, `len` is used as part of the inlined buffer.
//
// ## `cap: usize`
//
// The capacity of the handle's buffer view. When backed by a `Vec<u8>`, this is
// always the `Vec`'s capacity. The slice represented by `ptr+len` and `cap-len`
// may or may not be initialized memory.
//
// When in "inlined" mode, `cap` is used as part of the inlined buffer.
//
// ## `arc: AtomicPtr<Shared>`
//
// When `Inner` is in allocated mode (backed by Vec<u8> or Arc<Vec<u8>>), this
// will be the pointer to the `Arc` structure tracking the ref count for the
// underlying buffer. When the pointer is null, then the `Arc` has not been
// allocated yet and `self` is the only outstanding handle for the underlying
// buffer.
//
// The lower two bits of `arc` are used to track the storage mode of `Inner`.
// `0b01` indicates inline storage, `0b10` indicates static storage, and `0b11`
// indicates vector storage, not yet promoted to Arc.  Since pointers to
// allocated structures are aligned, the lower two bits of a pointer will always
// be 0. This allows disambiguating between a pointer and the two flags.
//
// When in "inlined" mode, the least significant byte of `arc` is also used to
// store the length of the buffer view (vs. the capacity, which is a constant).
//
// The rest of `arc`'s bytes are used as part of the inline buffer, which means
// that those bytes need to be located next to the `ptr`, `len`, and `cap`
// fields, which make up the rest of the inline buffer. This requires special
// casing the layout of `Inner` depending on if the target platform is bit or
// little endian.
//
// On little endian platforms, the `arc` field must be the first field in the
// struct. On big endian platforms, the `arc` field must be the last field in
// the struct. Since a deterministic struct layout is required, `Inner` is
// annotated with `#[repr(C)]`.
//
// # Thread safety
//
// `Bytes::clone()` returns a new `Bytes` handle with no copying. This is done
// by bumping the buffer ref count and returning a new struct pointing to the
// same buffer. However, the `Arc` structure is lazily allocated. This means
// that if `Bytes` is stored itself in an `Arc` (`Arc<Bytes>`), the `clone`
// function can be called concurrently from multiple threads. This is why an
// `AtomicPtr` is used for the `arc` field vs. a `*const`.
//
// Care is taken to ensure that the need for synchronization is minimized. Most
// operations do not require any synchronization.
//
#[cfg(target_endian = "little")]
#[repr(C)]
struct Inner {
    // WARNING: Do not access the fields directly unless you know what you are
    // doing. Instead, use the fns. See implementation comment above.
    arc: AtomicPtr<Shared>,
    ptr: *mut u8,
    len: usize,
    cap: usize,
}

#[cfg(target_endian = "big")]
#[repr(C)]
struct Inner {
    // WARNING: Do not access the fields directly unless you know what you are
    // doing. Instead, use the fns. See implementation comment above.
    ptr: *mut u8,
    len: usize,
    cap: usize,
    arc: AtomicPtr<Shared>,
}

// Thread-safe reference-counted container for the shared storage. This mostly
// the same as `std::sync::Arc` but without the weak counter. The ref counting
// fns are based on the ones found in `std`.
//
// The main reason to use `Shared` instead of `std::sync::Arc` is that it ends
// up making the overall code simpler and easier to reason about. This is due to
// some of the logic around setting `Inner::arc` and other ways the `arc` field
// is used. Using `Arc` ended up requiring a number of funky transmutes and
// other shenanigans to make it work.
struct Shared {
    vec: Vec<u8>,
    original_capacity_repr: usize,
    ref_count: AtomicUsize,
}

// Buffer storage strategy flags.
const KIND_ARC: usize = 0b00;
const KIND_INLINE: usize = 0b01;
const KIND_STATIC: usize = 0b10;
const KIND_VEC: usize = 0b11;
const KIND_MASK: usize = 0b11;

// The max original capacity value. Any `Bytes` allocated with a greater initial
// capacity will default to this.
const MAX_ORIGINAL_CAPACITY_WIDTH: usize = 17;
// The original capacity algorithm will not take effect unless the originally
// allocated capacity was at least 1kb in size.
const MIN_ORIGINAL_CAPACITY_WIDTH: usize = 10;
// The original capacity is stored in powers of 2 starting at 1kb to a max of
// 64kb. Representing it as such requires only 3 bits of storage.
const ORIGINAL_CAPACITY_MASK: usize = 0b11100;
const ORIGINAL_CAPACITY_OFFSET: usize = 2;

