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    1581

    use core::iter::{FromIterator, Iterator};
    use core::mem::{self, ManuallyDrop};
    use core::ops::{Deref, DerefMut};
    use core::ptr::{self, NonNull};
    use core::{cmp, fmt, hash, isize, slice, usize};

    use alloc::{
        borrow::{Borrow, BorrowMut},
        boxed::Box,
        string::String,
        vec::Vec,
    };

    use crate::buf::{IntoIter, UninitSlice};
    use crate::bytes::Vtable;
    #[allow(unused)]
    use crate::loom::sync::atomic::AtomicMut;
    use crate::loom::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
    use crate::{Buf, BufMut, Bytes};

    /// 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.
    ///
    /// `BytesMut` can be thought of as containing a `buf: Arc<Vec<u8>>`, an offset
    /// into `buf`, a slice length, and a guarantee that no other `BytesMut` for the
    /// same `buf` overlaps with its slice. That guarantee means that a write lock
    /// is not required.
    ///
    /// # Growth
    ///
    /// `BytesMut`'s `BufMut` implementation will implicitly grow its buffer as
    /// necessary. However, explicitly reserving the required space up-front before
    /// a series of inserts will be more efficient.
    ///
    /// # Examples
    ///
    /// ```
    /// use bytes::{BytesMut, BufMut};
    ///
    /// let mut buf = BytesMut::with_capacity(64);
    ///
    /// buf.put_u8(b'h');
    /// buf.put_u8(b'e');
    /// buf.put(&b"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 {
        ptr: NonNull<u8>,
        len: usize,
        cap: usize,
        data: *mut Shared,
    }

    // Thread-safe reference-counted container for the shared storage. This mostly
    // the same as `core::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 `core::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 = 0b0;
    const KIND_VEC: usize = 0b1;
    const KIND_MASK: usize = 0b1;

    // 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;

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

    /*
     *
     * ===== BytesMut =====
     *
     */

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

        /// Creates a new `BytesMut` with default capacity.
        ///
        /// Resulting object has length 0 and unspecified capacity.
        /// This function does not allocate.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::{BytesMut, BufMut};
        ///
        /// let mut bytes = BytesMut::new();
        ///
        /// assert_eq!(0, bytes.len());
        ///
        /// bytes.reserve(2);
        /// bytes.put_slice(b"xy");
        ///
        /// assert_eq!(&b"xy"[..], &bytes[..]);
        /// ```
        #[inline]
        pub fn new() -> BytesMut {
            BytesMut::with_capacity(0)
        }

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

        /// Returns true if the `BytesMut` has a length of 0.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let b = BytesMut::with_capacity(64);
        /// assert!(b.is_empty());
        /// ```
        #[inline]
        pub fn is_empty(&self) -> bool {
            self.len == 0
        }

        /// Returns the number of bytes the `BytesMut` can hold without reallocating.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let b = BytesMut::with_capacity(64);
        /// assert_eq!(b.capacity(), 64);
        /// ```
        #[inline]
        pub fn capacity(&self) -> usize {
            self.cap
        }

        /// Converts `self` into an immutable `Bytes`.
        ///
        /// The conversion is zero cost and is used to indicate that the slice
        /// referenced by the handle will no longer be mutated. Once the conversion
        /// is done, the handle can be cloned and shared across threads.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::{BytesMut, BufMut};
        /// use std::thread;
        ///
        /// let mut b = BytesMut::with_capacity(64);
        /// b.put(&b"hello world"[..]);
        /// let b1 = b.freeze();
        /// let b2 = b1.clone();
        ///
        /// let th = thread::spawn(move || {
        ///     assert_eq!(&b1[..], b"hello world");
        /// });
        ///
        /// assert_eq!(&b2[..], b"hello world");
        /// th.join().unwrap();
        /// ```
        #[inline]
        pub fn freeze(mut self) -> Bytes {
            if self.kind() == KIND_VEC {
                // Just re-use `Bytes` internal Vec vtable
                unsafe {
                    let (off, _) = self.get_vec_pos();
                    let vec = rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off);
                    mem::forget(self);
                    let mut b: Bytes = vec.into();
                    b.advance(off);
                    b
                }
            } else {
                debug_assert_eq!(self.kind(), KIND_ARC);

                let ptr = self.ptr.as_ptr();
                let len = self.len;
                let data = AtomicPtr::new(self.data as _);
                mem::forget(self);
                unsafe { Bytes::with_vtable(ptr, len, data, &SHARED_VTABLE) }
            }
        }

