linear_deque/

lib.rs

#![cfg_attr(not(any(feature = "std", test)), no_std)]
#![warn(missing_docs)]
#![doc(test(attr(deny(warnings))))]

//! A double-ended queue that can be sliced at any time without preparation.
//!
//! # [`LinearDeque`] vs [`VecDeque`]
//!
//! ## Slicing
//!
//! The standard [`VecDeque`] uses a ring buffer. It requires that the
//! [`make_contiguous`] method is called to ensure that the deque content can
//! all be referenced in a single slice. [`make_contiguous`] is only callable on
//! a mutable instance of the deque.
//!
//! The [`LinearDeque`] provided by this lib uses a linear buffer, keeping all
//! its content contiguous and allowing to have a slice with all the content at
//! any time, even when the deque is not mutable.
//!
//! ## Memory Usage
//!
//! By using a ring buffer, all spare memory allocated by the standard
//! [`VecDeque`] can be used for elements added at both the front and the back
//! ends.
//!
//! Using a linear buffer, each end of the [`LinearDeque`] have its own reserved
//! memory, so it tends to use more memory a do more re-allocations than [`VecDeque`].
//!
//!  # `no_std`, `alloc` and the `std` feature
//!
//! For backward compatibility, this crate uses [`std`] by default. The `std` feature
//! can be disabled to use this crate in a `no_std` environment. In that case, the crate
//! will use the [`alloc`] crate instead of [`std`] for heap allocation and other utilities.
//!
//!  The only functionality that is gated behind the `std` feature is the implementation of
//! [`Read`] and [`Write`] traits for `LinearDeque<u8>`.
//!
//! To disable the `std` feature, add the following to the `dependencies` in your `Cargo.toml`:
//!
//! ```toml
//! linear-deque = { version = "0.1", default-features = false }
//!  ```
//!
//! [`VecDeque`]: std::collections::VecDeque
//! [`make_contiguous`]: std::collections::VecDeque::make_contiguous

extern crate alloc;

use alloc::vec::Vec;
use core::cmp::Ordering;
use core::fmt::Debug;
use core::hash::Hash;
use core::marker::PhantomData;
use core::ops::{Bound, Deref, DerefMut, RangeBounds};
use core::ptr;
use core::{cmp, mem};
#[cfg(feature = "std")]
use std::io::{Read, Write};

use buffer::Buffer;
use iter::RawValIter;

mod buffer;
mod iter;

#[cfg(test)]
mod drop_tracker;

/// A double-ended queue implemented with a growable linear buffer.
///
/// A `LinearDeque` with a known list of items can be initialized from an array:
///
/// ```
/// use linear_deque::LinearDeque;
///
/// # #[allow(unused)]
/// let deq = LinearDeque::from([-1, 0, 1]);
/// ```
///
/// Since `LinearDeque` is a linear buffer, its elements are contiguous in
/// memory, and it can be coerced into a slice at any time.
pub struct LinearDeque<T> {
    buf: Buffer<T>,
    len: usize,
    off: usize,
}

unsafe impl<T: Send> Send for LinearDeque<T> {}
unsafe impl<T: Sync> Sync for LinearDeque<T> {}

impl<T> LinearDeque<T> {
    fn ptr(&self) -> *mut T {
        self.buf.ptr.as_ptr()
    }

    fn cap(&self) -> usize {
        self.buf.cap
    }

    /// Creates an empty deque.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    /// # #[allow(unused)]
    /// let deque: LinearDeque<u32> = LinearDeque::new();
    /// ```
    #[must_use]
    pub fn new() -> Self {
        Self::with_reserved_space(0, 0)
    }

    /// Creates an empty deque with reserved spaces at the front and the back.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let deque: LinearDeque<u32> = LinearDeque::with_reserved_space(3, 7);
    ///
    /// assert_eq!(deque.reserved_front_space(), 3);
    /// assert_eq!(deque.reserved_back_space(), 7);
    /// assert_eq!(deque.len(), 0);
    /// ```
    #[must_use]
    pub fn with_reserved_space(front: usize, back: usize) -> Self {
        let mut buf = Buffer::new();

        let cap = front + back;
        if cap > 0 && !is_zst::<T>() {
            buf.realloc(cap);
        }

        LinearDeque {
            buf,
            len: 0,
            off: front,
        }
    }

    /// Provides a reference to the front element, or `None` if the deque is
    /// empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// assert_eq!(d.front(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.front(), Some(&1));
    /// ```
    #[must_use]
    pub fn front(&self) -> Option<&T> {
        self.first()
    }

    /// Provides a mutable reference to the front element, or `None` if the
    /// deque is empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// assert_eq!(d.front_mut(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.front_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.front(), Some(&9));
    /// ```
    pub fn front_mut(&mut self) -> Option<&mut T> {
        self.first_mut()
    }

    /// Provides a reference to the back element, or `None` if the deque is
    /// empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.back(), Some(&2));
    /// ```
    #[must_use]
    pub fn back(&self) -> Option<&T> {
        self.last()
    }

    /// Provides a mutable reference to the back element, or `None` if the
    /// deque is empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.back_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.back(), Some(&9));
    /// ```
    pub fn back_mut(&mut self) -> Option<&mut T> {
        self.last_mut()
    }

    /// Prepends an element to the deque.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// d.push_front(1);
    /// d.push_front(2);
    /// assert_eq!(d.front(), Some(&2));
    /// ```
    pub fn push_front(&mut self, elem: T) {
        if !is_zst::<T>() {
            self.ensure_reserved_front_space(1);
            unsafe {
                self.off -= 1;
                ptr::write(self.ptr().add(self.off), elem);
            }
        }
        self.len += 1;
    }

    /// Appends an element to the back of the deque.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::new();
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(3, *buf.back().unwrap());
    /// ```
    pub fn push_back(&mut self, elem: T) {
        self.ensure_reserved_back_space(1);
        unsafe {
            ptr::write(self.ptr().add(self.off + self.len), elem);
        }

        self.len += 1;
    }

    /// Removes the first element and returns it, or `None` if the deque is
    /// empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut d = LinearDeque::new();
    /// d.push_back(1);
    /// d.push_back(2);
    ///
    /// assert_eq!(d.pop_front(), Some(1));
    /// assert_eq!(d.pop_front(), Some(2));
    /// assert_eq!(d.pop_front(), None);
    /// ```
    pub fn pop_front(&mut self) -> Option<T> {
        if self.len == 0 {
            None
        } else {
            self.len -= 1;
            self.off += 1;
            unsafe { Some(ptr::read(self.ptr().add(self.off - 1))) }
        }
    }

    /// Removes the last element from the deque and returns it, or `None` if
    /// it is empty.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::new();
    /// assert_eq!(buf.pop_back(), None);
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(buf.pop_back(), Some(3));
    /// ```
    pub fn pop_back(&mut self) -> Option<T> {
        if self.len == 0 {
            None
        } else {
            self.len -= 1;
            unsafe { Some(ptr::read(self.ptr().add(self.off + self.len))) }
        }
    }

    /// Inserts an element at `index` within the deque, shifting all elements
    /// before or after the index.
    ///
    /// If `index` is nearer to the front, the elements with indices lower than
    /// `index` are moved to the left; otherwise, the elements with indices
    /// grater than `index` are moved right.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Panics
    ///
    /// Panics if `index` is greater than deque's length
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::new();
    /// deque.push_back('a');
    /// deque.push_back('b');
    /// deque.push_back('c');
    /// assert_eq!(deque, &['a', 'b', 'c']);
    ///
    /// deque.insert(1, 'd');
    /// assert_eq!(deque, &['a', 'd', 'b', 'c']);
    /// ```
    pub fn insert(&mut self, index: usize, elem: T) {
        assert!(index <= self.len, "index out of bounds");

        if !is_zst::<T>() {
            if 2 * index < self.len {
                // near front
                unsafe {
                    let pending_copy = self.prepare_reserved_front_space(1);
                    let (mut front_copy, back_copy) = pending_copy.split(index);
                    front_copy.dst -= 1;
                    back_copy.perform(self.ptr());
                    front_copy.perform(self.ptr());
                    self.off -= 1;
                    ptr::write(self.ptr().add(self.off + index), elem);
                }
            } else {
                // near back
                unsafe {
                    let pending_copy = self.prepare_reserved_back_space(1);
                    let (front_copy, mut back_copy) = pending_copy.split(index);
                    back_copy.dst += 1;
                    front_copy.perform(self.ptr());
                    back_copy.perform(self.ptr());
                    ptr::write(self.ptr().add(self.off + index), elem);
                }
            }
        }
        self.len += 1;
    }