// When the storage is in the `Vec` representation, the pointer can be advanced
// at most this value. This is due to the amount of storage available to track
// the offset is usize - number of KIND bits and number of ORIGINAL_CAPACITY
// bits.
const VEC_POS_OFFSET: usize = 5;
const MAX_VEC_POS: usize = usize::MAX >> VEC_POS_OFFSET;
const NOT_VEC_POS_MASK: usize = 0b11111;

// Bit op constants for extracting the inline length value from the `arc` field.
const INLINE_LEN_MASK: usize = 0b11111100;
const INLINE_LEN_OFFSET: usize = 2;

// Byte offset from the start of `Inner` to where the inline buffer data
// starts. On little endian platforms, the first byte of the struct is the
// storage flag, so the data is shifted by a byte. On big endian systems, the
// data starts at the beginning of the struct.
#[cfg(target_endian = "little")]
const INLINE_DATA_OFFSET: isize = 1;
#[cfg(target_endian = "big")]
const INLINE_DATA_OFFSET: isize = 0;

#[cfg(target_pointer_width = "64")]
const PTR_WIDTH: usize = 64;
#[cfg(target_pointer_width = "32")]
const PTR_WIDTH: usize = 32;

// Inline buffer capacity. This is the size of `Inner` minus 1 byte for the
// metadata.
#[cfg(target_pointer_width = "64")]
const INLINE_CAP: usize = 4 * 8 - 1;
#[cfg(target_pointer_width = "32")]
const INLINE_CAP: usize = 4 * 4 - 1;

/*
 *
 * ===== Bytes =====
 *
 */

impl Bytes {
    /// Creates a new `Bytes` with the specified capacity.
    ///
    /// The returned `Bytes` will be able to hold at least `capacity` bytes
    /// without reallocating. If `capacity` is under `4 * size_of::<usize>() - 1`,
    /// then `BytesMut` will not allocate.
    ///
    /// It is important to note that this function does not specify the length
    /// of the returned `Bytes`, but only the capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut bytes = Bytes::with_capacity(64);
    ///
    /// // `bytes` contains no data, even though there is capacity
    /// assert_eq!(bytes.len(), 0);
    ///
    /// bytes.extend_from_slice(&b"hello world"[..]);
    ///
    /// assert_eq!(&bytes[..], b"hello world");
    /// ```
    #[inline]
    pub fn with_capacity(capacity: usize) -> Bytes {
        Bytes {
            inner: Inner::with_capacity(capacity),
        }
    }

    /// Creates a new empty `Bytes`.
    ///
    /// This will not allocate and the returned `Bytes` handle will be empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let b = Bytes::new();
    /// assert_eq!(&b[..], b"");
    /// ```
    #[inline]
    pub fn new() -> Bytes {
        Bytes::with_capacity(0)
    }

    /// Creates a new `Bytes` from a static slice.
    ///
    /// The returned `Bytes` will point directly to the static slice. There is
    /// no allocating or copying.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let b = Bytes::from_static(b"hello");
    /// assert_eq!(&b[..], b"hello");
    /// ```
    #[inline]
    pub fn from_static(bytes: &'static [u8]) -> Bytes {
        Bytes {
            inner: Inner::from_static(bytes),
        }
    }

    /// Returns the number of bytes contained in this `Bytes`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let b = Bytes::from(&b"hello"[..]);
    /// assert_eq!(b.len(), 5);
    /// ```
    #[inline]
    pub fn len(&self) -> usize {
        self.inner.len()
    }

    /// Returns true if the `Bytes` has a length of 0.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let b = Bytes::new();
    /// assert!(b.is_empty());
    /// ```
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }

