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    use std::cmp::Ordering;
    use std::collections::hash_map::RandomState;
    use std::collections::{HashMap, LinkedList};
    use std::fmt::{self, Debug, Formatter};
    use std::hash::{BuildHasher, Hash, Hasher};
    use std::iter::{FromIterator, FusedIterator};
    use std::marker::PhantomData;
    use std::num::NonZeroUsize;
    use std::{mem, ops};

    /// A semi-doubly linked list implemented with a vector.
    ///
    /// This provides many of the benefits of an actual linked list with a few tradeoffs. First, due to
    /// the use of an underlying vector, an individual insert operation may be O(n) due to allocating
    /// more space for the vector. However, it is amortized O(1) and it avoids the frequent allocation
    /// that traditional linked lists suffer from.
    ///
    /// Another tradeoff is that extending a traditional linked list with another list is O(1) but a
    /// vector based implementation is O(n). Splicing has a similar disadvantage.
    ///
    /// Lastly, the vector based implementation is likely to have better cache locality in general.
    #[derive(Clone)]
    pub struct VecList<EntryData> {
        /// The backing storage for the list. This includes both used and unused indices.`
        entries: Vec<Entry<EntryData>>,

        /// The current generation of the list. This is used to avoid the ABA problem.
        generation: u64,

        /// The index of the head of the list.
        head: Option<NonZeroUsize>,

        /// The length of the list since we cannot rely on the length of [`VecList::entries`] because
        /// it includes unused indices.
        length: usize,

        /// The index of the tail of the list.
        tail: Option<NonZeroUsize>,

        /// The index of the head of the vacant indices.
        vacant_head: Option<NonZeroUsize>,
    }

    impl<EntryData> VecList<EntryData> {
        /// Returns an immutable reference to the value at the back of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.back(), None);
        ///
        /// list.push_back(0);
        /// list.push_back(5);
        /// assert_eq!(list.back(), Some(&5));
        /// ```
        pub fn back(&self) -> Option<&EntryData> {
            let index = self.tail()?;

            match &self.entries[index] {
                Entry::Occupied(entry) => Some(&entry.value),
                _ => None,
            }
        }

        /// Returns a mutable reference to the value at the back of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.back_mut(), None);
        ///
        /// list.push_back(0);
        /// list.push_back(5);
        ///
        /// let mut back = list.back_mut().unwrap();
        /// assert_eq!(back, &mut 5);
        /// *back *= 2;
        ///
        /// assert_eq!(list.back(), Some(&10));
        /// ```
        pub fn back_mut(&mut self) -> Option<&mut EntryData> {
            let index = self.tail()?;

            match &mut self.entries[index] {
                Entry::Occupied(entry) => Some(&mut entry.value),
                _ => None,
            }
        }

        /// Returns the capacity of the list.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let list: VecList<u32> = VecList::new();
        /// assert_eq!(list.capacity(), 0);
        ///
        /// let list: VecList<u32> = VecList::with_capacity(10);
        /// assert_eq!(list.capacity(), 10);
        /// ```
        pub fn capacity(&self) -> usize {
            self.entries.capacity()
        }

        /// Removes all values from the list and invalidates all existing indices.
        ///
        /// Complexity: O(n)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        ///
        /// list.push_back(5);
        /// assert!(!list.is_empty());
        ///
        /// list.clear();
        /// assert!(list.is_empty());
        /// ```
        pub fn clear(&mut self) {
            self.entries.clear();
            self.generation = self.generation.wrapping_add(1);
            self.head = None;
            self.length = 0;
            self.tail = None;
            self.vacant_head = None;
        }

        /// Returns whether or not the list contains the given value.
        ///
        /// Complexity: O(n)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert!(!list.contains(&0));
        ///
        /// list.push_back(0);
        /// assert!(list.contains(&0));
        /// ```
        pub fn contains(&self, value: &EntryData) -> bool
        where
            EntryData: PartialEq,
        {
            self.iter().any(|entry| entry == value)
        }

        /// Creates a draining iterator that removes all values from the list and yields them in order.
        ///
        /// All values are removed even if the iterator is only partially consumed or not consumed at
        /// all.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_back(0);
        /// list.push_back(5);
        ///
        /// {
        ///     let mut iter = list.drain();
        ///     assert_eq!(iter.next(), Some(0));
        ///     assert_eq!(iter.next(), Some(5));
        ///     assert_eq!(iter.next(), None);
        /// }
        ///
        /// println!("{}", list.len());
        /// assert!(list.is_empty());
        /// ```
        pub fn drain(&mut self) -> Drain<EntryData> {
            Drain {
                head: self.head(),
                remaining: self.length,
                tail: self.tail(),
                list: self,
            }
        }

        /// Returns an immutable reference to the value at the front of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.front(), None);
        ///
        /// list.push_front(0);
        /// list.push_front(5);
        /// assert_eq!(list.front(), Some(&5));
        /// ```
        pub fn front(&self) -> Option<&EntryData> {
            let index = self.head()?;

            match &self.entries[index] {
                Entry::Occupied(entry) => Some(&entry.value),
                _ => None,
            }
        }

        /// Returns a mutable reference to the value at the front of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.front_mut(), None);
        ///
        /// list.push_front(0);
        /// list.push_front(5);
        ///
        /// let mut front = list.front_mut().unwrap();
        /// assert_eq!(front, &mut 5);
        /// *front *= 2;
        ///
        /// assert_eq!(list.front(), Some(&10));
        /// ```
        pub fn front_mut(&mut self) -> Option<&mut EntryData> {
            let index = self.head()?;

            match &mut self.entries[index] {
                Entry::Occupied(entry) => Some(&mut entry.value),
                _ => None,
            }
        }

        /// Returns an immutable reference to the value at the given index.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_front(0);
        /// assert_eq!(list.get(index), Some(&0));
        ///
        /// let index = list.push_front(5);
        /// assert_eq!(list.get(index), Some(&5));
        /// ```
        pub fn get(&self, index: Index<EntryData>) -> Option<&EntryData> {
            match self.entries.get(index.index)? {
                Entry::Occupied(entry) if entry.generation == index.generation => Some(&entry.value),
                _ => None,
            }
        }

        /// Returns a mutable reference to the value at the given index.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_front(0);
        /// let value = list.get_mut(index).unwrap();
        /// *value = 100;
        /// assert_eq!(list.get(index), Some(&100));
        /// ```
        pub fn get_mut(&mut self, index: Index<EntryData>) -> Option<&mut EntryData> {
            match self.entries.get_mut(index.index)? {
                Entry::Occupied(entry) if entry.generation == index.generation => {
                    Some(&mut entry.value)
                }
                _ => None,
            }
        }