    /// Removes and returns the element at `index` from the deque.
    /// Whichever end is closer to the removal point will be moved to make
    /// room, and all the affected elements will be moved to new positions.
    /// Returns `None` if `index` is out of bounds.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::new();
    /// buf.push_back(1);
    /// buf.push_back(2);
    /// buf.push_back(3);
    /// assert_eq!(buf, [1, 2, 3]);
    ///
    /// assert_eq!(buf.remove(1), Some(2));
    /// assert_eq!(buf, [1, 3]);
    /// ```
    pub fn remove(&mut self, index: usize) -> Option<T> {
        if index < self.len {
            unsafe {
                let start = self.ptr().add(self.off);
                let result = ptr::read(start.add(index));

                if index < self.len / 2 {
                    // near front
                    ptr::copy(start, start.add(1), index);
                    self.off += 1;
                } else {
                    // near back
                    ptr::copy(start.add(index + 1), start.add(index), self.len - index - 1);
                }
                self.len -= 1;

                Some(result)
            }
        } else {
            None
        }
    }

    /// Removes the specified range from the deque in bulk, returning all removed
    /// elements as an iterator. If the iterator is dropped before being fully
    /// consumed, it drops the remaining removed elements.
    ///
    /// The returned iterator keeps a mutable borrow on the queue to optimize
    /// its implementation.
    ///
    /// After draining, the remaining unused allocated memory is equally split
    /// as reserved space for both ends.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the deque.
    ///
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque: LinearDeque<_> = [1, 2, 3].into();
    /// let drained = deque.drain(2..).collect::<LinearDeque<_>>();
    /// assert_eq!(drained, [3]);
    /// assert_eq!(deque, [1, 2]);
    ///
    /// // A full range clears all contents, like `clear()` does
    /// deque.drain(..);
    /// assert!(deque.is_empty());
    /// ```
    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T>
    where
        R: RangeBounds<usize>,
    {
        let start = match range.start_bound() {
            Bound::Included(&start) => start,
            Bound::Excluded(start) => start.checked_add(1).expect("invalid start index"),
            Bound::Unbounded => 0,
        };

        let end = match range.end_bound() {
            Bound::Included(end) => end.checked_add(1).expect("invalid end index"),
            Bound::Excluded(&end) => end,
            Bound::Unbounded => self.len,
        };

        assert!(!(start > end || end > self.len), "invalid range");

        let iter = unsafe { RawValIter::new(&self[start..end]) };

        let front_len = start;
        let back_len = self.len - end;
        let count = end - start;

        let pending_copy = if front_len < back_len {
            let prev_off = self.off;
            self.off += count;
            PendingCopy {
                src: prev_off,
                dst: prev_off + count,
                count: front_len,
            }
        } else {
            PendingCopy {
                src: self.off + end,
                dst: self.off + start,
                count: back_len,
            }
        };

        self.len -= count;

        Drain {
            iter,
            pending_copy,
            ptr: self.ptr(),
            vec: PhantomData,
        }
    }

    /// Extends the deque at the front end.
    ///
    /// It's the same as iterating on the `iter` parameter, passing each element
    /// to [`push_front`].
    ///
    /// [`push_front`]: LinearDeque::push_front
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::from([2, 1]);
    /// deque.extend_at_front([3, 4]);
    /// assert_eq!(deque, [4, 3, 2, 1]);
    /// ```
    pub fn extend_at_front(&mut self, iter: impl IntoIterator<Item = T>) {
        let iter = iter.into_iter();
        let (lower, upper) = iter.size_hint();
        let count = upper.unwrap_or(lower);
        if count < usize::MAX {
            self.ensure_reserved_front_space(count);
        }
        for elem in iter {
            self.push_front(elem);
        }
    }

    /// Extends the deque at the back end.
    ///
    /// It's the same as iterating on the `iter` parameter, passing each element
    /// to [`push_back`].
    ///
    /// [`push_back`]: LinearDeque::push_back
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::from([1, 2]);
    /// deque.extend_at_back([3, 4]);
    /// assert_eq!(deque, [1, 2, 3, 4]);
    /// ```
    pub fn extend_at_back(&mut self, iter: impl IntoIterator<Item = T>) {
        let iter = iter.into_iter();
        let (lower, upper) = iter.size_hint();
        let count = upper.unwrap_or(lower);
        if count < usize::MAX {
            self.ensure_reserved_back_space(count);
        }
        for elem in iter {
            self.push_back(elem);
        }
    }

    /// Resizes the `LinearDeque` in-place at the front end so that `len` is
    /// equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `LinearDeque` is extended by the
    /// difference, with each additional slot at front filled with the result of
    /// calling the closure `f`. The return values from `f` will end up in the
    /// `LinearDeque` in the order they have been generated.
    ///
    /// If `new_len` is less than `len`, the `LinearDeque` is simply truncated.
    ///
    /// This method uses a closure to create new values on every push. If you'd
    /// rather [`Clone`] a given value, use [`resize_at_front`]. If you want to
    /// use the [`Default`] trait to generate values, you can pass
    /// [`Default::default`] as the second argument.
    ///
    /// [`resize_at_front`]: LinearDeque::resize_at_front
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::from([1, 2, 3]);
    /// deque.resize_at_front_with(5, Default::default);
    /// assert_eq!(deque, [0, 0, 1, 2, 3]);
    ///
    /// let mut deque = LinearDeque::new();
    /// let mut p = 1;
    /// deque.resize_at_front_with(4, || { p *= 2; p });
    /// assert_eq!(deque, [16, 8, 4, 2]);
    /// ```
    pub fn resize_at_front_with(&mut self, new_len: usize, mut f: impl FnMut() -> T) {
        match new_len.cmp(&self.len) {
            Ordering::Less => self.truncate_at_front(new_len),
            Ordering::Equal => (),
            Ordering::Greater => {
                let count = new_len - self.len;
                self.set_reserved_space(SetReservedSpace::GrowTo(count), SetReservedSpace::Keep);
                self.extend_at_front(core::iter::from_fn(|| Some(f())).take(count));
            }
        }
    }

    /// Resizes the `LinearDeque` in-place at the back end so that `len` is
    /// equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `LinearDeque` is extended by the
    /// difference, with each additional slot at back filled with the result of
    /// calling the closure `f`. The return values from `f` will end up in the
    /// `LinearDeque` in the order they have been generated.
    ///
    /// If `new_len` is less than `len`, the `LinearDeque` is simply truncated.
    ///
    /// This method uses a closure to create new values on every push. If you'd
    /// rather [`Clone`] a given value, use [`resize_at_back`]. If you want to
    /// use the [`Default`] trait to generate values, you can pass
    /// [`Default::default`] as the second argument.
    ///
    /// [`resize_at_back`]: LinearDeque::resize_at_back
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::from([1, 2, 3]);
    /// deque.resize_at_back_with(5, Default::default);
    /// assert_eq!(deque, [1, 2, 3, 0, 0]);
    ///
    /// let mut deque = LinearDeque::new();
    /// let mut p = 1;
    /// deque.resize_at_back_with(4, || { p *= 2; p });
    /// assert_eq!(deque, [2, 4, 8, 16]);
    /// ```
    pub fn resize_at_back_with(&mut self, new_len: usize, mut f: impl FnMut() -> T) {
        match new_len.cmp(&self.len) {
            Ordering::Less => self.truncate_at_back(new_len),
            Ordering::Equal => (),
            Ordering::Greater => {
                let count = new_len - self.len;
                self.set_reserved_space(SetReservedSpace::Keep, SetReservedSpace::GrowTo(count));
                self.extend_at_back(core::iter::from_fn(|| Some(f())).take(count));
            }
        }
    }

    /// Resizes the `LinearDeque` in-place at the back end so that `len` is
    /// equal to `new_len`.
    ///
    /// It is just an alias for [`resize_at_back_with`].
    ///
    /// [`resize_at_back_with`]: LinearDeque::resize_at_back_with
    #[inline]
    pub fn resize_with(&mut self, new_len: usize, f: impl FnMut() -> T) {
        self.resize_at_back_with(new_len, f);
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all elements `e` for which `f(&e)` returns false.
    /// This method operates in place, visiting each element exactly once in the
    /// original order, and preserves the order of the retained elements.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::from_iter(1..=5);
    /// buf.retain(|&x| x % 2 == 0);
    /// assert_eq!(buf, [2, 4]);
    /// ```
    ///
    /// Because the elements are visited exactly once in the original order,
    /// external state may be used to decide which elements to keep.
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::from_iter(1..6);
    ///
    /// let keep = [false, true, true, false, true];
    /// let mut iter = keep.iter();
    /// buf.retain(|_| *iter.next().unwrap());
    /// assert_eq!(buf, [2, 3, 5]);
    /// ```
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&T) -> bool,
    {
        self.retain_mut(|x| f(x));
    }