        /// Splits the bytes into two at the given index.
        ///
        /// Afterwards `self` contains elements `[0, at)`, and the returned
        /// `BytesMut` contains elements `[at, capacity)`.
        ///
        /// This is an `O(1)` operation that just increases the reference count
        /// and sets a few indices.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut a = BytesMut::from(&b"hello world"[..]);
        /// let mut b = a.split_off(5);
        ///
        /// a[0] = b'j';
        /// b[0] = b'!';
        ///
        /// assert_eq!(&a[..], b"jello");
        /// assert_eq!(&b[..], b"!world");
        /// ```
        ///
        /// # Panics
        ///
        /// Panics if `at > capacity`.
        #[must_use = "consider BytesMut::truncate if you don't need the other half"]
        pub fn split_off(&mut self, at: usize) -> BytesMut {
            assert!(
                at <= self.capacity(),
                "split_off out of bounds: {:?} <= {:?}",
                at,
                self.capacity(),
            );
            unsafe {
                let mut other = self.shallow_clone();
                other.set_start(at);
                self.set_end(at);
                other
            }
        }

        /// Removes the bytes from the current view, returning them in a new
        /// `BytesMut` handle.
        ///
        /// Afterwards, `self` will be empty, but will retain any additional
        /// capacity that it had before the operation. This is identical to
        /// `self.split_to(self.len())`.
        ///
        /// This is an `O(1)` operation that just increases the reference count and
        /// sets a few indices.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::{BytesMut, BufMut};
        ///
        /// let mut buf = BytesMut::with_capacity(1024);
        /// buf.put(&b"hello world"[..]);
        ///
        /// let other = buf.split();
        ///
        /// assert!(buf.is_empty());
        /// assert_eq!(1013, buf.capacity());
        ///
        /// assert_eq!(other, b"hello world"[..]);
        /// ```
        #[must_use = "consider BytesMut::advance(len()) if you don't need the other half"]
        pub fn split(&mut self) -> BytesMut {
            let len = self.len();
            self.split_to(len)
        }

        /// Splits the buffer into two at the given index.
        ///
        /// Afterwards `self` contains elements `[at, len)`, and the returned `BytesMut`
        /// contains elements `[0, at)`.
        ///
        /// This is an `O(1)` operation that just increases the reference count and
        /// sets a few indices.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut a = BytesMut::from(&b"hello world"[..]);
        /// let mut b = a.split_to(5);
        ///
        /// a[0] = b'!';
        /// b[0] = b'j';
        ///
        /// assert_eq!(&a[..], b"!world");
        /// assert_eq!(&b[..], b"jello");
        /// ```
        ///
        /// # Panics
        ///
        /// Panics if `at > len`.
        #[must_use = "consider BytesMut::advance if you don't need the other half"]
        pub fn split_to(&mut self, at: usize) -> BytesMut {
            assert!(
                at <= self.len(),
                "split_to out of bounds: {:?} <= {:?}",
                at,
                self.len(),
            );

            unsafe {
                let mut other = self.shallow_clone();
                other.set_end(at);
                self.set_start(at);
                other
            }
        }

        /// 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::BytesMut;
        ///
        /// let mut buf = BytesMut::from(&b"hello world"[..]);
        /// buf.truncate(5);
        /// assert_eq!(buf, b"hello"[..]);
        /// ```
        ///
        /// [`split_off`]: #method.split_off
        pub fn truncate(&mut self, len: usize) {
            if len <= self.len() {
                unsafe {
                    self.set_len(len);
                }
            }
        }

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

        /// Resizes the buffer so that `len` is equal to `new_len`.
        ///
        /// If `new_len` is greater than `len`, the buffer is extended by the
        /// difference with each additional byte set to `value`. If `new_len` is
        /// less than `len`, the buffer is simply truncated.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut buf = BytesMut::new();
        ///
        /// buf.resize(3, 0x1);
        /// assert_eq!(&buf[..], &[0x1, 0x1, 0x1]);
        ///
        /// buf.resize(2, 0x2);
        /// assert_eq!(&buf[..], &[0x1, 0x1]);
        ///
        /// buf.resize(4, 0x3);
        /// assert_eq!(&buf[..], &[0x1, 0x1, 0x3, 0x3]);
        /// ```
        pub fn resize(&mut self, new_len: usize, value: u8) {
            let len = self.len();
            if new_len > len {
                let additional = new_len - len;
                self.reserve(additional);
                unsafe {
                    let dst = self.chunk_mut().as_mut_ptr();
                    ptr::write_bytes(dst, value, additional);
                    self.set_len(new_len);
                }
            } else {
                self.truncate(new_len);
            }
        }

        /// Sets the length of the buffer.
        ///
        /// This will explicitly set the size of the buffer without actually
        /// modifying the data, so it is up to the caller to ensure that the data
        /// has been initialized.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut b = BytesMut::from(&b"hello world"[..]);
        ///
        /// unsafe {
        ///     b.set_len(5);
        /// }
        ///
        /// assert_eq!(&b[..], b"hello");
        ///
        /// unsafe {
        ///     b.set_len(11);
        /// }
        ///
        /// assert_eq!(&b[..], b"hello world");
        /// ```
        #[inline]
        pub unsafe fn set_len(&mut self, len: usize) {
            debug_assert!(len <= self.cap, "set_len out of bounds");
            self.len = len;
        }