        /// Returns the index of the value next to the value at the given index.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        ///
        /// let index_1 = list.push_back(0);
        /// assert_eq!(list.get_next_index(index_1), None);
        ///
        /// let index_2 = list.push_back(5);
        /// assert_eq!(list.get_next_index(index_1), Some(index_2));
        /// ```
        pub fn get_next_index(&self, index: Index<EntryData>) -> Option<Index<EntryData>> {
            match self.entries.get(index.index)? {
                Entry::Occupied(entry) if entry.generation == index.generation => {
                    let next_index = entry.next?;
                    let next_entry = self.entries[next_index].occupied_ref();
                    Some(Index::new(next_index, next_entry.generation))
                }
                _ => None,
            }
        }

        /// Returns the index of the value previous to the value at the given index.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        ///
        /// let index_1 = list.push_front(0);
        /// assert_eq!(list.get_previous_index(index_1), None);
        ///
        /// let index_2 = list.push_front(5);
        /// assert_eq!(list.get_previous_index(index_1), Some(index_2));
        /// ```
        pub fn get_previous_index(&self, index: Index<EntryData>) -> Option<Index<EntryData>> {
            match self.entries.get(index.index)? {
                Entry::Occupied(entry) if entry.generation == index.generation => {
                    let previous_index = entry.previous?;
                    let previous_entry = self.entries[previous_index].occupied_ref();
                    Some(Index::new(previous_index, previous_entry.generation))
                }
                _ => None,
            }
        }

        /// Convenience function for returning the actual head index.
        fn head(&self) -> Option<usize> {
            self.head.map(|head| head.get() - 1)
        }

        /// Creates an indices iterator which will yield all indices of the list in order.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_front(0);
        /// list.push_front(5);
        ///
        /// let mut indices = list.indices();
        /// let index = indices.next().unwrap();
        /// assert_eq!(list.get(index), Some(&5));
        ///
        /// let index = indices.next().unwrap();
        /// assert_eq!(list.get(index), Some(&0));
        ///
        /// assert_eq!(indices.next(), None);
        /// ```
        pub fn indices(&self) -> Indices<EntryData> {
            Indices {
                entries: &self.entries,
                head: self.head(),
                remaining: self.length,
                tail: self.tail(),
            }
        }

        /// Inserts the given value after the value at the given index.
        ///
        /// The index of the newly inserted value will be returned.
        ///
        /// Complexity: amortized O(1)
        ///
        /// # Panics
        ///
        /// Panics if the index refers to an index not in the list anymore or if the index has been
        /// invalidated. This is enforced because this function will consume the value to be inserted,
        /// and if it cannot be inserted (due to the index not being valid), then it will be lost.
        ///
        /// Also panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_front(0);
        /// let index_1 = list.push_front(5);
        /// list.push_front(10);
        ///
        /// let index_2 = list.insert_after(index_1, 1000);
        /// assert_eq!(list.get_next_index(index_1), Some(index_2));
        /// ```
        pub fn insert_after(&mut self, index: Index<EntryData>, value: EntryData) -> Index<EntryData> {
            let next_index = match &mut self.entries[index.index] {
                Entry::Occupied(entry) if entry.generation == index.generation => entry.next,
                _ => panic!("expected occupied entry with correct generation"),
            };
            let new_index = self.insert_new(value, Some(index.index), next_index);
            let entry = self.entries[index.index].occupied_mut();
            entry.next = Some(new_index);

            if Some(index.index) == self.tail() {
                self.set_tail(new_index);
            }

            if let Some(next_index) = next_index {
                self.entries[next_index].occupied_mut().previous = Some(new_index);
            }

            Index::new(new_index, self.generation)
        }

        /// Inserts the given value before the value at the given index.
        ///
        /// The index of the newly inserted value will be returned.
        ///
        /// Complexity: amortized O(1)
        ///
        /// # Panics
        ///
        /// Panics if the index refers to an index not in the list anymore or if the index has been
        /// invalidated. This is enforced because this function will consume the value to be inserted,
        /// and if it cannot be inserted (due to the index not being valid), then it will be lost.
        ///
        /// Also panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_front(0);
        /// let index_1 = list.push_front(5);
        /// list.push_front(10);
        ///
        /// let index_2 = list.insert_before(index_1, 1000);
        /// assert_eq!(list.get_previous_index(index_1), Some(index_2));
        /// ```
        pub fn insert_before(&mut self, index: Index<EntryData>, value: EntryData) -> Index<EntryData> {
            let previous_index = match &mut self.entries[index.index] {
                Entry::Occupied(entry) if entry.generation == index.generation => entry.previous,
                _ => panic!("expected occupied entry with correct generation"),
            };
            let new_index = self.insert_new(value, previous_index, Some(index.index));
            let entry = self.entries[index.index].occupied_mut();
            entry.previous = Some(new_index);

            if Some(index.index) == self.head() {
                self.set_head(new_index);
            }

            if let Some(previous_index) = previous_index {
                self.entries[previous_index].occupied_mut().next = Some(new_index);
            }

            Index::new(new_index, self.generation)
        }

        /// Inserts the given value into the list with the assumption that it is currently empty.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        fn insert_empty(&mut self, value: EntryData) -> Index<EntryData> {
            let generation = self.generation;
            let index = self.insert_new(value, None, None);
            self.set_head(index);
            self.set_tail(index);
            Index::new(index, generation)
        }

        /// Inserts the given value into the list with its expected previous and next value indices.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        fn insert_new(
            &mut self,
            value: EntryData,
            previous: Option<usize>,
            next: Option<usize>,
        ) -> usize {
            use self::Entry::*;

            self.length += 1;

            if self.length == usize::max_value() {
                panic!("reached maximum possible length");
            }

            match self.vacant_head() {
                Some(index) => {
                    self.set_vacant_head(self.entries[index].vacant_ref().next);
                    self.entries[index] =
                        Occupied(OccupiedEntry::new(self.generation, previous, next, value));
                    index
                }
                None => {
                    self.entries.push(Occupied(OccupiedEntry::new(
                        self.generation,
                        previous,
                        next,
                        value,
                    )));
                    self.entries.len() - 1
                }
            }
        }

        /// Returns whether or not the list is empty.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert!(list.is_empty());
        ///
        /// list.push_back(0);
        /// assert!(!list.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            self.length == 0
        }

        /// Creates an iterator that yields immutable references to values in the list in order.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_back(0);
        /// list.push_back(10);
        /// list.push_back(200);
        /// list.push_back(-10);
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&10));
        /// assert_eq!(iter.next(), Some(&200));
        /// assert_eq!(iter.next(), Some(&-10));
        /// assert_eq!(iter.next(), None);
        /// ```
        pub fn iter(&self) -> Iter<EntryData> {
            Iter {
                entries: &self.entries,
                head: self.head(),
                remaining: self.length,
                tail: self.tail(),
            }
        }