    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
    ///
    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
    /// This method operates in place, visiting each element exactly once in the
    /// original order, and preserves the order of the retained elements.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::from([1, 2, 3, 4, 5]);
    /// deque.retain_mut(|x| if *x % 2 == 0 {
    ///     *x += 1;
    ///     true
    /// } else {
    ///     false
    /// });
    /// assert_eq!(deque, [3, 5]);
    /// ```
    pub fn retain_mut<F>(&mut self, mut f: F)
    where
        F: FnMut(&mut T) -> bool,
    {
        unsafe {
            let mut dropped_index_option = None;

            let base = self.ptr().add(self.off);
            for i in 0..self.len {
                let p = base.add(i);
                if f(&mut *p) {
                    if let Some(dropped_index) = dropped_index_option {
                        ptr::copy_nonoverlapping(p, base.add(dropped_index), 1);
                        dropped_index_option = Some(dropped_index + 1);
                    }
                } else {
                    p.drop_in_place();
                    if dropped_index_option.is_none() {
                        dropped_index_option = Some(i);
                    }
                }
            }

            if let Some(dropped_index) = dropped_index_option {
                self.len = dropped_index;
            }
        }
    }

    /// Shortens the deque, keeping the last `len` elements and dropping
    /// the rest.
    ///
    /// If `len` is greater than the deque's current length, this has no
    /// effect.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// assert_eq!(buf, [5, 10, 15]);
    /// buf.truncate_at_front(1);
    /// assert_eq!(buf, [15]);
    /// ```
    pub fn truncate_at_front(&mut self, len: usize) {
        if len < self.len {
            unsafe {
                let count = self.len() - len;
                let front = self.get_unchecked_mut(..count);
                ptr::drop_in_place(front);
                self.off += count;
                self.len = len;
            }
        }
    }

    /// Shortens the deque, keeping the first `len` elements and dropping
    /// the rest.
    ///
    /// If `len` is greater than the deque's current length, this has no
    /// effect.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// assert_eq!(buf, [5, 10, 15]);
    /// buf.truncate_at_back(1);
    /// assert_eq!(buf, [5]);
    /// ```
    pub fn truncate_at_back(&mut self, len: usize) {
        if len < self.len {
            unsafe {
                let back = self.get_unchecked_mut(len..);
                ptr::drop_in_place(back);
                self.len = len;
            }
        }
    }

    /// Shortens the deque, keeping the first `len` elements and dropping
    /// the rest.
    ///
    /// It is just an alias for [`truncate_at_back`].
    ///
    /// [`truncate_at_back`]: LinearDeque::truncate_at_back
    #[inline]
    pub fn truncate(&mut self, len: usize) {
        self.truncate_at_back(len);
    }

    /// Clears the deque, removing all values.
    ///
    /// After clearing, the reserved space is equally distributed to the front
    /// and back ends.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut deque = LinearDeque::new();
    /// deque.push_back(1);
    /// deque.clear();
    /// assert!(deque.is_empty());
    /// ```
    pub fn clear(&mut self) {
        self.truncate_at_back(0);
        self.off = self.cap() / 2;
    }

    /// Sets the reserved space on both ends of the deque.
    ///
    /// When the reserved front space is changed, the existing elements are moved.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::{LinearDeque, SetReservedSpace};
    ///
    /// let mut deque: LinearDeque<i32> = LinearDeque::with_reserved_space(3, 7);
    /// assert_eq!(deque.reserved_front_space(), 3);
    /// assert_eq!(deque.reserved_back_space(), 7);
    ///
    /// deque.set_reserved_space(SetReservedSpace::GrowTo(8), SetReservedSpace::Keep);
    /// assert_eq!(deque.reserved_front_space(), 8);
    /// assert_eq!(deque.reserved_back_space(), 7);
    /// ```
    pub fn set_reserved_space(&mut self, front: SetReservedSpace, back: SetReservedSpace) {
        fn new_space(current: usize, set: SetReservedSpace) -> usize {
            match set {
                SetReservedSpace::Keep => current,
                SetReservedSpace::ShrinkTo(new) => current.min(new),
                SetReservedSpace::GrowTo(new) => current.max(new),
                SetReservedSpace::Exact(new) => new,
            }
        }

        if is_zst::<T>() {
            return;
        }

        let front_space = self.reserved_front_space();
        let back_space = self.reserved_back_space();
        let cap = self.cap();

        let new_front_space = new_space(front_space, front);
        let new_back_space = new_space(back_space, back);
        let new_cap = new_front_space + self.len + new_back_space;

        if new_cap > cap {
            self.buf.realloc(new_cap);
        }

        if new_front_space != front_space {
            unsafe {
                ptr::copy(
                    self.ptr().add(front_space),
                    self.ptr().add(new_front_space),
                    self.len,
                );
            }
            self.off = new_front_space;
        }

        if new_cap < cap {
            self.buf.realloc(new_cap);
        }
    }

    fn ensure_reserved_front_space(&mut self, count: usize) {
        unsafe {
            let pending_copy = self.prepare_reserved_front_space(count);
            pending_copy.perform(self.ptr());
        }
    }

    unsafe fn prepare_reserved_front_space(&mut self, count: usize) -> PendingCopy {
        let mut pending_copy = PendingCopy {
            src: self.off,
            dst: self.off,
            count: self.len,
        };

        if self.reserved_front_space() >= count {
            // do nothing
        } else {
            let total_reserved_space = self.cap() - self.len;
            self.off = if total_reserved_space >= count {
                usize::midpoint(total_reserved_space, count)
            } else {
                let growth = cmp::max(1, self.len);
                let new_cap = if count > total_reserved_space + growth {
                    self.len + count
                } else {
                    self.cap() + growth
                };
                self.buf.realloc(new_cap);
                new_cap - self.len
            };
            pending_copy.dst = self.off;
        }

        debug_assert!(self.reserved_front_space() >= count);

        pending_copy
    }

    fn ensure_reserved_back_space(&mut self, count: usize) {
        unsafe {
            let pending_copy = self.prepare_reserved_back_space(count);
            pending_copy.perform(self.ptr());
        }
    }

    unsafe fn prepare_reserved_back_space(&mut self, count: usize) -> PendingCopy {
        let mut pending_copy = PendingCopy {
            src: self.off,
            dst: self.off,
            count: self.len,
        };

        if self.reserved_back_space() >= count {
            // do nothing
        } else {
            let total_reserved_space = self.cap() - self.len;
            self.off = if total_reserved_space >= count {
                (total_reserved_space - count) / 2
            } else {
                let growth = cmp::max(1, self.len);
                let new_cap = if count > total_reserved_space + growth {
                    self.len + count
                } else {
                    self.cap() + growth
                };
                self.buf.realloc(new_cap);
                0
            };
            pending_copy.dst = self.off;
        }

        debug_assert!(self.reserved_back_space() >= count);

        pending_copy
    }

    /// Returns the number of elements can be put at the front of the deque
    /// without reallocating or moving existing elements.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let buf: LinearDeque<i32> = LinearDeque::with_reserved_space(7, 3);
    /// assert!(buf.reserved_front_space() == 7);
    /// ```
    #[must_use]
    pub fn reserved_front_space(&self) -> usize {
        if is_zst::<T>() { usize::MAX } else { self.off }
    }

    /// Returns the number of elements can be put at the back of the deque
    /// without reallocating or moving existing elements.
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let buf: LinearDeque<i32> = LinearDeque::with_reserved_space(7, 3);
    /// assert!(buf.reserved_back_space() == 3);
    /// ```
    #[must_use]
    pub fn reserved_back_space(&self) -> usize {
        if is_zst::<T>() {
            usize::MAX
        } else {
            self.cap() - self.len - self.off
        }
    }
}

impl<T: Clone> LinearDeque<T> {
    /// Resizes the `LinearDeque` in-place at the front end so that `len` is
    /// equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `LinearDeque` is extended by the
    /// difference, with clones of the `value` pushed at the front. If `new_len`
    /// is less than `len`, the `LinearDeque` is simply truncated at the front.
    ///
    /// This method requires `T` to implement [`Clone`], in order to be able to
    /// clone the passed value. If you need more flexibility (or want to rely on
    /// [`Default`] instead of [`Clone`]), use [`resize_at_front_with`]. If you
    /// only need to resize to a smaller size, use [`truncate_at_front`].
    ///
    /// [`resize_at_front_with`]: LinearDeque::resize_at_front_with
    /// [`truncate_at_front`]: LinearDeque::truncate_at_front
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::from([5, 10, 15]);
    ///
    /// buf.resize_at_front(2, 0);
    /// assert_eq!(buf, [10, 15]);
    ///
    /// buf.resize_at_front(5, 20);
    /// assert_eq!(buf, [20, 20, 20, 10, 15]);
    /// ```
    pub fn resize_at_front(&mut self, new_len: usize, value: T) {
        self.resize_at_front_with(new_len, || value.clone());
    }