    /// Returns a slice of self for the index range `[begin..end)`.
    ///
    /// This will increment the reference count for the underlying memory and
    /// return a new `Bytes` handle set to the slice.
    ///
    /// This operation is `O(1)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let a = Bytes::from(&b"hello world"[..]);
    /// let b = a.slice(2, 5);
    ///
    /// assert_eq!(&b[..], b"llo");
    /// ```
    ///
    /// # Panics
    ///
    /// Requires that `begin <= end` and `end <= self.len()`, otherwise slicing
    /// will panic.
    pub fn slice(&self, begin: usize, end: usize) -> Bytes {
        assert!(begin <= end);
        assert!(end <= self.len());

        if end - begin <= INLINE_CAP {
            return Bytes::from(&self[begin..end]);
        }

        let mut ret = self.clone();

        unsafe {
            ret.inner.set_end(end);
            ret.inner.set_start(begin);
        }

        ret
    }

    /// Returns a slice of self for the index range `[begin..self.len())`.
    ///
    /// This will increment the reference count for the underlying memory and
    /// return a new `Bytes` handle set to the slice.
    ///
    /// This operation is `O(1)` and is equivalent to `self.slice(begin,
    /// self.len())`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let a = Bytes::from(&b"hello world"[..]);
    /// let b = a.slice_from(6);
    ///
    /// assert_eq!(&b[..], b"world");
    /// ```
    ///
    /// # Panics
    ///
    /// Requires that `begin <= self.len()`, otherwise slicing will panic.
    pub fn slice_from(&self, begin: usize) -> Bytes {
        self.slice(begin, self.len())
    }

    /// Returns a slice of self for the index range `[0..end)`.
    ///
    /// This will increment the reference count for the underlying memory and
    /// return a new `Bytes` handle set to the slice.
    ///
    /// This operation is `O(1)` and is equivalent to `self.slice(0, end)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let a = Bytes::from(&b"hello world"[..]);
    /// let b = a.slice_to(5);
    ///
    /// assert_eq!(&b[..], b"hello");
    /// ```
    ///
    /// # Panics
    ///
    /// Requires that `end <= self.len()`, otherwise slicing will panic.
    pub fn slice_to(&self, end: usize) -> Bytes {
        self.slice(0, end)
    }

    /// Returns a slice of self that is equivalent to the given `subset`.
    ///
    /// When processing a `Bytes` buffer with other tools, one often gets a
    /// `&[u8]` which is in fact a slice of the `Bytes`, i.e. a subset of it.
    /// This function turns that `&[u8]` into another `Bytes`, as if one had
    /// called `self.slice()` with the offsets that correspond to `subset`.
    ///
    /// This operation is `O(1)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let bytes = Bytes::from(&b"012345678"[..]);
    /// let as_slice = bytes.as_ref();
    /// let subset = &as_slice[2..6];
    /// let subslice = bytes.slice_ref(&subset);
    /// assert_eq!(&subslice[..], b"2345");
    /// ```
    ///
    /// # Panics
    ///
    /// Requires that the given `sub` slice is in fact contained within the
    /// `Bytes` buffer; otherwise this function will panic.
    pub fn slice_ref(&self, subset: &[u8]) -> Bytes {
        let bytes_p = self.as_ptr() as usize;
        let bytes_len = self.len();

        let sub_p = subset.as_ptr() as usize;
        let sub_len = subset.len();

        assert!(sub_p >= bytes_p);
        assert!(sub_p + sub_len <= bytes_p + bytes_len);

        let sub_offset = sub_p - bytes_p;

        self.slice(sub_offset, sub_offset + sub_len)
    }

    /// Splits the bytes into two at the given index.
    ///
    /// Afterwards `self` contains elements `[0, at)`, and the returned `Bytes`
    /// contains elements `[at, len)`.
    ///
    /// This is an `O(1)` operation that just increases the reference count and
    /// sets a few indices.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut a = Bytes::from(&b"hello world"[..]);
    /// let b = a.split_off(5);
    ///
    /// assert_eq!(&a[..], b"hello");
    /// assert_eq!(&b[..], b" world");
    /// ```
    ///
    /// # Panics
    ///
    /// Panics if `at > len`.
    pub fn split_off(&mut self, at: usize) -> Bytes {
        assert!(at <= self.len());

        if at == self.len() {
            return Bytes::new();
        }

        if at == 0 {
            return mem::replace(self, Bytes::new());
        }

        Bytes {
            inner: self.inner.split_off(at),
        }
    }

    /// Splits the bytes into two at the given index.
    ///
    /// Afterwards `self` contains elements `[at, len)`, and the returned
    /// `Bytes` contains elements `[0, at)`.
    ///
    /// This is an `O(1)` operation that just increases the reference count and
    /// sets a few indices.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut a = Bytes::from(&b"hello world"[..]);
    /// let b = a.split_to(5);
    ///
    /// assert_eq!(&a[..], b" world");
    /// assert_eq!(&b[..], b"hello");
    /// ```
    ///
    /// # Panics
    ///
    /// Panics if `at > len`.
    pub fn split_to(&mut self, at: usize) -> Bytes {
        assert!(at <= self.len());