        /// Reserves capacity for at least `additional` more bytes to be inserted
        /// into the given `BytesMut`.
        ///
        /// More than `additional` bytes may be reserved in order to avoid frequent
        /// reallocations. A call to `reserve` may result in an allocation.
        ///
        /// Before allocating new buffer space, the function will attempt to reclaim
        /// space in the existing buffer. If the current handle references a small
        /// view in the original buffer and all other handles have been dropped,
        /// and the requested capacity is less than or equal to the existing
        /// buffer's capacity, then the current view will be copied to the front of
        /// the buffer and the handle will take ownership of the full buffer.
        ///
        /// # Examples
        ///
        /// In the following example, a new buffer is allocated.
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut buf = BytesMut::from(&b"hello"[..]);
        /// buf.reserve(64);
        /// assert!(buf.capacity() >= 69);
        /// ```
        ///
        /// In the following example, the existing buffer is reclaimed.
        ///
        /// ```
        /// use bytes::{BytesMut, BufMut};
        ///
        /// let mut buf = BytesMut::with_capacity(128);
        /// buf.put(&[0; 64][..]);
        ///
        /// let ptr = buf.as_ptr();
        /// let other = buf.split();
        ///
        /// assert!(buf.is_empty());
        /// assert_eq!(buf.capacity(), 64);
        ///
        /// drop(other);
        /// buf.reserve(128);
        ///
        /// assert_eq!(buf.capacity(), 128);
        /// assert_eq!(buf.as_ptr(), ptr);
        /// ```
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        #[inline]
        pub fn reserve(&mut self, additional: usize) {
            let len = self.len();
            let rem = self.capacity() - len;

            if additional <= rem {
                // The handle can already store at least `additional` more bytes, so
                // there is no further work needed to be done.
                return;
            }

            self.reserve_inner(additional);
        }

        // In separate function to allow the short-circuits in `reserve` to
        // be inline-able. Significant helps performance.
        fn reserve_inner(&mut self, additional: usize) {
            let len = self.len();
            let kind = self.kind();

            if kind == KIND_VEC {
                // If there's enough free space before the start of the buffer, then
                // just copy the data backwards and reuse the already-allocated
                // space.
                //
                // Otherwise, since backed by a vector, use `Vec::reserve`
                unsafe {
                    let (off, prev) = self.get_vec_pos();

                    // Only reuse space if we can satisfy the requested additional space.
                    if self.capacity() - self.len() + off >= additional {
                        // There's space - reuse it
                        //
                        // Just move the pointer back to the start after copying
                        // data back.
                        let base_ptr = self.ptr.as_ptr().offset(-(off as isize));
                        ptr::copy(self.ptr.as_ptr(), base_ptr, self.len);
                        self.ptr = vptr(base_ptr);
                        self.set_vec_pos(0, prev);

                        // Length stays constant, but since we moved backwards we
                        // can gain capacity back.
                        self.cap += off;
                    } else {
                        // No space - allocate more
                        let mut v =
                            ManuallyDrop::new(rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off));
                        v.reserve(additional);

                        // Update the info
                        self.ptr = vptr(v.as_mut_ptr().offset(off as isize));
                        self.len = v.len() - off;
                        self.cap = v.capacity() - off;
                    }

                    return;
                }
            }

            debug_assert_eq!(kind, KIND_ARC);
            let shared: *mut Shared = self.data as _;

            // Reserving involves abandoning the currently shared buffer and
            // allocating a new vector with the requested capacity.
            //
            // Compute the new capacity
            let mut new_cap = len.checked_add(additional).expect("overflow");

            let original_capacity;
            let original_capacity_repr;

            unsafe {
                original_capacity_repr = (*shared).original_capacity_repr;
                original_capacity = original_capacity_from_repr(original_capacity_repr);

                // First, try to reclaim the buffer. This is possible if the current
                // handle is the only outstanding handle pointing to the buffer.
                if (*shared).is_unique() {
                    // This is the only handle to the buffer. It can be reclaimed.
                    // However, before doing the work of copying data, check to make
                    // sure that the vector has enough capacity.
                    let v = &mut (*shared).vec;

                    if v.capacity() >= new_cap {
                        // The capacity is sufficient, reclaim the buffer
                        let ptr = v.as_mut_ptr();

                        ptr::copy(self.ptr.as_ptr(), ptr, len);

                        self.ptr = vptr(ptr);
                        self.cap = v.capacity();

                        return;
                    }

                    // The vector capacity is not sufficient. The reserve request is
                    // asking for more than the initial buffer capacity. Allocate more
                    // than requested if `new_cap` is not much bigger than the current
                    // capacity.
                    //
                    // There are some situations, using `reserve_exact` that the
                    // buffer capacity could be below `original_capacity`, so do a
                    // check.
                    let double = v.capacity().checked_shl(1).unwrap_or(new_cap);

                    new_cap = cmp::max(cmp::max(double, new_cap), original_capacity);
                } else {
                    new_cap = cmp::max(new_cap, original_capacity);
                }
            }