    /// Resizes the `LinearDeque` in-place at the back end so that `len` is
    /// equal to `new_len`.
    ///
    /// If `new_len` is greater than `len`, the `LinearDeque` is extended by the
    /// difference, with clones of the `value` pushed at the back. If `new_len`
    /// is less than `len`, the `LinearDeque` is simply truncated at the back.
    ///
    /// This method requires `T` to implement [`Clone`], in order to be able to
    /// clone the passed value. If you need more flexibility (or want to rely on
    /// [`Default`] instead of [`Clone`]), use [`resize_at_back_with`]. If you
    /// only need to resize to a smaller size, use [`truncate_at_back`].
    ///
    /// [`resize_at_back_with`]: LinearDeque::resize_at_back_with
    /// [`truncate_at_back`]: LinearDeque::truncate_at_back
    ///
    /// # Example
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let mut buf = LinearDeque::from([5, 10, 15]);
    ///
    /// buf.resize_at_back(2, 0);
    /// assert_eq!(buf, [5, 10]);
    ///
    /// buf.resize_at_back(5, 20);
    /// assert_eq!(buf, [5, 10, 20, 20, 20]);
    /// ```
    pub fn resize_at_back(&mut self, new_len: usize, value: T) {
        self.resize_at_back_with(new_len, || value.clone());
    }

    /// Resizes the `LinearDeque` in-place at the back end so that `len` is
    /// equal to `new_len`.
    ///
    /// It is just an alias for [`resize_at_back`].
    ///
    /// [`resize_at_back`]: LinearDeque::resize_at_back
    #[inline]
    pub fn resize(&mut self, new_len: usize, value: T) {
        self.resize_at_back(new_len, value);
    }
}

impl<T> Default for LinearDeque<T> {
    fn default() -> Self {
        Self::new()
    }
}

impl<T> Drop for LinearDeque<T> {
    fn drop(&mut self) {
        while self.pop_back().is_some() {}
    }
}

impl<T> Deref for LinearDeque<T> {
    type Target = [T];

    fn deref(&self) -> &Self::Target {
        unsafe { core::slice::from_raw_parts(self.ptr().add(self.off), self.len) }
    }
}

impl<T> DerefMut for LinearDeque<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe { core::slice::from_raw_parts_mut(self.ptr().add(self.off), self.len) }
    }
}

impl<T> Extend<T> for LinearDeque<T> {
    /// Extends the deque with the contents of the iterator.
    ///
    /// It is just an alias for [`extend_at_back`].
    ///
    /// [`extend_at_back`]: LinearDeque::extend_at_back
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        self.extend_at_back(iter);
    }
}

impl<'a, T> Extend<&'a T> for LinearDeque<T>
where
    T: 'a + Copy,
{
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().copied());
    }
}

impl<T> IntoIterator for LinearDeque<T> {
    type Item = T;

    type IntoIter = IntoIter<T>;

    fn into_iter(self) -> Self::IntoIter {
        unsafe {
            let iter = RawValIter::new(&self);

            let buf = ptr::read(&raw const self.buf);
            mem::forget(self);

            IntoIter { iter, _buf: buf }
        }
    }
}

macro_rules! impl_partial_eq {
    ([$($n:tt)*] $rhs:ty) => {
        impl<T, U, $($n)*> PartialEq<$rhs> for LinearDeque<T>
        where
            T: PartialEq<U>,
        {
            fn eq(&self, other: & $rhs) -> bool {
                self.deref() == other.deref()
            }
        }
    };
}

impl_partial_eq!([const N: usize] [U; N]);
impl_partial_eq!([const N: usize] &[U; N]);
impl_partial_eq!([const N: usize] &mut [U; N]);
impl_partial_eq!([] & [U]);
impl_partial_eq!([] &mut [U]);
impl_partial_eq!([] Vec<U>);
impl_partial_eq!([] LinearDeque<U>);

impl<T: Eq> Eq for LinearDeque<T> {}

impl<T: PartialOrd> PartialOrd for LinearDeque<T> {
    fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
        self.iter().partial_cmp(other.iter())
    }
}

impl<T: Ord> Ord for LinearDeque<T> {
    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
        self.iter().cmp(other.iter())
    }
}

impl<T: Hash> Hash for LinearDeque<T> {
    fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
        self.deref().hash(state);
    }
}

impl<T, const N: usize> From<[T; N]> for LinearDeque<T> {
    /// Converts a `[T; N]` into a `LinearDeque<T>`.
    ///
    /// ```
    /// use linear_deque::LinearDeque;
    ///
    /// let deq = LinearDeque::from([1, 2, 3, 4]);
    /// assert_eq!(deq, [1, 2, 3, 4]);
    /// ```
    fn from(value: [T; N]) -> Self {
        Self::from_iter(value)
    }
}

impl<T> From<Vec<T>> for LinearDeque<T> {
    /// Turn a [`Vec<T>`] into a [`LinearDeque<T>`].
    fn from(value: Vec<T>) -> Self {
        Self::from_iter(value)
    }
}

impl<T> FromIterator<T> for LinearDeque<T> {
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
        let iter = iter.into_iter();
        let (lower, upper) = iter.size_hint();
        let size = upper.unwrap_or(lower);
        let mut deque = Self::with_reserved_space(0, size);
        for elem in iter {
            deque.push_back(elem);
        }
        deque
    }
}

#[cfg(feature = "std")]
impl Read for LinearDeque<u8> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        let result = (&*self as &[u8]).read(buf);
        if let Ok(ref count) = result {
            self.truncate_at_front(self.len - count);
        }
        result
    }
}

#[cfg(feature = "std")]
impl Write for LinearDeque<u8> {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        self.extend(buf);
        Ok(buf.len())
    }

    fn flush(&mut self) -> std::io::Result<()> {
        Ok(())
    }
}

impl<T: Clone> Clone for LinearDeque<T> {
    fn clone(&self) -> Self {
        let mut clone = Self::with_reserved_space(0, self.len);
        for elem in self.iter() {
            clone.push_back(elem.clone());
        }
        clone
    }
}

impl<T: Debug> Debug for LinearDeque<T> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("LinearDeque")
            .field("values", &&**self)
            .field(
                "reserved_spaces",
                &(self.reserved_front_space(), self.reserved_back_space()),
            )
            .finish()
    }
}

#[derive(Clone, Copy, Debug)]
#[must_use]
struct PendingCopy {
    src: usize,
    dst: usize,
    count: usize,
}

impl PendingCopy {
    unsafe fn perform<T>(self, ptr: *mut T) {
        unsafe {
            if self.count > 0 && self.src != self.dst {
                ptr::copy(ptr.add(self.src), ptr.add(self.dst), self.count);
            }
        }
    }

    fn split(self, index: usize) -> (PendingCopy, PendingCopy) {
        let low = PendingCopy {
            count: index,
            ..self
        };
        let high = PendingCopy {
            src: self.src + index,
            dst: self.dst + index,
            count: self.count - index,
        };
        (low, high)
    }
}

/// Defines what happens to the reserved space of one of the deque ends on a call
/// to [`set_reserved_space`].
///
/// [`set_reserved_space`]: LinearDeque::set_reserved_space
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum SetReservedSpace {
    /// The reserved space is not changed.
    Keep,

    /// The reserved space is shrunk to the specified size.
    ///
    /// If the current space is less than the wanted size, then this is a no-op.
    ShrinkTo(usize),

    /// The reserved space is grown to the specified size.
    ///
    /// If the current space is greater than the wanted size, then this is a no-op.
    GrowTo(usize),

    /// The reserved space is unconditionally changed to the specified size.
    Exact(usize),
}

/// An owning iterator over the elements of a `LinearDeque`.
///
/// This `struct` is created by the [`into_iter`] method on [`LinearDeque`]
/// (provided by the [`IntoIterator`] trait). See its documentation for more.
///
/// [`into_iter`]: LinearDeque::into_iter
/// [`IntoIterator`]: core::iter::IntoIterator
pub struct IntoIter<T> {
    _buf: Buffer<T>,
    iter: RawValIter<T>,
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<T> DoubleEndedIterator for IntoIter<T> {
    fn next_back(&mut self) -> Option<Self::Item> {
        self.iter.next_back()
    }
}

impl<T> Drop for IntoIter<T> {
    fn drop(&mut self) {
        for _ in &mut *self {}
    }
}