        /// Creates an iterator that yields mutable references to values in the list in order.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_back(0);
        /// list.push_back(10);
        /// list.push_back(200);
        /// list.push_back(-10);
        ///
        /// let mut iter = list.iter_mut();
        /// assert_eq!(iter.next(), Some(&mut 0));
        /// assert_eq!(iter.next(), Some(&mut 10));
        /// assert_eq!(iter.next(), Some(&mut 200));
        /// assert_eq!(iter.next(), Some(&mut -10));
        /// assert_eq!(iter.next(), None);
        /// ```
        pub fn iter_mut(&mut self) -> IterMut<EntryData> {
            IterMut {
                entries: &mut self.entries as *mut _,
                head: self.head(),
                phantom: PhantomData,
                remaining: self.length,
                tail: self.tail(),
            }
        }

        /// Returns the number of values in the list.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.len(), 0);
        ///
        /// list.push_back(0);
        /// list.push_back(1);
        /// list.push_back(2);
        /// assert_eq!(list.len(), 3);
        /// ```
        pub fn len(&self) -> usize {
            self.length
        }

        /// Creates a new list with no initial capacity.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_back(0);
        /// assert_eq!(list.get(index), Some(&0));
        /// ```
        pub fn new() -> Self {
            VecList::default()
        }

        /// Reorganizes the existing values to ensure maximum cache locality and shrinks the list such
        /// that the capacity is exactly [`minimum_capacity`].
        ///
        /// This function can be used to actually increase the capacity of the list.
        ///
        /// Complexity: O(n)
        ///
        /// # Panics
        ///
        /// Panics if the given minimum capacity is less than the current length of the list.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index_1 = list.push_back(5);
        /// let index_2 = list.push_back(10);
        /// let index_3 = list.push_front(100);
        /// list.remove(index_1);
        ///
        /// assert!(list.capacity() >= 3);
        ///
        /// let mut map = list.pack_to(list.len() + 5);
        /// assert_eq!(list.capacity(), 7);
        /// assert_eq!(map.len(), 2);
        ///
        /// let index_2 = map.remove(&index_2).unwrap();
        /// let index_3 = map.remove(&index_3).unwrap();
        ///
        /// assert_eq!(list.get(index_2), Some(&10));
        /// assert_eq!(list.get(index_3), Some(&100));
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&100));
        /// assert_eq!(iter.next(), Some(&10));
        /// assert_eq!(iter.next(), None);
        /// ```
        pub fn pack_to(
            &mut self,
            minimum_capacity: usize,
        ) -> HashMap<Index<EntryData>, Index<EntryData>> {
            assert!(
                minimum_capacity >= self.length,
                "cannot shrink to capacity lower than current length"
            );

            let mut count = 0;
            let mut entries = Vec::with_capacity(minimum_capacity);
            let generation = create_initial_generation();
            let length = self.length;
            let mut map = HashMap::with_capacity(length);
            let mut next_index = self.head();

            while let Some(index) = next_index {
                let mut entry = self.remove_entry(index).expect("expected occupied entry");
                next_index = entry.next;
                map.insert(
                    Index::new(index, entry.generation),
                    Index::new(count, generation),
                );

                entry.generation = generation;
                entry.previous = if count > 0 { Some(count - 1) } else { None };
                entry.next = if count < length - 1 {
                    Some(count + 1)
                } else {
                    None
                };

                entries.push(Entry::Occupied(entry));
                count += 1;
            }

            self.entries = entries;
            self.generation = generation;
            self.length = length;
            self.vacant_head = None;

            if self.length > 0 {
                self.set_head(0);
                self.set_tail(length - 1);
            } else {
                self.head = None;
                self.tail = None;
            }

            map
        }

        /// Reorganizes the existing values to ensure maximum cache locality and shrinks the list such
        /// that no additional capacity exists.
        ///
        /// This is equivalent to calling [`VecList::pack_to`] with the current length.
        ///
        /// Complexity: O(n)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index_1 = list.push_back(5);
        /// let index_2 = list.push_back(10);
        /// let index_3 = list.push_front(100);
        /// list.remove(index_1);
        ///
        /// assert!(list.capacity() >= 3);
        ///
        /// let mut map = list.pack_to_fit();
        /// assert_eq!(list.capacity(), 2);
        /// assert_eq!(map.len(), 2);
        ///
        /// let index_2 = map.remove(&index_2).unwrap();
        /// let index_3 = map.remove(&index_3).unwrap();
        ///
        /// assert_eq!(list.get(index_2), Some(&10));
        /// assert_eq!(list.get(index_3), Some(&100));
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&100));
        /// assert_eq!(iter.next(), Some(&10));
        /// assert_eq!(iter.next(), None);
        /// ```
        pub fn pack_to_fit(&mut self) -> HashMap<Index<EntryData>, Index<EntryData>> {
            self.pack_to(self.length)
        }

        /// Removes and returns the value at the back of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.pop_back(), None);
        ///
        /// list.push_back(0);
        /// list.push_back(1);
        /// list.push_back(2);
        /// assert_eq!(list.len(), 3);
        ///
        /// assert_eq!(list.pop_back(), Some(2));
        /// assert_eq!(list.len(), 2);
        /// ```
        pub fn pop_back(&mut self) -> Option<EntryData> {
            self.remove_entry(self.tail()?).map(|entry| entry.value)
        }

        /// Removes and returns the value at the front of the list, if it exists.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// assert_eq!(list.pop_front(), None);
        ///
        /// list.push_front(0);
        /// list.push_front(1);
        /// list.push_front(2);
        /// assert_eq!(list.len(), 3);
        ///
        /// assert_eq!(list.pop_front(), Some(2));
        /// assert_eq!(list.len(), 2);
        /// ```
        pub fn pop_front(&mut self) -> Option<EntryData> {
            self.remove_entry(self.head()?).map(|entry| entry.value)
        }

        /// Inserts the given value to the back of the list.
        ///
        /// The index of the newly inserted value will be returned.
        ///
        /// Complexity: amortized O(1)
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_back(0);
        /// assert_eq!(list.get(index), Some(&0));
        /// ```
        pub fn push_back(&mut self, value: EntryData) -> Index<EntryData> {
            let tail_index = match self.tail() {
                Some(index) => index,
                None => return self.insert_empty(value),
            };
            let index = self.insert_new(value, Some(tail_index), None);
            self.entries[tail_index].occupied_mut().next = Some(index);
            self.set_tail(index);
            Index::new(index, self.generation)
        }