            // Create a new vector to store the data
            let mut v = ManuallyDrop::new(Vec::with_capacity(new_cap));

            // Copy the bytes
            v.extend_from_slice(self.as_ref());

            // Release the shared handle. This must be done *after* the bytes are
            // copied.
            unsafe { release_shared(shared) };

            // Update self
            let data = (original_capacity_repr << ORIGINAL_CAPACITY_OFFSET) | KIND_VEC;
            self.data = data as _;
            self.ptr = vptr(v.as_mut_ptr());
            self.len = v.len();
            self.cap = v.capacity();
        }

        /// Appends given bytes to this `BytesMut`.
        ///
        /// If this `BytesMut` object does not have enough capacity, it is resized
        /// first.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut buf = BytesMut::with_capacity(0);
        /// buf.extend_from_slice(b"aaabbb");
        /// buf.extend_from_slice(b"cccddd");
        ///
        /// assert_eq!(b"aaabbbcccddd", &buf[..]);
        /// ```
        pub fn extend_from_slice(&mut self, extend: &[u8]) {
            let cnt = extend.len();
            self.reserve(cnt);

            unsafe {
                let dst = self.uninit_slice();
                // Reserved above
                debug_assert!(dst.len() >= cnt);

                ptr::copy_nonoverlapping(extend.as_ptr(), dst.as_mut_ptr() as *mut u8, cnt);
            }

            unsafe {
                self.advance_mut(cnt);
            }
        }

        /// Absorbs a `BytesMut` that was previously split off.
        ///
        /// If the two `BytesMut` objects were previously contiguous, i.e., if
        /// `other` was created by calling `split_off` on this `BytesMut`, then
        /// this is an `O(1)` operation that just decreases a reference
        /// count and sets a few indices. Otherwise this method degenerates to
        /// `self.extend_from_slice(other.as_ref())`.
        ///
        /// # Examples
        ///
        /// ```
        /// use bytes::BytesMut;
        ///
        /// let mut buf = BytesMut::with_capacity(64);
        /// buf.extend_from_slice(b"aaabbbcccddd");
        ///
        /// let split = buf.split_off(6);
        /// assert_eq!(b"aaabbb", &buf[..]);
        /// assert_eq!(b"cccddd", &split[..]);
        ///
        /// buf.unsplit(split);
        /// assert_eq!(b"aaabbbcccddd", &buf[..]);
        /// ```
        pub fn unsplit(&mut self, other: BytesMut) {
            if self.is_empty() {
                *self = other;
                return;
            }

            if let Err(other) = self.try_unsplit(other) {
                self.extend_from_slice(other.as_ref());
            }
        }

        // private

        // For now, use a `Vec` to manage the memory for us, but we may want to
        // change that in the future to some alternate allocator strategy.
        //
        // Thus, we don't expose an easy way to construct from a `Vec` since an
        // internal change could make a simple pattern (`BytesMut::from(vec)`)
        // suddenly a lot more expensive.
        #[inline]
        pub(crate) fn from_vec(mut vec: Vec<u8>) -> BytesMut {
            let ptr = vptr(vec.as_mut_ptr());
            let len = vec.len();
            let cap = vec.capacity();
            mem::forget(vec);

            let original_capacity_repr = original_capacity_to_repr(cap);
            let data = (original_capacity_repr << ORIGINAL_CAPACITY_OFFSET) | KIND_VEC;

            BytesMut {
                ptr,
                len,
                cap,
                data: data as *mut _,
            }
        }

        #[inline]
        fn as_slice(&self) -> &[u8] {
            unsafe { slice::from_raw_parts(self.ptr.as_ptr(), self.len) }
        }

        #[inline]
        fn as_slice_mut(&mut self) -> &mut [u8] {
            unsafe { slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len) }
        }

        unsafe fn set_start(&mut self, start: usize) {
            // Setting the start to 0 is a no-op, so return early if this is the
            // case.
            if start == 0 {
                return;
            }

            debug_assert!(start <= self.cap, "internal: set_start out of bounds");

            let kind = self.kind();

            if kind == KIND_VEC {
                // Setting the start when in vec representation is a little more
                // complicated. First, we have to track how far ahead the
                // "start" of the byte buffer from the beginning of the vec. We
                // also have to ensure that we don't exceed the maximum shift.
                let (mut pos, prev) = self.get_vec_pos();
                pos += start;

                if pos <= MAX_VEC_POS {
                    self.set_vec_pos(pos, prev);
                } else {
                    // The repr must be upgraded to ARC. This will never happen
                    // on 64 bit systems and will only happen on 32 bit systems
                    // when shifting past 134,217,727 bytes. As such, we don't
                    // worry too much about performance here.
                    self.promote_to_shared(/*ref_count = */ 1);
                }
            }