/// A draining iterator over the elements of a `LinearDeque`.
///
/// This `struct` is created by the [`drain`] method on [`LinearDeque`]. See its
/// documentation for more.
///
/// [`drain`]: LinearDeque::drain
pub struct Drain<'a, T: 'a> {
    vec: PhantomData<&'a mut LinearDeque<T>>,
    iter: RawValIter<T>,
    pending_copy: PendingCopy,
    ptr: *mut T,
}

impl<T> Iterator for Drain<'_, T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<T> Drop for Drain<'_, T> {
    fn drop(&mut self) {
        for _ in &mut *self {}
        unsafe {
            self.pending_copy.perform(self.ptr);
        }
    }
}

fn is_zst<T>() -> bool {
    mem::size_of::<T>() == 0
}

#[cfg(test)]
mod tests {
    use core::fmt::Debug;
    use core::hash::{Hash as _, Hasher};
    use core::{mem, ptr};
    use std::collections::hash_map::DefaultHasher;
    use std::io::{Read as _, Write as _};

    use crate::drop_tracker::DropTracker;
    use crate::{LinearDeque, SetReservedSpace};

    macro_rules! assert_deque {
        ($deque:ident, $expected_reserved_front_space:expr, $expected_elems:expr, $expected_reserved_back_space:expr $(,)?) => {{
            let expected_reserved_front_space = $expected_reserved_front_space;
            let expected_elems: Vec<_> = $expected_elems.into_iter().collect();
            let expected_reserved_back_space = $expected_reserved_back_space;

            let expected_len = expected_elems.len();
            let expected_capacity =
                expected_reserved_front_space + expected_len + expected_reserved_back_space;
            let expected_off = expected_reserved_front_space;

            assert_eq!($deque.cap(), expected_capacity, "cap");
            assert_eq!($deque.len, expected_len, "len");
            assert_eq!($deque.off, expected_off, "off");
            for (i, expected_elem) in expected_elems.into_iter().enumerate() {
                let elem = unsafe { ptr::read($deque.ptr().add($deque.off + i)) };
                assert_eq!(elem, expected_elem, "index {i}");
            }
            assert_eq!(
                $deque.reserved_front_space(),
                expected_reserved_front_space,
                "front space"
            );
            assert_eq!(
                $deque.reserved_back_space(),
                expected_reserved_back_space,
                "back_space"
            );
        }};
    }

    fn prepare_deque<T: Clone + Eq + Debug>(
        reserved_front_space: usize,
        elems: impl IntoIterator<Item = T>,
        reserved_back_space: usize,
    ) -> LinearDeque<T> {
        let elems: Vec<_> = elems.into_iter().collect();
        let capacity = reserved_front_space + elems.len() + reserved_back_space;

        let mut deque: LinearDeque<T> = LinearDeque::new();
        if capacity != 0 {
            deque.buf.realloc(capacity);
            deque.len = elems.len();
            deque.off = reserved_front_space;
            for (i, elem) in elems.iter().enumerate() {
                unsafe {
                    ptr::write(deque.ptr().add(deque.off + i), elem.clone());
                }
            }
        }

        assert_deque!(deque, reserved_front_space, elems, reserved_back_space);

        deque
    }

    macro_rules! assert_zst_deque {
        ($deque:ident, $expected_len:expr) => {{
            fn assert_zst_deque<T>(_: &LinearDeque<T>) {
                assert_eq!(mem::size_of::<T>(), 0);
            }
            assert_zst_deque(&$deque);
        }
        assert_eq!($deque.cap(), usize::MAX);
        assert_eq!($deque.len, $expected_len);};
    }

    fn prepare_zst_deque<T>(len: usize) -> LinearDeque<T> {
        assert_eq!(mem::size_of::<T>(), 0);
        let mut deque = LinearDeque::new();
        deque.len = len;
        deque
    }

    #[test]
    fn new_zst() {
        let deque: LinearDeque<()> = LinearDeque::new();

        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn with_reserved_space() {
        let deque: LinearDeque<char> = LinearDeque::with_reserved_space(3, 4);

        assert_deque!(deque, 3, [], 4);
    }

    #[test]
    fn with_reserved_space_zst() {
        let deque: LinearDeque<()> = LinearDeque::with_reserved_space(3, 4);

        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn front_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(0);
        assert_eq!(deque.front(), None);

        let deque: LinearDeque<()> = prepare_zst_deque(2);
        assert_eq!(deque.front(), Some(&()));
    }

    #[test]
    fn back_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(0);
        assert_eq!(deque.back(), None);

        let deque: LinearDeque<()> = prepare_zst_deque(2);
        assert_eq!(deque.back(), Some(&()));
    }

    #[test]
    fn push_front() {
        let mut deque: LinearDeque<char> = prepare_deque(2, [], 1);
        // |--[]-|

        deque.push_front('A');

        // |-[A]-|
        assert_deque!(deque, 1, ['A'], 1);

        deque.push_front('B');

        // |[BA]-|
        assert_deque!(deque, 0, ['B', 'A'], 1);

        deque.push_front('C');

        // |[CBA]|
        assert_deque!(deque, 0, ['C', 'B', 'A'], 0);

        deque.push_front('D');

        // |--[DCBA]|
        assert_deque!(deque, 2, ['D', 'C', 'B', 'A'], 0);

        deque.push_front('E');

        // |-[EDCBA]|
        assert_deque!(deque, 1, ['E', 'D', 'C', 'B', 'A'], 0);
    }

    #[test]
    fn push_front_zst() {
        let mut deque = prepare_zst_deque(0);

        deque.push_front(());
        deque.push_front(());

        assert_zst_deque!(deque, 2);
    }

    #[test]
    fn push_back() {
        let mut deque: LinearDeque<char> = prepare_deque(1, [], 2);
        // |-[]--|

        deque.push_back('A');

        // |-[A]-|
        assert_deque!(deque, 1, ['A'], 1);

        deque.push_back('B');

        // |-[AB]|
        assert_deque!(deque, 1, ['A', 'B'], 0);

        deque.push_back('C');

        // |[ABC]|
        assert_deque!(deque, 0, ['A', 'B', 'C'], 0);

        deque.push_back('D');

        // |[ABCD]--|
        assert_deque!(deque, 0, ['A', 'B', 'C', 'D'], 2);

        deque.push_back('E');

        // |[ABCDE]-|
        assert_deque!(deque, 0, ['A', 'B', 'C', 'D', 'E'], 1);
    }

    #[test]
    fn push_back_zst() {
        let mut deque = prepare_zst_deque(0);

        deque.push_back(());
        deque.push_back(());

        assert_zst_deque!(deque, 2);
    }

    #[test]
    fn pop_front() {
        let mut deque = prepare_deque(1, ['B', 'A'], 2);
        // |-[AB]--|

        let popped = deque.pop_front();

        assert_eq!(popped, Some('B'));
        // |--[A]--|
        assert_deque!(deque, 2, ['A'], 2);

        let popped = deque.pop_front();

        assert_eq!(popped, Some('A'));
        // |---[]--|
        assert_deque!(deque, 3, [], 2);

        let popped = deque.pop_front();

        assert_eq!(popped, None);
        assert_deque!(deque, 3, [], 2);
    }

    #[test]
    fn pop_front_zst() {
        let mut deque = prepare_zst_deque(2);

        let popped = deque.pop_front();
        assert_eq!(popped, Some(()));
        assert_zst_deque!(deque, 1);

        let popped = deque.pop_front();
        assert_eq!(popped, Some(()));
        assert_zst_deque!(deque, 0);

        let popped = deque.pop_front();
        assert_eq!(popped, None);
        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn pop_back() {
        let mut deque = prepare_deque(2, ['A', 'B'], 1);
        // |--[AB]-|

        let popped = deque.pop_back();

        assert_eq!(popped, Some('B'));
        // |--[A]--|
        assert_deque!(deque, 2, ['A'], 2);

        let popped = deque.pop_back();

        assert_eq!(popped, Some('A'));
        // |--[]---|
        assert_deque!(deque, 2, [], 3);

        let popped = deque.pop_back();

        assert_eq!(popped, None);
        assert_deque!(deque, 2, [], 3);
    }

    #[test]
    fn pop_back_zst() {
        let mut deque = prepare_zst_deque(2);

        let popped = deque.pop_back();
        assert_eq!(popped, Some(()));
        assert_zst_deque!(deque, 1);

        let popped = deque.pop_back();
        assert_eq!(popped, Some(()));
        assert_zst_deque!(deque, 0);

        let popped = deque.pop_back();
        assert_eq!(popped, None);
        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn insert_near_front() {
        let mut deque = prepare_deque(1, ['A', 'B', 'C'], 1);
        // |-[ABC]-|

        deque.insert(1, 'x');

        // |[AxBC]-|
        assert_deque!(deque, 0, ['A', 'x', 'B', 'C'], 1);

        deque.insert(1, 'y');

        // |[AyxBC]|
        assert_deque!(deque, 0, ['A', 'y', 'x', 'B', 'C'], 0);

        deque.insert(1, 'z');