        if at == self.len() {
            return mem::replace(self, Bytes::new());
        }

        if at == 0 {
            return Bytes::new();
        }

        Bytes {
            inner: self.inner.split_to(at),
        }
    }

    #[deprecated(since = "0.4.1", note = "use split_to instead")]
    #[doc(hidden)]
    pub fn drain_to(&mut self, at: usize) -> Bytes {
        self.split_to(at)
    }

    /// Shortens the buffer, keeping the first `len` bytes and dropping the
    /// rest.
    ///
    /// If `len` is greater than the buffer's current length, this has no
    /// effect.
    ///
    /// The [`split_off`] method can emulate `truncate`, but this causes the
    /// excess bytes to be returned instead of dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut buf = Bytes::from(&b"hello world"[..]);
    /// buf.truncate(5);
    /// assert_eq!(buf, b"hello"[..]);
    /// ```
    ///
    /// [`split_off`]: #method.split_off
    pub fn truncate(&mut self, len: usize) {
        self.inner.truncate(len);
    }

    /// Shortens the buffer, dropping the first `cnt` bytes and keeping the
    /// rest.
    ///
    /// This is the same function as `Buf::advance`, and in the next breaking
    /// release of `bytes`, this implementation will be removed in favor of
    /// having `Bytes` implement `Buf`.
    ///
    /// # Panics
    ///
    /// This function panics if `cnt` is greater than `self.len()`
    #[inline]
    pub fn advance(&mut self, cnt: usize) {
        assert!(cnt <= self.len(), "cannot advance past `remaining`");
        unsafe { self.inner.set_start(cnt); }
    }

    /// Clears the buffer, removing all data.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut buf = Bytes::from(&b"hello world"[..]);
    /// buf.clear();
    /// assert!(buf.is_empty());
    /// ```
    pub fn clear(&mut self) {
        self.truncate(0);
    }

    /// Attempts to convert into a `BytesMut` handle.
    ///
    /// This will only succeed if there are no other outstanding references to
    /// the underlying chunk of memory. `Bytes` handles that contain inlined
    /// bytes will always be convertable to `BytesMut`.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let a = Bytes::from(&b"Mary had a little lamb, little lamb, little lamb..."[..]);
    ///
    /// // Create a shallow clone
    /// let b = a.clone();
    ///
    /// // This will fail because `b` shares a reference with `a`
    /// let a = a.try_mut().unwrap_err();
    ///
    /// drop(b);
    ///
    /// // This will succeed
    /// let mut a = a.try_mut().unwrap();
    ///
    /// a[0] = b'b';
    ///
    /// assert_eq!(&a[..4], b"bary");
    /// ```
    pub fn try_mut(mut self) -> Result<BytesMut, Bytes> {
        if self.inner.is_mut_safe() {
            Ok(BytesMut { inner: self.inner })
        } else {
            Err(self)
        }
    }

    /// Appends given bytes to this object.
    ///
    /// If this `Bytes` object has not enough capacity, it is resized first.
    /// If it is shared (`refcount > 1`), it is copied first.
    ///
    /// This operation can be less effective than the similar operation on
    /// `BytesMut`, especially on small additions.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::Bytes;
    ///
    /// let mut buf = Bytes::from("aabb");
    /// buf.extend_from_slice(b"ccdd");
    /// buf.extend_from_slice(b"eeff");
    ///
    /// assert_eq!(b"aabbccddeeff", &buf[..]);
    /// ```
    pub fn extend_from_slice(&mut self, extend: &[u8]) {
        if extend.is_empty() {
            return;
        }

        let new_cap = self.len().checked_add(extend.len()).expect("capacity overflow");

        let result = match mem::replace(self, Bytes::new()).try_mut() {
            Ok(mut bytes_mut) => {
                bytes_mut.extend_from_slice(extend);
                bytes_mut
            },
            Err(bytes) => {
                let mut bytes_mut = BytesMut::with_capacity(new_cap);
                bytes_mut.put_slice(&bytes);
                bytes_mut.put_slice(extend);
                bytes_mut
            }
        };