            // Updating the start of the view is setting `ptr` to point to the
            // new start and updating the `len` field to reflect the new length
            // of the view.
            self.ptr = vptr(self.ptr.as_ptr().offset(start as isize));

            if self.len >= start {
                self.len -= start;
            } else {
                self.len = 0;
            }

            self.cap -= start;
        }

        unsafe fn set_end(&mut self, end: usize) {
            debug_assert_eq!(self.kind(), KIND_ARC);
            assert!(end <= self.cap, "set_end out of bounds");

            self.cap = end;
            self.len = cmp::min(self.len, end);
        }

        fn try_unsplit(&mut self, other: BytesMut) -> Result<(), BytesMut> {
            if other.is_empty() {
                return Ok(());
            }

            let ptr = unsafe { self.ptr.as_ptr().offset(self.len as isize) };
            if ptr == other.ptr.as_ptr()
                && self.kind() == KIND_ARC
                && other.kind() == KIND_ARC
                && self.data == other.data
            {
                // Contiguous blocks, just combine directly
                self.len += other.len;
                self.cap += other.cap;
                Ok(())
            } else {
                Err(other)
            }
        }

        #[inline]
        fn kind(&self) -> usize {
            self.data as usize & KIND_MASK
        }

        unsafe fn promote_to_shared(&mut self, ref_cnt: usize) {
            debug_assert_eq!(self.kind(), KIND_VEC);
            debug_assert!(ref_cnt == 1 || ref_cnt == 2);

            let original_capacity_repr =
                (self.data as usize & ORIGINAL_CAPACITY_MASK) >> ORIGINAL_CAPACITY_OFFSET;

            // The vec offset cannot be concurrently mutated, so there
            // should be no danger reading it.
            let off = (self.data as usize) >> VEC_POS_OFFSET;

            // First, allocate a new `Shared` instance containing the
            // `Vec` fields. It's important to note that `ptr`, `len`,
            // and `cap` cannot be mutated without having `&mut self`.
            // This means that these fields will not be concurrently
            // updated and since the buffer hasn't been promoted to an
            // `Arc`, those three fields still are the components of the
            // vector.
            let shared = Box::new(Shared {
                vec: rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off),
                original_capacity_repr,
                ref_count: AtomicUsize::new(ref_cnt),
            });

            let shared = Box::into_raw(shared);

            // The pointer should be aligned, so this assert should
            // always succeed.
            debug_assert_eq!(shared as usize & KIND_MASK, KIND_ARC);

            self.data = shared as _;
        }

        /// Makes an exact shallow clone of `self`.
        ///
        /// The kind of `self` doesn't matter, but this is unsafe
        /// because the clone will have the same offsets. You must
        /// be sure the returned value to the user doesn't allow
        /// two views into the same range.
        #[inline]
        unsafe fn shallow_clone(&mut self) -> BytesMut {
            if self.kind() == KIND_ARC {
                increment_shared(self.data);
                ptr::read(self)
            } else {
                self.promote_to_shared(/*ref_count = */ 2);
                ptr::read(self)
            }
        }

        #[inline]
        unsafe fn get_vec_pos(&mut self) -> (usize, usize) {
            debug_assert_eq!(self.kind(), KIND_VEC);

            let prev = self.data as usize;
            (prev >> VEC_POS_OFFSET, prev)
        }

        #[inline]
        unsafe fn set_vec_pos(&mut self, pos: usize, prev: usize) {
            debug_assert_eq!(self.kind(), KIND_VEC);
            debug_assert!(pos <= MAX_VEC_POS);

            self.data = ((pos << VEC_POS_OFFSET) | (prev & NOT_VEC_POS_MASK)) as *mut _;
        }

        #[inline]
        fn uninit_slice(&mut self) -> &mut UninitSlice {
            unsafe {
                let ptr = self.ptr.as_ptr().offset(self.len as isize);
                let len = self.cap - self.len;

                UninitSlice::from_raw_parts_mut(ptr, len)
            }
        }
    }

    impl Drop for BytesMut {
        fn drop(&mut self) {
            let kind = self.kind();

            if kind == KIND_VEC {
                unsafe {
                    let (off, _) = self.get_vec_pos();

                    // Vector storage, free the vector
                    let _ = rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off);
                }
            } else if kind == KIND_ARC {
                unsafe { release_shared(self.data as _) };
            }
        }
    }

    impl Buf for BytesMut {
        #[inline]
        fn remaining(&self) -> usize {
            self.len()
        }

        #[inline]
        fn chunk(&self) -> &[u8] {
            self.as_slice()
        }

        #[inline]
        fn advance(&mut self, cnt: usize) {
            assert!(
                cnt <= self.remaining(),
                "cannot advance past `remaining`: {:?} <= {:?}",
                cnt,
                self.remaining(),
            );
            unsafe {
                self.set_start(cnt);
            }
        }

        fn copy_to_bytes(&mut self, len: usize) -> crate::Bytes {
            self.split_to(len).freeze()
        }
    }

    unsafe impl BufMut for BytesMut {
        #[inline]
        fn remaining_mut(&self) -> usize {
            usize::MAX - self.len()
        }