        // |----[AzyxBC]|
        assert_deque!(deque, 4, ['A', 'z', 'y', 'x', 'B', 'C'], 0);
    }

    #[test]
    fn insert_near_back() {
        let mut deque = prepare_deque(1, ['A', 'B', 'C'], 1);
        // |-[ABC]-|

        deque.insert(2, 'x');

        // |-[ABxC]|
        assert_deque!(deque, 1, ['A', 'B', 'x', 'C'], 0);

        deque.insert(3, 'y');

        // |[ABxyC]|
        assert_deque!(deque, 0, ['A', 'B', 'x', 'y', 'C'], 0);

        deque.insert(4, 'z');

        // |[ABxyzC]----|
        assert_deque!(deque, 0, ['A', 'B', 'x', 'y', 'z', 'C'], 4);
    }

    #[test]
    fn insert_zst() {
        let mut deque = LinearDeque::new();

        deque.insert(0, ());
        deque.insert(0, ());
        deque.insert(2, ());
        deque.insert(2, ());

        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn remove_near_front() {
        let mut deque = prepare_deque(0, ['A', 'B', 'C', 'D'], 0);
        // |[ABCD]|

        let removed = deque.remove(1);

        assert_eq!(removed, Some('B'));
        // |-[ACD]|
        assert_deque!(deque, 1, ['A', 'C', 'D'], 0);
    }

    #[test]
    fn remove_near_back() {
        let mut deque = prepare_deque(0, ['A', 'B', 'C', 'D'], 0);
        // |[ABCD]|

        let removed = deque.remove(2);

        assert_eq!(removed, Some('C'));
        // |[ABD]-|
        assert_deque!(deque, 0, ['A', 'B', 'D'], 1);
    }

    #[test]
    fn remove_zst() {
        let mut deque = prepare_zst_deque(3);

        let removed = deque.remove(1);
        assert_eq!(removed, Some(()));
        assert_zst_deque!(deque, 2);

        let removed = deque.remove(1);
        assert_eq!(removed, Some(()));
        assert_zst_deque!(deque, 1);

        let removed = deque.remove(0);
        assert_eq!(removed, Some(()));
        assert_zst_deque!(deque, 0);

        let removed = deque.remove(0);
        assert_eq!(removed, None);
        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn drain_all() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(..);

        assert_eq!(drained.collect::<Vec<_>>(), Vec::from_iter('A'..='H'));
        // There is no requirement for exact reserved space distribution in this case
        // |--..........--]
        assert_eq!(deque.cap(), 12);
        assert!(deque.reserved_front_space() >= 2);
        assert!(deque.reserved_back_space() >= 2);
        assert!(deque.is_empty());
    }

    #[test]
    fn drain_start() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(..3);

        assert_eq!(drained.collect::<Vec<_>>(), Vec::from_iter('A'..='C'));
        // |-----[DEFGH]--]
        assert_deque!(deque, 5, 'D'..='H', 2);
    }

    #[test]
    fn drain_end() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(5..);

        assert_eq!(drained.collect::<Vec<_>>(), Vec::from_iter('F'..='H'));
        // |--[ABCDE]-----]
        assert_deque!(deque, 2, 'A'..='E', 5);
    }

    #[test]
    fn drain_near_start() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(1..5);

        assert_eq!(drained.collect::<Vec<_>>(), Vec::from_iter('B'..='E'));
        // |------[AFGH]--]
        assert_deque!(deque, 6, ['A', 'F', 'G', 'H'], 2);
    }

    #[test]
    fn drain_near_end() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(3..7);

        assert_eq!(drained.collect::<Vec<_>>(), Vec::from_iter('D'..='G'));
        // |--[ABCH]------]
        assert_deque!(deque, 2, ['A', 'B', 'C', 'H'], 6);
    }

    #[test]
    fn drain_nothing() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--]

        let drained = deque.drain(3..3);

        assert_eq!(drained.count(), 0);
        // |--[ABCDEFGH]--]
        assert_deque!(deque, 2, 'A'..='H', 2);
    }

    #[test]
    fn drain_not_fully_consumed() {
        let mut deque = prepare_deque(2, 'A'..='H', 2);
        // |--[ABCDEFGH]--|

        let mut drained = deque.drain(3..7);
        assert_eq!(drained.next(), Some('D'));
        drop(drained);

        // |--[ABCH]------]
        assert_deque!(deque, 2, ['A', 'B', 'C', 'H'], 6);
    }

    #[test]
    fn drain_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(8);

        let iter = deque.drain(3..5);
        assert_eq!(iter.count(), 2);
        assert_zst_deque!(deque, 6);

        let iter = deque.drain(4..);
        assert_eq!(iter.count(), 2);
        assert_zst_deque!(deque, 4);

        let iter = deque.drain(..2);
        assert_eq!(iter.count(), 2);
        assert_zst_deque!(deque, 2);

        let iter = deque.drain(..);
        assert_eq!(iter.count(), 2);
        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn extend_at_front() {
        let mut deque = prepare_deque(1, ['C', 'D'], 1);
        // |-[CD]-|

        deque.extend_at_front(['B', 'A']);

        // |[ABCD]|
        assert_deque!(deque, 0, 'A'..='D', 0);
    }

    #[test]
    fn extend_at_front_zst() {
        let mut deque = prepare_zst_deque(2);

        deque.extend_at_front(std::iter::repeat(()).take(4));

        assert_zst_deque!(deque, 6);
    }

    #[test]
    fn extend_at_back() {
        let mut deque = prepare_deque(1, ['A', 'B'], 1);
        // |-[AB]-|

        deque.extend_at_back(['C', 'D']);

        // |[ABCD]|
        assert_deque!(deque, 0, 'A'..='D', 0);
    }

    #[test]
    fn extend_at_back_zst() {
        let mut deque = prepare_zst_deque(2);

        deque.extend_at_back(std::iter::repeat(()).take(4));

        assert_zst_deque!(deque, 6);
    }

    #[test]
    fn reserved_front_space() {
        let deque = prepare_deque(3, 'A'..='D', 4);

        assert_eq!(deque.reserved_front_space(), 3);
    }

    #[test]
    fn reserved_front_space_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(5);

        assert_eq!(deque.reserved_front_space(), usize::MAX);
    }

    #[test]
    fn reserved_back_space() {
        let deque = prepare_deque(3, 'A'..='D', 4);

        assert_eq!(deque.reserved_back_space(), 4);
    }

    #[test]
    fn reserved_back_space_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(5);

        assert_eq!(deque.reserved_back_space(), usize::MAX);
    }

    #[test]
    fn deref_empty() {
        let deque: LinearDeque<char> = prepare_deque(6, [], 4);

        assert!(deque.is_empty());
        assert_eq!(deque.len(), 0);
    }

    #[test]
    fn deref_non_empty() {
        let deque = prepare_deque(2, ['A', 'B', 'C'], 4);

        assert!(!deque.is_empty());
        assert_eq!(deque.len(), 3);
        assert_eq!(deque[0], 'A');
        assert_eq!(deque[1], 'B');
        assert_eq!(deque[2], 'C');
    }

    #[test]
    fn deref_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(4);

        assert!(!deque.is_empty());
        assert_eq!(deque.len(), 4);
        assert_eq!(deque[0], ());
        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn into_iter() {
        let deque = prepare_deque(2, ['A', 'B', 'C'], 4);

        let mut iter = deque.into_iter();

        assert_eq!(iter.next(), Some('A'));
        assert_eq!(iter.next(), Some('B'));
        assert_eq!(iter.next(), Some('C'));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn into_iter_double_ended() {
        let deque = prepare_deque(2, ['A', 'B', 'C'], 4);

        let mut iter = deque.into_iter();

        assert_eq!(iter.next_back(), Some('C'));
        assert_eq!(iter.next_back(), Some('B'));
        assert_eq!(iter.next_back(), Some('A'));
        assert_eq!(iter.next_back(), None);
    }

    #[test]
    fn into_iter_mixed() {
        let deque = prepare_deque(2, ['A', 'B', 'C'], 4);

        let mut iter = deque.into_iter();

        assert_eq!(iter.next_back(), Some('C'));
        assert_eq!(iter.next(), Some('A'));
        assert_eq!(iter.next_back(), Some('B'));
        assert_eq!(iter.next(), None);
        assert_eq!(iter.next_back(), None);
    }

    #[test]
    fn into_iter_zst() {
        let deque: LinearDeque<()> = prepare_zst_deque(4);

        let iter = deque.into_iter();

        assert_eq!(iter.count(), 4);
    }

    #[test]
    fn resize_at_front_larger() {
        let mut deque = prepare_deque(2, ['A', 'B', 'C', 'D'], 3);
        // |--[ABCD]---|

        deque.resize_at_front(10, 'x');