        mem::replace(self, result.freeze());
    }
}

impl IntoBuf for Bytes {
    type Buf = Cursor<Self>;

    fn into_buf(self) -> Self::Buf {
        Cursor::new(self)
    }
}

impl<'a> IntoBuf for &'a Bytes {
    type Buf = Cursor<Self>;

    fn into_buf(self) -> Self::Buf {
        Cursor::new(self)
    }
}

impl Clone for Bytes {
    fn clone(&self) -> Bytes {
        Bytes {
            inner: unsafe { self.inner.shallow_clone(false) },
        }
    }
}

impl AsRef<[u8]> for Bytes {
    #[inline]
    fn as_ref(&self) -> &[u8] {
        self.inner.as_ref()
    }
}

impl ops::Deref for Bytes {
    type Target = [u8];

    #[inline]
    fn deref(&self) -> &[u8] {
        self.inner.as_ref()
    }
}

impl From<BytesMut> for Bytes {
    fn from(src: BytesMut) -> Bytes {
        src.freeze()
    }
}

impl From<Vec<u8>> for Bytes {
    fn from(src: Vec<u8>) -> Bytes {
        BytesMut::from(src).freeze()
    }
}

impl From<String> for Bytes {
    fn from(src: String) -> Bytes {
        BytesMut::from(src).freeze()
    }
}

impl<'a> From<&'a [u8]> for Bytes {
    fn from(src: &'a [u8]) -> Bytes {
        BytesMut::from(src).freeze()
    }
}

impl<'a> From<&'a str> for Bytes {
    fn from(src: &'a str) -> Bytes {
        BytesMut::from(src).freeze()
    }
}

impl FromIterator<u8> for BytesMut {
    fn from_iter<T: IntoIterator<Item = u8>>(into_iter: T) -> Self {
        let iter = into_iter.into_iter();
        let (min, maybe_max) = iter.size_hint();

        let mut out = BytesMut::with_capacity(maybe_max.unwrap_or(min));

        for i in iter {
            out.reserve(1);
            out.put(i);
        }

        out
    }
}

impl FromIterator<u8> for Bytes {
    fn from_iter<T: IntoIterator<Item = u8>>(into_iter: T) -> Self {
        BytesMut::from_iter(into_iter).freeze()
    }
}

impl PartialEq for Bytes {
    fn eq(&self, other: &Bytes) -> bool {
        self.inner.as_ref() == other.inner.as_ref()
    }
}

impl PartialOrd for Bytes {
    fn partial_cmp(&self, other: &Bytes) -> Option<cmp::Ordering> {
        self.inner.as_ref().partial_cmp(other.inner.as_ref())
    }
}

impl Ord for Bytes {
    fn cmp(&self, other: &Bytes) -> cmp::Ordering {
        self.inner.as_ref().cmp(other.inner.as_ref())
    }
}

impl Eq for Bytes {
}

impl Default for Bytes {
    #[inline]
    fn default() -> Bytes {
        Bytes::new()
    }
}

impl fmt::Debug for Bytes {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt::Debug::fmt(&debug::BsDebug(&self.inner.as_ref()), fmt)
    }
}

impl hash::Hash for Bytes {
    fn hash<H>(&self, state: &mut H) where H: hash::Hasher {
        let s: &[u8] = self.as_ref();
        s.hash(state);
    }
}

impl Borrow<[u8]> for Bytes {
    fn borrow(&self) -> &[u8] {
        self.as_ref()
    }
}

impl IntoIterator for Bytes {
    type Item = u8;
    type IntoIter = Iter<Cursor<Bytes>>;

    fn into_iter(self) -> Self::IntoIter {
        self.into_buf().iter()
    }
}

impl<'a> IntoIterator for &'a Bytes {
    type Item = u8;
    type IntoIter = Iter<Cursor<&'a Bytes>>;

    fn into_iter(self) -> Self::IntoIter {
        self.into_buf().iter()
    }
}

impl Extend<u8> for Bytes {
    fn extend<T>(&mut self, iter: T) where T: IntoIterator<Item = u8> {
        let iter = iter.into_iter();

        let (lower, upper) = iter.size_hint();

        // Avoid possible conversion into mut if there's nothing to add