        /// Inserts the given value to the front of the list.
        ///
        /// The index of the newly inserted value will be returned.
        ///
        /// Complexity: amortized O(1)
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_front(0);
        /// assert_eq!(list.get(index), Some(&0));
        /// ```
        pub fn push_front(&mut self, value: EntryData) -> Index<EntryData> {
            let head_index = match self.head() {
                Some(index) => index,
                None => return self.insert_empty(value),
            };
            let index = self.insert_new(value, None, Some(head_index));
            self.entries[head_index].occupied_mut().previous = Some(index);
            self.set_head(index);
            Index::new(index, self.generation)
        }

        /// Removes and returns the value at the given index, if it exists.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned and the list will be unaffected.
        ///
        /// Complexity: O(1)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// let index = list.push_back(0);
        /// assert_eq!(list.remove(index), Some(0));
        /// assert_eq!(list.remove(index), None);
        /// ```
        pub fn remove(&mut self, index: Index<EntryData>) -> Option<EntryData> {
            let (previous_index, next_index) = match &self.entries[index.index] {
                Entry::Occupied(entry) if entry.generation == index.generation => {
                    (entry.previous, entry.next)
                }
                _ => return None,
            };
            Some(
                self.remove_helper(previous_index, index.index, next_index)
                    .value,
            )
        }

        /// Removes and returns the entry at the given index, if it exists.
        ///
        /// If the index refers to an index not in the list anymore or if the index has been
        /// invalidated, then [`None`] will be returned and the list will be unaffected.
        fn remove_entry(&mut self, index: usize) -> Option<OccupiedEntry<EntryData>> {
            use self::Entry::*;

            let (previous_index, next_index) = match &self.entries[index] {
                Occupied(entry) => (entry.previous, entry.next),
                Vacant(_) => return None,
            };
            Some(self.remove_helper(previous_index, index, next_index))
        }

        /// Removes and returns the entry at the given index with the entries previous and next index
        /// values.
        ///
        /// It is assumed that there is an entry at the given index.
        ///
        /// # Panics
        ///
        /// Panics if called when the list is empty. Behavior is undefined if provided indices do not
        /// follow the expected constraints.
        fn remove_helper(
            &mut self,
            previous_index: Option<usize>,
            index: usize,
            next_index: Option<usize>,
        ) -> OccupiedEntry<EntryData> {
            let head_index = self.head().expect("expected head index");
            let tail_index = self.tail().expect("expected tail index");
            let vacant_head = self.vacant_head();
            let removed_entry = mem::replace(
                &mut self.entries[index],
                Entry::Vacant(VacantEntry::new(vacant_head)),
            );

            self.generation = self.generation.wrapping_add(1);
            self.length -= 1;
            self.set_vacant_head(Some(index));

            if index == head_index && index == tail_index {
                self.head = None;
                self.tail = None;
            } else if index == head_index {
                self.entries[next_index.expect("expected next entry to exist")]
                    .occupied_mut()
                    .previous = None;
                self.head = next_index.map(|index| NonZeroUsize::new(index + 1).unwrap());
            } else if index == tail_index {
                self.entries[previous_index.expect("expected previous entry to exist")]
                    .occupied_mut()
                    .next = None;
                self.tail = previous_index.map(|index| NonZeroUsize::new(index + 1).unwrap());
            } else {
                self.entries[next_index.expect("expected next entry to exist")]
                    .occupied_mut()
                    .previous = previous_index;
                self.entries[previous_index.expect("expected previous entry to exist")]
                    .occupied_mut()
                    .next = next_index;
            }

            removed_entry.occupied()
        }

        /// Reserves capacity for the given expected size increase.
        ///
        /// The collection may reserve more space to avoid frequent reallocations. After calling this
        /// function, capacity will be greater than or equal to `self.len() + additional_capacity`.
        /// Does nothing if the current capacity is already sufficient.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list: VecList<u32> = VecList::new();
        /// assert_eq!(list.capacity(), 0);
        ///
        /// list.reserve(10);
        /// assert!(list.capacity() >= 10);
        /// ```
        pub fn reserve(&mut self, additional_capacity: usize) {
            self.entries.reserve(additional_capacity);
        }

        /// Removes all elements from the list not satisfying the given predicate.
        ///
        /// Complexity: O(n)
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list = VecList::new();
        /// list.push_back(0);
        /// list.push_back(-1);
        /// list.push_back(1);
        /// list.push_back(-2);
        /// list.retain(|&mut value| value >= 0);
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), None);
        /// ```
        pub fn retain<Predicate>(&mut self, mut predicate: Predicate)
        where
            Predicate: FnMut(&mut EntryData) -> bool,
        {
            let mut next_index = self.head();

            while let Some(index) = next_index {
                let entry = &mut self.entries[index].occupied_mut();
                next_index = entry.next;

                if !predicate(&mut entry.value) {
                    self.remove_entry(index);
                }
            }
        }

        /// Convenience function for returning setting the offset head index.
        fn set_head(&mut self, index: usize) {
            self.head = Some(NonZeroUsize::new(index + 1).expect("head should not be 0"));
        }

        /// Convenience function for returning setting the offset tail index.
        fn set_tail(&mut self, index: usize) {
            self.tail = Some(NonZeroUsize::new(index + 1).expect("tail should not be 0"));
        }

        /// Convenience function for returning setting the offset vacant head index.
        fn set_vacant_head(&mut self, index: Option<usize>) {
            self.vacant_head =
                index.map(|index| NonZeroUsize::new(index + 1).expect("vacant head should not be 0"));
        }

        /// Convenience function for returning the actual tail index.
        fn tail(&self) -> Option<usize> {
            self.tail.map(|tail| tail.get() - 1)
        }

        /// Convenience function for returning the actual vacant head index.
        fn vacant_head(&self) -> Option<usize> {
            self.vacant_head.map(|vacant_head| vacant_head.get() - 1)
        }