        // |[xxxxxxABCD]---|
        assert_deque!(
            deque,
            0,
            ['x', 'x', 'x', 'x', 'x', 'x', 'A', 'B', 'C', 'D'],
            3
        );
    }

    #[test]
    fn resize_at_front_smaller() {
        let mut deque = prepare_deque(2, ['A', 'B', 'C', 'D'], 3);
        // |--[ABCD]---|

        deque.resize_at_front(3, 'x');

        // |---[BCD]---|
        assert_deque!(deque, 3, ['B', 'C', 'D'], 3);
    }

    #[test]
    fn resize_at_front_zst() {
        let mut deque = prepare_zst_deque(5);

        deque.resize_at_front(2, ());
        assert_zst_deque!(deque, 2);

        deque.resize_at_front(4, ());
        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn resize_at_back_larger() {
        let mut deque = prepare_deque(3, ['A', 'B', 'C', 'D'], 2);
        // |---[ABCD]--|

        deque.resize_at_back(10, 'x');

        // |---[ABCDxxxxxx]|
        assert_deque!(
            deque,
            3,
            ['A', 'B', 'C', 'D', 'x', 'x', 'x', 'x', 'x', 'x'],
            0
        );
    }

    #[test]
    fn resize_at_back_smaller() {
        let mut deque = prepare_deque(3, ['A', 'B', 'C', 'D'], 2);
        // |---[ABCD]--|

        deque.resize_at_back(3, 'x');

        // |---[ABC]---|
        assert_deque!(deque, 3, ['A', 'B', 'C'], 3);
    }

    #[test]
    fn resize_at_back_zst() {
        let mut deque = prepare_zst_deque(5);

        deque.resize_at_back(2, ());
        assert_zst_deque!(deque, 2);

        deque.resize_at_back(4, ());
        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn resize_at_front_with_larger() {
        let mut deque = prepare_deque(2, 'A'..='D', 3);
        // |--[ABCD]---|
        let mut chars = 'a'..;

        deque.resize_at_front_with(7, || chars.next().unwrap());

        // |[cbaABCD]---|
        assert_deque!(deque, 0, ['c', 'b', 'a', 'A', 'B', 'C', 'D'], 3);
    }

    #[test]
    fn resize_at_front_with_smaller() {
        let mut deque = prepare_deque(2, 'A'..='D', 3);
        // |--[ABCD]---|
        let mut chars = 'a'..;

        deque.resize_at_front_with(3, || chars.next().unwrap());

        // |---[BCD]---|
        assert_deque!(deque, 3, ['B', 'C', 'D'], 3);
    }

    #[test]
    fn resize_at_front_with_zst() {
        let mut deque = prepare_zst_deque(5);

        deque.resize_at_front_with(2, || ());
        assert_zst_deque!(deque, 2);

        deque.resize_at_front_with(4, || ());
        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn resize_at_back_with_larger() {
        let mut deque = prepare_deque(3, 'A'..='D', 2);
        // |---[ABCD]--|
        let mut chars = 'a'..;

        deque.resize_at_back_with(7, || chars.next().unwrap());

        // |---[ABCDabc]|
        assert_deque!(deque, 3, ['A', 'B', 'C', 'D', 'a', 'b', 'c'], 0);
    }

    #[test]
    fn resize_at_back_with_smaller() {
        let mut deque = prepare_deque(2, 'A'..='D', 3);
        // |--[ABCD]---|
        let mut chars = 'a'..;

        deque.resize_at_back_with(3, || chars.next().unwrap());

        // |--[ABC]----|
        assert_deque!(deque, 2, ['A', 'B', 'C'], 4);
    }

    #[test]
    fn resize_at_back_with_zst() {
        let mut deque = prepare_zst_deque(5);

        deque.resize_at_back_with(2, || ());
        assert_zst_deque!(deque, 2);

        deque.resize_at_back_with(4, || ());
        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn retain() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(2, drop_tracker.wrap_iter(['A', 'b', 'c', 'D', 'e']), 1);
        // |--[AbcDe]-|

        let (dropped, _) = drop_tracker.track(|| {
            deque.retain(|c| c.value.is_lowercase());
        });

        // |--[bce]---|
        assert_deque!(deque, 2, drop_tracker.wrap_iter(['b', 'c', 'e']), 3);
        assert_eq!(dropped, ['A', 'D']);
    }

    #[test]
    fn retain_mut() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(2, drop_tracker.wrap_iter(['A', 'b', 'c', 'D', 'e']), 1);
        // |--[AbcDe]-|

        let (dropped, _) = drop_tracker.track(|| {
            deque.retain_mut(|c| {
                if c.value.is_lowercase() {
                    c.value = c.value.to_uppercase().next().unwrap();
                    true
                } else {
                    false
                }
            });
        });

        // |--[BCE]---|
        assert_deque!(deque, 2, drop_tracker.wrap_iter(['B', 'C', 'E']), 3);
        assert_eq!(dropped, ['A', 'D']);
    }

    #[test]
    fn retain_mut_drop_none() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(2, drop_tracker.wrap_iter('A'..='E'), 1);
        // |--[ABCDE]-|

        let (dropped, _) = drop_tracker.track(|| {
            deque.retain_mut(|_| true);
        });

        // |--[ABCDE]-|
        assert_deque!(deque, 2, drop_tracker.wrap_iter('A'..='E'), 1);
        assert!(dropped.is_empty());
    }

    #[test]
    fn retain_mut_drop_all() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(2, drop_tracker.wrap_iter('A'..='E'), 1);
        // |--[ABCDE]-|

        let (dropped, _) = drop_tracker.track(|| {
            deque.retain_mut(|_| false);
        });

        // |--[]------|
        assert_deque!(deque, 2, [], 6);
        assert_eq!(dropped, Vec::from_iter('A'..='E'));
    }

    #[test]
    fn retain_mut_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(4);

        let mut p = false;
        deque.retain(|_| {
            p = !p;
            p
        });

        assert_zst_deque!(deque, 2);
    }

    #[test]
    fn truncate_at_front() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(5, drop_tracker.wrap_iter('A'..='F'), 1);
        // |-----[ABCDEF]-|

        let (dropped, _) = drop_tracker.track(|| {
            deque.truncate_at_front(2);
        });

        // |---------[EF]-|
        assert_deque!(deque, 9, drop_tracker.wrap_iter('E'..='F'), 1);
        assert_eq!(dropped, Vec::from_iter('A'..='D'));
    }

    #[test]
    fn truncate_at_front_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(4);

        deque.truncate_at_front(3);

        assert_zst_deque!(deque, 3);
    }

    #[test]
    fn truncate_at_back() {
        let mut drop_tracker = DropTracker::new();
        let mut deque = prepare_deque(1, drop_tracker.wrap_iter('A'..='F'), 5);
        // |-[ABCDEF]-----|

        let (dropped, _) = drop_tracker.track(|| {
            deque.truncate_at_back(2);
        });

        // |-[AB]---------|
        assert_deque!(deque, 1, drop_tracker.wrap_iter('A'..='B'), 9);
        assert_eq!(dropped, Vec::from_iter('C'..='F'));
    }

    #[test]
    fn truncate_at_back_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(4);

        deque.truncate_at_back(3);

        assert_zst_deque!(deque, 3);
    }

    #[test]
    fn clear() {
        let mut deque = prepare_deque(2, 'A'..='D', 4);
        // |--[ABCD]----|

        deque.clear();

        // |-----[]-----|
        assert_deque!(deque, 5, [], 5);
    }

    #[test]
    fn clear_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(4);

        deque.clear();

        assert_zst_deque!(deque, 0);
    }

    #[test]
    fn set_reserved_space_keeping() {
        let mut deque = prepare_deque(2, 'A'..='D', 5);
        // |--[ABCD]-----|

        deque.set_reserved_space(SetReservedSpace::Keep, SetReservedSpace::Keep);

        // |--[ABCD]-----|
        assert_deque!(deque, 2, 'A'..='D', 5);
    }

    #[test]
    fn set_reserved_space_growing() {
        let mut deque = prepare_deque(3, 'A'..='D', 1);
        // |---[ABCD]-|

        deque.set_reserved_space(SetReservedSpace::GrowTo(6), SetReservedSpace::GrowTo(2));

        // |------[ABCD]--|
        assert_deque!(deque, 6, 'A'..='D', 2);
    }

    #[test]
    fn set_reserved_space_not_growing() {
        let mut deque = prepare_deque(3, 'A'..='D', 1);
        // |---[ABCD]-|

        deque.set_reserved_space(SetReservedSpace::GrowTo(2), SetReservedSpace::GrowTo(1));