        #[inline]
        unsafe fn advance_mut(&mut self, cnt: usize) {
            let new_len = self.len() + cnt;
            assert!(
                new_len <= self.cap,
                "new_len = {}; capacity = {}",
                new_len,
                self.cap
            );
            self.len = new_len;
        }

        #[inline]
        fn chunk_mut(&mut self) -> &mut UninitSlice {
            if self.capacity() == self.len() {
                self.reserve(64);
            }
            self.uninit_slice()
        }

        // Specialize these methods so they can skip checking `remaining_mut`
        // and `advance_mut`.

        fn put<T: crate::Buf>(&mut self, mut src: T)
        where
            Self: Sized,
        {
            while src.has_remaining() {
                let s = src.chunk();
                let l = s.len();
                self.extend_from_slice(s);
                src.advance(l);
            }
        }

        fn put_slice(&mut self, src: &[u8]) {
            self.extend_from_slice(src);
        }
    }

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

    impl Deref for BytesMut {
        type Target = [u8];

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

    impl AsMut<[u8]> for BytesMut {
        #[inline]
        fn as_mut(&mut self) -> &mut [u8] {
            self.as_slice_mut()
        }
    }

    impl DerefMut for BytesMut {
        #[inline]
        fn deref_mut(&mut self) -> &mut [u8] {
            self.as_mut()
        }
    }

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

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

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

    impl PartialEq for BytesMut {
        fn eq(&self, other: &BytesMut) -> bool {
            self.as_slice() == other.as_slice()
        }
    }

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

    impl Ord for BytesMut {
        fn cmp(&self, other: &BytesMut) -> cmp::Ordering {
            self.as_slice().cmp(other.as_slice())
        }
    }

    impl Eq for BytesMut {}

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

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

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

    impl BorrowMut<[u8]> for BytesMut {
        fn borrow_mut(&mut self) -> &mut [u8] {
            self.as_mut()
        }
    }

    impl fmt::Write for BytesMut {
        #[inline]
        fn write_str(&mut self, s: &str) -> fmt::Result {
            if self.remaining_mut() >= s.len() {
                self.put_slice(s.as_bytes());
                Ok(())
            } else {
                Err(fmt::Error)
            }
        }

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

    impl Clone for BytesMut {
        fn clone(&self) -> BytesMut {
            BytesMut::from(&self[..])
        }
    }

    impl IntoIterator for BytesMut {
        type Item = u8;
        type IntoIter = IntoIter<BytesMut>;

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

    impl<'a> IntoIterator for &'a BytesMut {
        type Item = &'a u8;
        type IntoIter = core::slice::Iter<'a, u8>;

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

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

            let (lower, _) = iter.size_hint();
            self.reserve(lower);

            // TODO: optimize
            // 1. If self.kind() == KIND_VEC, use Vec::extend
            // 2. Make `reserve` inline-able
            for b in iter {
                self.reserve(1);
                self.put_u8(b);
            }
        }
    }

    impl<'a> Extend<&'a u8> for BytesMut {
        fn extend<T>(&mut self, iter: T)
        where
            T: IntoIterator<Item = &'a u8>,
        {
            self.extend(iter.into_iter().map(|b| *b))
        }
    }

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

    impl<'a> FromIterator<&'a u8> for BytesMut {
        fn from_iter<T: IntoIterator<Item = &'a u8>>(into_iter: T) -> Self {
            BytesMut::from_iter(into_iter.into_iter().map(|b| *b))
        }
    }

    /*
     *
     * ===== Inner =====
     *
     */

    unsafe fn increment_shared(ptr: *mut Shared) {
        let old_size = (*ptr).ref_count.fetch_add(1, Ordering::Relaxed);

        if old_size > isize::MAX as usize {
            crate::abort();
        }
    }

    unsafe fn release_shared(ptr: *mut Shared) {
        // `Shared` storage... follow the drop steps from Arc.
        if (*ptr).ref_count.fetch_sub(1, Ordering::Release) != 1 {
            return;
        }

        // This fence is needed to prevent reordering of use of the data and
        // deletion of the data.  Because it is marked `Release`, the decreasing
        // of the reference count synchronizes with this `Acquire` fence. This
        // means that use of the data happens before decreasing the reference
        // count, which happens before this fence, which happens before the
        // deletion of the data.
        //
        // As explained in the [Boost documentation][1],
        //
        // > It is important to enforce any possible access to the object in one
        // > thread (through an existing reference) to *happen before* deleting
        // > the object in a different thread. This is achieved by a "release"
        // > operation after dropping a reference (any access to the object
        // > through this reference must obviously happened before), and an
        // > "acquire" operation before deleting the object.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        atomic::fence(Ordering::Acquire);