        /// Creates a new list with the given capacity.
        ///
        /// # Examples
        ///
        /// ```
        /// use dlv_list::VecList;
        ///
        /// let mut list: VecList<u32> = VecList::new();
        /// assert_eq!(list.capacity(), 0);
        ///
        /// let mut list: VecList<u32> = VecList::with_capacity(10);
        /// assert_eq!(list.capacity(), 10);
        /// ```
        pub fn with_capacity(capacity: usize) -> Self {
            VecList {
                entries: Vec::with_capacity(capacity),
                generation: create_initial_generation(),
                head: None,
                length: 0,
                tail: None,
                vacant_head: None,
            }
        }
    }

    impl<EntryData> Debug for VecList<EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.debug_list().entries(self.iter()).finish()
        }
    }

    impl<EntryData> Default for VecList<EntryData> {
        fn default() -> Self {
            VecList {
                entries: Vec::default(),
                generation: create_initial_generation(),
                head: None,
                length: 0,
                tail: None,
                vacant_head: None,
            }
        }
    }

    impl<EntryData> Eq for VecList<EntryData> where EntryData: Eq {}

    impl<EntryData> Extend<EntryData> for VecList<EntryData> {
        fn extend<Iter>(&mut self, iter: Iter)
        where
            Iter: IntoIterator<Item = EntryData>,
        {
            let iter = iter.into_iter();
            self.reserve(iter.size_hint().0);

            for value in iter {
                self.push_back(value);
            }
        }
    }

    impl<'entries, EntryData> Extend<&'entries EntryData> for VecList<EntryData>
    where
        EntryData: 'entries + Copy,
    {
        fn extend<Iter>(&mut self, iter: Iter)
        where
            Iter: IntoIterator<Item = &'entries EntryData>,
        {
            self.extend(iter.into_iter().cloned())
        }
    }

    impl<EntryData> FromIterator<EntryData> for VecList<EntryData> {
        fn from_iter<Iter>(iter: Iter) -> Self
        where
            Iter: IntoIterator<Item = EntryData>,
        {
            let mut list = VecList::new();
            list.extend(iter);
            list
        }
    }

    impl<EntryData> Hash for VecList<EntryData>
    where
        EntryData: Hash,
    {
        fn hash<StateHasher>(&self, state: &mut StateHasher)
        where
            StateHasher: Hasher,
        {
            self.len().hash(state);

            for value in self {
                value.hash(state);
            }
        }
    }

    impl<EntryData> ops::Index<Index<EntryData>> for VecList<EntryData> {
        type Output = EntryData;

        fn index(&self, index: Index<EntryData>) -> &Self::Output {
            self.get(index).expect("expected entry at index")
        }
    }

    impl<EntryData> ops::IndexMut<Index<EntryData>> for VecList<EntryData> {
        fn index_mut(&mut self, index: Index<EntryData>) -> &mut Self::Output {
            self.get_mut(index).expect("expected entry at index")
        }
    }

    impl<EntryData> IntoIterator for VecList<EntryData> {
        type IntoIter = IntoIter<EntryData>;
        type Item = EntryData;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter {
                head: self.head(),
                remaining: self.length,
                tail: self.tail(),
                list: self,
            }
        }
    }

    impl<'entries, EntryData> IntoIterator for &'entries VecList<EntryData> {
        type IntoIter = Iter<'entries, EntryData>;
        type Item = &'entries EntryData;

        fn into_iter(self) -> Self::IntoIter {
            Iter {
                entries: &self.entries,
                head: self.head(),
                remaining: self.length,
                tail: self.tail(),
            }
        }
    }

    impl<'entries, EntryData> IntoIterator for &'entries mut VecList<EntryData> {
        type IntoIter = IterMut<'entries, EntryData>;
        type Item = &'entries mut EntryData;

        fn into_iter(self) -> Self::IntoIter {
            IterMut {
                entries: &mut self.entries as *mut _,
                head: self.head(),
                phantom: PhantomData,
                remaining: self.length,
                tail: self.tail(),
            }
        }
    }

    impl<EntryData> Ord for VecList<EntryData>
    where
        EntryData: Ord,
    {
        fn cmp(&self, other: &Self) -> Ordering {
            self.iter().cmp(other)
        }
    }

    impl<EntryData> PartialEq for VecList<EntryData>
    where
        EntryData: PartialEq,
    {
        fn eq(&self, other: &Self) -> bool {
            self.len() == other.len() && self.iter().eq(other)
        }
    }

    impl<EntryData> PartialEq<LinkedList<EntryData>> for VecList<EntryData>
    where
        EntryData: PartialEq,
    {
        fn eq(&self, other: &LinkedList<EntryData>) -> bool {
            self.len() == other.len() && self.iter().eq(other)
        }
    }

    impl<EntryData> PartialEq<Vec<EntryData>> for VecList<EntryData>
    where
        EntryData: PartialEq,
    {
        fn eq(&self, other: &Vec<EntryData>) -> bool {
            self.len() == other.len() && self.iter().eq(other)
        }
    }

    impl<'slice, EntryData> PartialEq<&'slice [EntryData]> for VecList<EntryData>
    where
        EntryData: PartialEq,
    {
        fn eq(&self, other: &&'slice [EntryData]) -> bool {
            self.len() == other.len() && self.iter().eq(other.iter())
        }
    }

    impl<EntryData> PartialOrd for VecList<EntryData>
    where
        EntryData: PartialOrd<EntryData>,
    {
        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
            self.iter().partial_cmp(other)
        }
    }

    /// A wrapper type that indicates an index into the list.
    ///
    /// This index may be invalidated by operations on the list itself.
    pub struct Index<EntryData> {
        /// The generation of the entry currently at this index. This is used to avoid the ABA problem.
        generation: u64,

        /// The actual index into the entry list.
        index: usize,

        /// This type is parameterized on the entry data type to avoid indices being used across
        /// differently typed lists.
        phantom: PhantomData<EntryData>,
    }

    impl<EntryData> Clone for Index<EntryData> {
        fn clone(&self) -> Self {
            Index {
                generation: self.generation,
                index: self.index,
                phantom: PhantomData,
            }
        }
    }

    impl<EntryData> Copy for Index<EntryData> {}

    impl<EntryData> Debug for Index<EntryData> {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter
                .debug_tuple("Index")
                .field(&self.index)
                .field(&self.generation)
                .finish()
        }
    }

    impl<EntryData> Eq for Index<EntryData> {}

    impl<EntryData> Hash for Index<EntryData> {
        fn hash<StateHasher>(&self, hasher: &mut StateHasher)
        where
            StateHasher: Hasher,
        {
            self.index.hash(hasher);
            self.generation.hash(hasher);
        }
    }

    impl<EntryData> PartialEq for Index<EntryData> {
        fn eq(&self, other: &Self) -> bool {
            self.generation == other.generation && self.index == other.index
        }
    }

    impl<EntryData> Index<EntryData> {
        /// Convenience function for creating new index.
        pub(self) fn new(index: usize, generation: u64) -> Index<EntryData> {
            Index {
                generation,
                index,
                phantom: PhantomData,
            }
        }
    }

    /// An entry in the list. This can be either occupied or vacant.
    #[derive(Clone)]
    enum Entry<EntryData> {
        /// An occupied entry contains actual entry data inserted by the user.
        Occupied(OccupiedEntry<EntryData>),