        // |---[ABCD]-|
        assert_deque!(deque, 3, 'A'..='D', 1);
    }

    #[test]
    fn set_reserved_space_shrinking() {
        let mut deque = prepare_deque(5, 'A'..='D', 2);
        // |-----[ABCD]--|

        deque.set_reserved_space(SetReservedSpace::ShrinkTo(2), SetReservedSpace::ShrinkTo(1));

        // |--[ABCD]-|
        assert_deque!(deque, 2, 'A'..='D', 1);
    }

    #[test]
    fn set_reserved_space_not_shrinking() {
        let mut deque = prepare_deque(5, 'A'..='D', 2);
        // |-----[ABCD]--|

        deque.set_reserved_space(SetReservedSpace::ShrinkTo(6), SetReservedSpace::ShrinkTo(3));

        // |-----[ABCD]--|
        assert_deque!(deque, 5, 'A'..='D', 2);
    }

    #[test]
    fn set_reserved_space_exactly_growing() {
        let mut deque = prepare_deque(2, 'A'..='D', 1);
        // |--[ABCD]-|

        deque.set_reserved_space(SetReservedSpace::Exact(5), SetReservedSpace::Exact(2));

        // |-----[ABCD]--|
        assert_deque!(deque, 5, 'A'..='D', 2);
    }

    #[test]
    fn set_reserved_space_exactly_shrinking() {
        let mut deque = prepare_deque(5, 'A'..='D', 2);
        // |-----[ABCD]--|

        deque.set_reserved_space(SetReservedSpace::Exact(2), SetReservedSpace::Exact(1));

        // |--[ABCD]-|
        assert_deque!(deque, 2, 'A'..='D', 1);
    }

    #[test]
    fn ensure_reserved_front_space_doing_nothing_1() {
        let mut deque = prepare_deque(3, 'A'..='C', 1);
        // |---[ABC]-|

        deque.ensure_reserved_front_space(1);

        // |---[ABC]-|
        assert_deque!(deque, 3, 'A'..='C', 1);
    }

    #[test]
    fn ensure_reserved_front_space_doing_nothing_2() {
        let mut deque = prepare_deque(3, 'A'..='C', 1);
        // |---[ABC]-|

        deque.ensure_reserved_front_space(3);

        // |---[ABC]-|
        assert_deque!(deque, 3, 'A'..='C', 1);
    }

    #[test]
    fn ensure_reserved_front_space_moving_1() {
        let mut deque = prepare_deque(1, 'A'..='C', 3);
        // |-[ABC]---|

        deque.ensure_reserved_front_space(2);

        // |-**[ABC]-|
        assert_deque!(deque, 3, 'A'..='C', 1);
    }

    #[test]
    fn ensure_reserved_front_space_moving_2() {
        let mut deque = prepare_deque(1, 'A'..='C', 3);
        // |-[ABC]---|

        deque.ensure_reserved_front_space(4);

        // |****[ABC]|
        assert_deque!(deque, 4, 'A'..='C', 0);
    }

    #[test]
    fn ensure_reserved_front_space_reallocating_1() {
        let mut deque = prepare_deque(2, 'A'..='C', 2);
        // |--[ABC]--|

        deque.ensure_reserved_front_space(5);

        // |--*****[ABC]|
        assert_deque!(deque, 7, 'A'..='C', 0);
    }

    #[test]
    fn ensure_reserved_front_space_reallocating_2() {
        let mut deque = prepare_deque(2, 'A'..='C', 2);
        // |--[ABC]--|

        deque.ensure_reserved_front_space(8);

        // |********[ABC]|
        assert_deque!(deque, 8, 'A'..='C', 0);
    }

    #[test]
    fn ensure_reserved_back_space_doing_nothing_1() {
        let mut deque = prepare_deque(1, 'A'..='C', 3);
        // |-[ABC]---|

        deque.ensure_reserved_back_space(1);

        // |-[ABC]---|
        assert_deque!(deque, 1, 'A'..='C', 3);
    }

    #[test]
    fn ensure_reserved_back_space_doing_nothing_2() {
        let mut deque = prepare_deque(1, 'A'..='C', 3);
        // |-[ABC]---|

        deque.ensure_reserved_back_space(3);

        // |-[ABC]---|
        assert_deque!(deque, 1, 'A'..='C', 3);
    }

    #[test]
    fn ensure_reserved_back_space_moving_1() {
        let mut deque = prepare_deque(3, 'A'..='C', 1);
        // |---[ABC]-|

        deque.ensure_reserved_back_space(2);

        // |-[ABC]**-|
        assert_deque!(deque, 1, 'A'..='C', 3);
    }

    #[test]
    fn ensure_reserved_back_space_moving_2() {
        let mut deque = prepare_deque(3, 'A'..='C', 1);
        // |---[ABC]-|

        deque.ensure_reserved_back_space(4);

        // |[ABC]****|
        assert_deque!(deque, 0, 'A'..='C', 4);
    }

    #[test]
    fn ensure_reserved_back_space_reallocating_1() {
        let mut deque = prepare_deque(2, 'A'..='C', 2);
        // |--[ABC]--|

        deque.ensure_reserved_back_space(5);

        // |[ABC]*****--|
        assert_deque!(deque, 0, 'A'..='C', 7);
    }

    #[test]
    fn ensure_reserved_back_space_reallocating_2() {
        let mut deque = prepare_deque(2, 'A'..='C', 2);
        // |--[ABC]--|

        deque.ensure_reserved_back_space(8);

        // |[ABC]********|
        assert_deque!(deque, 0, 'A'..='C', 8);
    }

    #[test]
    fn set_reserved_space_zst() {
        let mut deque: LinearDeque<()> = prepare_zst_deque(5);

        deque.set_reserved_space(SetReservedSpace::Exact(3), SetReservedSpace::Exact(4));

        assert_zst_deque!(deque, 5);
    }

    #[test]
    fn eq() {
        let mut array = ['A', 'B'];
        let mut array_x = ['B', 'A'];

        let deque = prepare_deque(1, array, 5);

        {
            let slice: &[_] = &array;
            let slice_x: &[_] = &array_x;

            assert!(deque == slice);
            assert!(deque != slice_x);
        }

        assert!(deque == &array);
        assert!(deque != &array_x);

        {
            let slice_mut: &mut [_] = &mut array;
            let slice_mut_x: &mut [_] = &mut array_x;

            assert!(deque == slice_mut);
            assert!(deque != slice_mut_x);
        }

        assert!(deque == &mut array);
        assert!(deque != &mut array_x);

        assert!(deque == array);
        assert!(deque != array_x);

        assert!(deque == Vec::from(array));
        assert!(deque != Vec::from(array_x));

        assert!(deque == prepare_deque(5, array, 0));
        assert!(deque != prepare_deque(1, array_x, 5));
    }

    #[test]
    fn hash() {
        let deque1 = prepare_deque(3, ['A', 'B', 'C'], 5);
        let deque2 = {
            let mut d = LinearDeque::new();
            d.push_back('B');
            d.push_front('A');
            d.push_back('C');
            d
        };
        let mut hasher1 = DefaultHasher::new();
        let mut hasher2 = DefaultHasher::new();

        deque1.hash(&mut hasher1);
        deque2.hash(&mut hasher2);

        assert_eq!(hasher1.finish(), hasher2.finish());
    }

    #[test]
    fn from_iter() {
        let deque = LinearDeque::from_iter('A'..='D');

        assert_deque!(deque, 0, ['A', 'B', 'C', 'D'], 0);
    }

    #[test]
    fn from_iter_zst() {
        let deque = LinearDeque::from_iter(std::iter::repeat(()).take(4));

        assert_zst_deque!(deque, 4);
    }

    #[test]
    fn read() {
        let mut deque = prepare_deque(1, b'A'..=b'E', 2);
        // |-[ABCDE]--|
        let mut buf = [b'0'; 3];

        let result = deque.read(&mut buf);

        assert_eq!(result.unwrap(), 3);
        assert_eq!(buf, [b'A', b'B', b'C']);
        // |----[DE]--|
        assert_deque!(deque, 4, [b'D', b'E'], 2);
    }

    #[test]
    fn write() {
        let mut deque = prepare_deque(2, b'A'..=b'C', 1);
        // |--[ABC]-|
        let buf: Vec<_> = (b'D'..=b'H').collect();

        let result = deque.write(&buf);

        assert_eq!(result.unwrap(), 5);
        // |[ABCDEFGH]-|
        assert_deque!(deque, 0, b'A'..=b'H', 1);
    }

    #[test]
    fn clone() {
        let mut deque = prepare_deque(2, 'A'..='C', 1);
        // |--[ABC]-|

        let mut clone = deque.clone();

        deque[1] = 'x';
        clone[1] = 'y';
        assert_deque!(deque, 2, ['A', 'x', 'C'], 1);
        assert_deque!(clone, 0, ['A', 'y', 'C'], 0);
    }
}