        // Drop the data
        Box::from_raw(ptr);
    }

    impl Shared {
        fn is_unique(&self) -> bool {
            // The goal is to check if the current handle is the only handle
            // that currently has access to the buffer. This is done by
            // checking if the `ref_count` is currently 1.
            //
            // The `Acquire` ordering synchronizes with the `Release` as
            // part of the `fetch_sub` in `release_shared`. The `fetch_sub`
            // operation guarantees that any mutations done in other threads
            // are ordered before the `ref_count` is decremented. As such,
            // this `Acquire` will guarantee that those mutations are
            // visible to the current thread.
            self.ref_count.load(Ordering::Acquire) == 1
        }
    }

    fn original_capacity_to_repr(cap: usize) -> usize {
        let width = PTR_WIDTH - ((cap >> MIN_ORIGINAL_CAPACITY_WIDTH).leading_zeros() as usize);
        cmp::min(
            width,
            MAX_ORIGINAL_CAPACITY_WIDTH - MIN_ORIGINAL_CAPACITY_WIDTH,
        )
    }

    fn original_capacity_from_repr(repr: usize) -> usize {
        if repr == 0 {
            return 0;
        }

        1 << (repr + (MIN_ORIGINAL_CAPACITY_WIDTH - 1))
    }

    /*
    #[test]
    fn test_original_capacity_to_repr() {
        assert_eq!(original_capacity_to_repr(0), 0);

        let max_width = 32;

        for width in 1..(max_width + 1) {
            let cap = 1 << width - 1;

            let expected = if width < MIN_ORIGINAL_CAPACITY_WIDTH {
                0
            } else if width < MAX_ORIGINAL_CAPACITY_WIDTH {
                width - MIN_ORIGINAL_CAPACITY_WIDTH
            } else {
                MAX_ORIGINAL_CAPACITY_WIDTH - MIN_ORIGINAL_CAPACITY_WIDTH
            };

            assert_eq!(original_capacity_to_repr(cap), expected);

            if width > 1 {
                assert_eq!(original_capacity_to_repr(cap + 1), expected);
            }

            //  MIN_ORIGINAL_CAPACITY_WIDTH must be bigger than 7 to pass tests below
            if width == MIN_ORIGINAL_CAPACITY_WIDTH + 1 {
                assert_eq!(original_capacity_to_repr(cap - 24), expected - 1);
                assert_eq!(original_capacity_to_repr(cap + 76), expected);
            } else if width == MIN_ORIGINAL_CAPACITY_WIDTH + 2 {
                assert_eq!(original_capacity_to_repr(cap - 1), expected - 1);
                assert_eq!(original_capacity_to_repr(cap - 48), expected - 1);
            }
        }
    }

    #[test]
    fn test_original_capacity_from_repr() {
        assert_eq!(0, original_capacity_from_repr(0));

        let min_cap = 1 << MIN_ORIGINAL_CAPACITY_WIDTH;

        assert_eq!(min_cap, original_capacity_from_repr(1));
        assert_eq!(min_cap * 2, original_capacity_from_repr(2));
        assert_eq!(min_cap * 4, original_capacity_from_repr(3));
        assert_eq!(min_cap * 8, original_capacity_from_repr(4));
        assert_eq!(min_cap * 16, original_capacity_from_repr(5));
        assert_eq!(min_cap * 32, original_capacity_from_repr(6));
        assert_eq!(min_cap * 64, original_capacity_from_repr(7));
    }
    */

    unsafe impl Send for BytesMut {}
    unsafe impl Sync for BytesMut {}

    /*
     *
     * ===== PartialEq / PartialOrd =====
     *
     */

    impl PartialEq<[u8]> for BytesMut {
        fn eq(&self, other: &[u8]) -> bool {
            &**self == other
        }
    }

    impl PartialOrd<[u8]> for BytesMut {
        fn partial_cmp(&self, other: &[u8]) -> Option<cmp::Ordering> {
            (**self).partial_cmp(other)
        }
    }

    impl PartialEq<BytesMut> for [u8] {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

    impl PartialOrd<BytesMut> for [u8] {
        fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
            <[u8] as PartialOrd<[u8]>>::partial_cmp(self, other)
        }
    }

    impl PartialEq<str> for BytesMut {
        fn eq(&self, other: &str) -> bool {
            &**self == other.as_bytes()
        }
    }

    impl PartialOrd<str> for BytesMut {
        fn partial_cmp(&self, other: &str) -> Option<cmp::Ordering> {
            (**self).partial_cmp(other.as_bytes())
        }
    }

    impl PartialEq<BytesMut> for str {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

    impl PartialOrd<BytesMut> for str {
        fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
            <[u8] as PartialOrd<[u8]>>::partial_cmp(self.as_bytes(), other)
        }
    }

    impl PartialEq<Vec<u8>> for BytesMut {
        fn eq(&self, other: &Vec<u8>) -> bool {
            *self == &other[..]
        }
    }

    impl PartialOrd<Vec<u8>> for BytesMut {
        fn partial_cmp(&self, other: &Vec<u8>) -> Option<cmp::Ordering> {
            (**self).partial_cmp(&other[..])
        }
    }

    impl PartialEq<BytesMut> for Vec<u8> {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