        /// A vacant entry is one that can be reused.
        Vacant(VacantEntry),
    }

    impl<EntryData> Entry<EntryData> {
        /// Returns the occupied entry by moving it out of the entry.
        ///
        /// # Panics
        ///
        /// Panics if the variant is actually [`Entry::Vacant`].
        pub fn occupied(self) -> OccupiedEntry<EntryData> {
            use self::Entry::*;

            match self {
                Occupied(entry) => entry,
                Vacant(_) => panic!("expected occupied entry"),
            }
        }

        /// Returns an immutable reference to the occupied entry.
        ///
        /// # Panics
        ///
        /// Panics if the variant is actually [`Entry::Vacant`].
        pub fn occupied_ref(&self) -> &OccupiedEntry<EntryData> {
            use self::Entry::*;

            match self {
                Occupied(entry) => entry,
                Vacant(_) => panic!("expected occupied entry"),
            }
        }

        /// Returns a mutable reference to the occupied entry.
        ///
        /// # Panics
        ///
        /// Panics if the variant is actually [`Entry::Vacant`].
        pub fn occupied_mut(&mut self) -> &mut OccupiedEntry<EntryData> {
            use self::Entry::*;

            match self {
                Occupied(entry) => entry,
                Vacant(_) => panic!("expected occupied entry"),
            }
        }

        /// Returns an immutable reference to the vacant entry.
        ///
        /// # Panics
        ///
        /// Panics if the variant is actually [`Entry::Occupied`].
        pub fn vacant_ref(&self) -> &VacantEntry {
            use self::Entry::*;

            match self {
                Vacant(entry) => entry,
                Occupied(_) => panic!("expected vacant entry"),
            }
        }
    }

    /// An occupied entry in the list.
    #[derive(Clone)]
    struct OccupiedEntry<EntryData> {
        /// The generation of when this entry was inserted. This is used to avoid the ABA problem.
        generation: u64,

        /// The index of the next occupied entry in the list.
        next: Option<usize>,

        /// The index of the previous occupied entry in the list.
        previous: Option<usize>,

        /// The actual value being stored in this entry.
        value: EntryData,
    }

    impl<EntryData> OccupiedEntry<EntryData> {
        /// Convenience function for creating a new occupied entry.
        pub fn new(
            generation: u64,
            previous: Option<usize>,
            next: Option<usize>,
            value: EntryData,
        ) -> OccupiedEntry<EntryData> {
            OccupiedEntry {
                generation,
                next,
                previous,
                value,
            }
        }
    }

    /// A vacant entry in the list.
    #[derive(Clone, Debug)]
    struct VacantEntry {
        /// The index of the next vacant entry in the list.
        next: Option<usize>,
    }

    impl VacantEntry {
        /// Convenience function for creating a new vacant entry.
        pub fn new(next: Option<usize>) -> VacantEntry {
            VacantEntry { next }
        }
    }

    /// An iterator that yields and removes all entries from the list.
    pub struct Drain<'entries, EntryData> {
        /// The index of the head of the unvisited portion of the list.
        head: Option<usize>,

        /// A reference to the entry list.
        list: &'entries mut VecList<EntryData>,

        /// The number of entries that have not been visited.
        remaining: usize,

        /// The index of the tail of the unvisited portion of the list.
        tail: Option<usize>,
    }

    impl<EntryData> Drain<'_, EntryData> {
        /// Creates an iterator that yields immutable references to entries in the list.
        pub fn iter(&self) -> Iter<EntryData> {
            Iter {
                entries: &self.list.entries,
                head: self.head,
                remaining: self.remaining,
                tail: self.tail,
            }
        }
    }

    impl<EntryData> Debug for Drain<'_, EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.write_str("Drain(")?;
            formatter.debug_list().entries(self.iter()).finish()?;
            formatter.write_str(")")
        }
    }

    impl<EntryData> DoubleEndedIterator for Drain<'_, EntryData> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.tail.map(|index| {
                    let entry = self
                        .list
                        .remove_entry(index)
                        .expect("expected occupied entry");
                    self.tail = entry.previous;
                    self.remaining -= 1;
                    entry.value
                })
            }
        }
    }

    impl<EntryData> Drop for Drain<'_, EntryData> {
        fn drop(&mut self) {
            self.list.clear();
        }
    }

    impl<EntryData> ExactSizeIterator for Drain<'_, EntryData> {}

    impl<EntryData> FusedIterator for Drain<'_, EntryData> {}

    impl<EntryData> Iterator for Drain<'_, EntryData> {
        type Item = EntryData;

        fn next(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.head.map(|index| {
                    let entry = self
                        .list
                        .remove_entry(index)
                        .expect("expected occupied entry");
                    self.head = entry.next;
                    self.remaining -= 1;
                    entry.value
                })
            }
        }

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

    /// An iterator that yields all indices in the list.
    pub struct Indices<'entries, EntryData> {
        /// A reference to the actual storage for the entry list.
        entries: &'entries Vec<Entry<EntryData>>,

        /// The index of the head of the unvisited portion of the list.
        head: Option<usize>,

        /// The number of entries that have not been visited.
        remaining: usize,

        /// The index of the tail of the unvisited portion of the list.
        tail: Option<usize>,
    }

    impl<EntryData> Clone for Indices<'_, EntryData> {
        fn clone(&self) -> Self {
            Indices {
                entries: self.entries,
                head: self.head,
                remaining: self.remaining,
                tail: self.tail,
            }
        }
    }

    impl<EntryData> Debug for Indices<'_, EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.write_str("Indices(")?;
            formatter.debug_list().entries(self.clone()).finish()?;
            formatter.write_str(")")
        }
    }

    impl<EntryData> DoubleEndedIterator for Indices<'_, EntryData> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.tail.map(|index| {
                    let entry = self.entries[index].occupied_ref();
                    let index = Index::new(index, entry.generation);
                    self.tail = entry.previous;
                    self.remaining -= 1;
                    index
                })
            }
        }
    }

    impl<EntryData> ExactSizeIterator for Indices<'_, EntryData> {}

    impl<EntryData> FusedIterator for Indices<'_, EntryData> {}

    impl<EntryData> Iterator for Indices<'_, EntryData> {
        type Item = Index<EntryData>;

        fn next(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.head.map(|index| {
                    let entry = self.entries[index].occupied_ref();
                    let index = Index::new(index, entry.generation);
                    self.head = entry.next;
                    self.remaining -= 1;
                    index
                })
            }
        }

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

    /// An iterator that moves all entries out of the entry list.
    #[derive(Clone)]
    pub struct IntoIter<EntryData> {
        /// The index of the head of the unvisited portion of the list.
        head: Option<usize>,

        /// The entry list from which entries are yielded.
        list: VecList<EntryData>,

        /// The number of entries that have not been visited.
        remaining: usize,

        /// The index of the tail of the unvisited portion of the list.
        tail: Option<usize>,
    }

    impl<EntryData> IntoIter<EntryData> {
        /// Creates an iterator that yields immutable references to entries in the list.
        pub fn iter(&self) -> Iter<EntryData> {
            Iter {
                entries: &self.list.entries,
                head: self.head,
                remaining: self.remaining,
                tail: self.tail,
            }
        }
    }

    impl<EntryData> Debug for IntoIter<EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.write_str("IntoIter(")?;
            formatter.debug_list().entries(self.iter()).finish()?;
            formatter.write_str(")")
        }
    }

    impl<EntryData> DoubleEndedIterator for IntoIter<EntryData> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.tail.map(|index| {
                    let entry = self
                        .list
                        .remove_entry(index)
                        .expect("expected occupied entry");
                    self.tail = entry.previous;
                    self.remaining -= 1;
                    entry.value
                })
            }
        }
    }

    impl<EntryData> ExactSizeIterator for IntoIter<EntryData> {}

    impl<EntryData> FusedIterator for IntoIter<EntryData> {}

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

        fn next(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.head.map(|index| {
                    let entry = self
                        .list
                        .remove_entry(index)
                        .expect("expected occupied entry");
                    self.head = entry.next;
                    self.remaining -= 1;
                    entry.value
                })
            }
        }

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

    /// An iterator that yields immutable references to entries in the list.
    pub struct Iter<'entries, EntryData> {
        /// A reference to the actual storage for the entry list.
        entries: &'entries Vec<Entry<EntryData>>,

        /// The index of the head of the unvisited portion of the list.
        head: Option<usize>,

        /// The number of entries that have not been visited.
        remaining: usize,

        /// The index of the tail of the unvisited portion of the list.
        tail: Option<usize>,
    }

    impl<'entries, EntryData> Clone for Iter<'entries, EntryData> {
        fn clone(&self) -> Iter<'entries, EntryData> {
            Iter {
                entries: self.entries,
                head: self.head,
                remaining: self.remaining,
                tail: self.tail,
            }
        }
    }

    impl<EntryData> Debug for Iter<'_, EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.write_str("Iter(")?;
            formatter.debug_list().entries(self.clone()).finish()?;
            formatter.write_str(")")
        }
    }

    impl<EntryData> DoubleEndedIterator for Iter<'_, EntryData> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.tail.map(|index| {
                    let entry = self.entries[index].occupied_ref();
                    self.tail = entry.previous;
                    self.remaining -= 1;
                    &entry.value
                })
            }
        }
    }

    impl<EntryData> ExactSizeIterator for Iter<'_, EntryData> {}

    impl<EntryData> FusedIterator for Iter<'_, EntryData> {}

    impl<'entries, EntryData> Iterator for Iter<'entries, EntryData> {
        type Item = &'entries EntryData;

        fn next(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.head.map(|index| {
                    let entry = self.entries[index].occupied_ref();
                    self.head = entry.next;
                    self.remaining -= 1;
                    &entry.value
                })
            }
        }

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

    /// An iterator that yields mutable references to entries in the list.
    pub struct IterMut<'entries, EntryData> {
        entries: *mut Vec<Entry<EntryData>>,

        /// The index of the head of the unvisited portion of the list.
        head: Option<usize>,

        /// Because [`IterMut::entries`] is a pointer, we need to have a phantom data here for the
        /// lifetime parameter.
        phantom: PhantomData<&'entries mut Vec<Entry<EntryData>>>,

        /// The number of entries that have not been visited.
        remaining: usize,

        /// The index of the tail of the unvisited portion of the list.
        tail: Option<usize>,
    }

    impl<EntryData> IterMut<'_, EntryData> {
        /// Creates an iterator that yields immutable references to entries in the list.
        pub fn iter(&self) -> Iter<EntryData> {
            Iter {
                entries: unsafe { &*self.entries },
                head: self.head,
                remaining: self.remaining,
                tail: self.tail,
            }
        }
    }

    impl<EntryData> Debug for IterMut<'_, EntryData>
    where
        EntryData: Debug,
    {
        fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
            formatter.write_str("IterMut(")?;
            formatter.debug_list().entries(self.iter()).finish()?;
            formatter.write_str(")")
        }
    }

    impl<EntryData> DoubleEndedIterator for IterMut<'_, EntryData> {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.tail.map(|index| {
                    let entry = unsafe { &mut (*self.entries)[index] }.occupied_mut();
                    self.tail = entry.previous;
                    self.remaining -= 1;
                    &mut entry.value
                })
            }
        }
    }

    impl<EntryData> ExactSizeIterator for IterMut<'_, EntryData> {}

    impl<EntryData> FusedIterator for IterMut<'_, EntryData> {}

    impl<'entries, EntryData> Iterator for IterMut<'entries, EntryData> {
        type Item = &'entries mut EntryData;

        fn next(&mut self) -> Option<Self::Item> {
            if self.remaining == 0 {
                None
            } else {
                self.head.map(|index| {
                    let entry = unsafe { &mut (*self.entries)[index] }.occupied_mut();
                    self.head = entry.next;
                    self.remaining -= 1;
                    &mut entry.value
                })
            }
        }

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

    unsafe impl<EntryData> Send for IterMut<'_, EntryData> where EntryData: Send {}

    unsafe impl<EntryData> Sync for IterMut<'_, EntryData> where EntryData: Sync {}

    /// Creates the initial generation seeded by the current time.
    fn create_initial_generation() -> u64 {
        let mut hasher = RandomState::new().build_hasher();
        hasher.write_u32(0);
        hasher.finish()
    }

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn test_bounds() {
            fn check_bounds<Type: Send + Sync>() {}

            check_bounds::<VecList<()>>();
            check_bounds::<Index<()>>();
            check_bounds::<Drain<()>>();
            check_bounds::<Indices<()>>();
            check_bounds::<IntoIter<()>>();
            check_bounds::<Iter<()>>();
            check_bounds::<IterMut<()>>();
        }

        #[test]
        fn test_drain_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let drain = list.drain();
            assert_eq!(format!("{:?}", drain), "Drain([0, 1, -1, 2, -2])");
        }

        #[test]
        fn test_drain_double_ended() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut drain = list.drain();
            assert_eq!(drain.next(), Some(0));
            assert_eq!(drain.next_back(), Some(-2));
            assert_eq!(drain.next(), Some(1));
            assert_eq!(drain.next_back(), Some(2));
            assert_eq!(drain.next(), Some(-1));
            assert_eq!(drain.next_back(), None);
        }