    impl PartialOrd<BytesMut> for Vec<u8> {
        fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
            other.partial_cmp(self)
        }
    }

    impl PartialEq<String> for BytesMut {
        fn eq(&self, other: &String) -> bool {
            *self == &other[..]
        }
    }

    impl PartialOrd<String> for BytesMut {
        fn partial_cmp(&self, other: &String) -> Option<cmp::Ordering> {
            (**self).partial_cmp(other.as_bytes())
        }
    }

    impl PartialEq<BytesMut> for String {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

    impl PartialOrd<BytesMut> for String {
        fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
            <[u8] as PartialOrd<[u8]>>::partial_cmp(self.as_bytes(), other)
        }
    }

    impl<'a, T: ?Sized> PartialEq<&'a T> for BytesMut
    where
        BytesMut: PartialEq<T>,
    {
        fn eq(&self, other: &&'a T) -> bool {
            *self == **other
        }
    }

    impl<'a, T: ?Sized> PartialOrd<&'a T> for BytesMut
    where
        BytesMut: PartialOrd<T>,
    {
        fn partial_cmp(&self, other: &&'a T) -> Option<cmp::Ordering> {
            self.partial_cmp(*other)
        }
    }

    impl PartialEq<BytesMut> for &[u8] {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

    impl PartialOrd<BytesMut> for &[u8] {
        fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
            <[u8] as PartialOrd<[u8]>>::partial_cmp(self, other)
        }
    }

    impl PartialEq<BytesMut> for &str {
        fn eq(&self, other: &BytesMut) -> bool {
            *other == *self
        }
    }

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

    impl PartialEq<BytesMut> for Bytes {
        fn eq(&self, other: &BytesMut) -> bool {
            &other[..] == &self[..]
        }
    }

    impl PartialEq<Bytes> for BytesMut {
        fn eq(&self, other: &Bytes) -> bool {
            &other[..] == &self[..]
        }
    }

    fn vptr(ptr: *mut u8) -> NonNull<u8> {
        if cfg!(debug_assertions) {
            NonNull::new(ptr).expect("Vec pointer should be non-null")
        } else {
            unsafe { NonNull::new_unchecked(ptr) }
        }
    }

    unsafe fn rebuild_vec(ptr: *mut u8, mut len: usize, mut cap: usize, off: usize) -> Vec<u8> {
        let ptr = ptr.offset(-(off as isize));
        len += off;
        cap += off;

        Vec::from_raw_parts(ptr, len, cap)
    }

    // ===== impl SharedVtable =====

    static SHARED_VTABLE: Vtable = Vtable {
        clone: shared_v_clone,
        drop: shared_v_drop,
    };

    unsafe fn shared_v_clone(data: &AtomicPtr<()>, ptr: *const u8, len: usize) -> Bytes {
        let shared = data.load(Ordering::Relaxed) as *mut Shared;
        increment_shared(shared);

        let data = AtomicPtr::new(shared as _);
        Bytes::with_vtable(ptr, len, data, &SHARED_VTABLE)
    }

    unsafe fn shared_v_drop(data: &mut AtomicPtr<()>, _ptr: *const u8, _len: usize) {
        data.with_mut(|shared| {
            release_shared(*shared as *mut Shared);
        });
    }

    // compile-fails

    /// ```compile_fail
    /// use bytes::BytesMut;
    /// #[deny(unused_must_use)]
    /// {
    ///     let mut b1 = BytesMut::from("hello world");
    ///     b1.split_to(6);
    /// }
    /// ```
    fn _split_to_must_use() {}

    /// ```compile_fail
    /// use bytes::BytesMut;
    /// #[deny(unused_must_use)]
    /// {
    ///     let mut b1 = BytesMut::from("hello world");
    ///     b1.split_off(6);
    /// }
    /// ```
    fn _split_off_must_use() {}

    /// ```compile_fail
    /// use bytes::BytesMut;
    /// #[deny(unused_must_use)]
    /// {
    ///     let mut b1 = BytesMut::from("hello world");
    ///     b1.split();
    /// }
    /// ```
    fn _split_must_use() {}

    // fuzz tests
    #[cfg(all(test, loom))]
    mod fuzz {
        use loom::sync::Arc;
        use loom::thread;

        use super::BytesMut;
        use crate::Bytes;

        #[test]
        fn bytes_mut_cloning_frozen() {
            loom::model(|| {
                let a = BytesMut::from(&b"abcdefgh"[..]).split().freeze();
                let addr = a.as_ptr() as usize;

                // test the Bytes::clone is Sync by putting it in an Arc
                let a1 = Arc::new(a);
                let a2 = a1.clone();

                let t1 = thread::spawn(move || {
                    let b: Bytes = (*a1).clone();
                    assert_eq!(b.as_ptr() as usize, addr);
                });

                let t2 = thread::spawn(move || {
                    let b: Bytes = (*a2).clone();
                    assert_eq!(b.as_ptr() as usize, addr);
                });

                t1.join().unwrap();
                t2.join().unwrap();
            });
        }
    }