        #[test]
        fn test_drain_empty() {
            let mut list: VecList<i32> = VecList::new();
            let mut drain = list.drain();
            assert_eq!(drain.next(), None);
        }

        #[test]
        fn test_drain_fused() {
            let mut list: VecList<i32> = VecList::new();
            list.push_back(0);
            let mut drain = list.drain();
            assert_eq!(drain.next(), Some(0));
            assert_eq!(drain.next(), None);
            assert_eq!(drain.next(), None);
            assert_eq!(drain.next(), None);
        }

        #[test]
        fn test_drain_size_hint() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut drain = list.drain();

            assert_eq!(drain.size_hint(), (5, Some(5)));
            drain.next();
            assert_eq!(drain.size_hint(), (4, Some(4)));
            drain.next();
            assert_eq!(drain.size_hint(), (3, Some(3)));
            drain.next();
            assert_eq!(drain.size_hint(), (2, Some(2)));
            drain.next();
            assert_eq!(drain.size_hint(), (1, Some(1)));
            drain.next();
            assert_eq!(drain.size_hint(), (0, Some(0)));
        }

        #[test]
        fn test_index_debug() {
            let mut list = VecList::new();
            let index = list.push_back(5);

            assert_eq!(
                format!("{:?}", index),
                format!("Index(0, {})", index.generation)
            );
        }

        #[test]
        fn test_index_equality() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.indices().next().unwrap();

            assert_eq!(index_1, index_2);
        }

        #[test]
        fn test_indices_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let indices = list.indices();
            assert_eq!(
                format!("{:?}", indices),
                format!(
                    "Indices([Index(0, {}), Index(1, {}), Index(2, {}), Index(3, {}), Index(4, {})])",
                    list.generation, list.generation, list.generation, list.generation, list.generation
                )
            );
        }

        #[test]
        fn test_indices_double_ended() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut indices = list.indices();
            assert_eq!(indices.next().unwrap().index, 0);
            assert_eq!(indices.next_back().unwrap().index, 4);
            assert_eq!(indices.next().unwrap().index, 1);
            assert_eq!(indices.next_back().unwrap().index, 3);
            assert_eq!(indices.next().unwrap().index, 2);
            assert_eq!(indices.next_back(), None);
        }

        #[test]
        fn test_indices_empty() {
            let list: VecList<i32> = VecList::new();
            let mut indices = list.indices();
            assert_eq!(indices.next(), None);
        }

        #[test]
        fn test_indices_fused() {
            let mut list: VecList<i32> = VecList::new();
            list.push_back(0);
            let mut indices = list.indices();
            assert_eq!(indices.next().unwrap().index, 0);
            assert_eq!(indices.next(), None);
            assert_eq!(indices.next(), None);
            assert_eq!(indices.next(), None);
        }

        #[test]
        fn test_indices_size_hint() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut indices = list.indices();

            assert_eq!(indices.size_hint(), (5, Some(5)));
            indices.next();
            assert_eq!(indices.size_hint(), (4, Some(4)));
            indices.next();
            assert_eq!(indices.size_hint(), (3, Some(3)));
            indices.next();
            assert_eq!(indices.size_hint(), (2, Some(2)));
            indices.next();
            assert_eq!(indices.size_hint(), (1, Some(1)));
            indices.next();
            assert_eq!(indices.size_hint(), (0, Some(0)));
        }

        #[test]
        fn test_into_iter_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let iter = list.into_iter();
            assert_eq!(format!("{:?}", iter), "IntoIter([0, 1, -1, 2, -2])");
        }

        #[test]
        fn test_into_iter_double_ended() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.into_iter();
            assert_eq!(iter.next(), Some(0));
            assert_eq!(iter.next_back(), Some(-2));
            assert_eq!(iter.next(), Some(1));
            assert_eq!(iter.next_back(), Some(2));
            assert_eq!(iter.next(), Some(-1));
            assert_eq!(iter.next_back(), None);
        }

        #[test]
        fn test_into_iter_empty() {
            let list: VecList<i32> = VecList::new();
            let mut iter = list.into_iter();
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_into_iter_fused() {
            let mut list: VecList<i32> = VecList::new();
            list.push_back(0);
            let mut iter = list.into_iter();
            assert_eq!(iter.next(), Some(0));
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_into_iter_size_hint() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.into_iter();

            assert_eq!(iter.size_hint(), (5, Some(5)));
            iter.next();
            assert_eq!(iter.size_hint(), (4, Some(4)));
            iter.next();
            assert_eq!(iter.size_hint(), (3, Some(3)));
            iter.next();
            assert_eq!(iter.size_hint(), (2, Some(2)));
            iter.next();
            assert_eq!(iter.size_hint(), (1, Some(1)));
            iter.next();
            assert_eq!(iter.size_hint(), (0, Some(0)));
        }

        #[test]
        fn test_iter_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let iter = list.iter();
            assert_eq!(format!("{:?}", iter), "Iter([0, 1, -1, 2, -2])");
        }

        #[test]
        fn test_iter_double_ended() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.iter();
            assert_eq!(iter.next(), Some(&0));
            assert_eq!(iter.next_back(), Some(&-2));
            assert_eq!(iter.next(), Some(&1));
            assert_eq!(iter.next_back(), Some(&2));
            assert_eq!(iter.next(), Some(&-1));
            assert_eq!(iter.next_back(), None);
        }

        #[test]
        fn test_iter_empty() {
            let list: VecList<i32> = VecList::new();
            let mut iter = list.iter();
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_iter_fused() {
            let mut list: VecList<i32> = VecList::new();
            list.push_back(0);
            let mut iter = list.iter();
            assert_eq!(iter.next(), Some(&0));
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_iter_size_hint() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.iter();

            assert_eq!(iter.size_hint(), (5, Some(5)));
            iter.next();
            assert_eq!(iter.size_hint(), (4, Some(4)));
            iter.next();
            assert_eq!(iter.size_hint(), (3, Some(3)));
            iter.next();
            assert_eq!(iter.size_hint(), (2, Some(2)));
            iter.next();
            assert_eq!(iter.size_hint(), (1, Some(1)));
            iter.next();
            assert_eq!(iter.size_hint(), (0, Some(0)));
        }

        #[test]
        fn test_iter_mut_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let iter = list.iter_mut();
            assert_eq!(format!("{:?}", iter), "IterMut([0, 1, -1, 2, -2])");
        }

        #[test]
        fn test_iter_mut_double_ended() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.iter_mut();
            assert_eq!(iter.next(), Some(&mut 0));
            assert_eq!(iter.next_back(), Some(&mut -2));
            assert_eq!(iter.next(), Some(&mut 1));
            assert_eq!(iter.next_back(), Some(&mut 2));
            assert_eq!(iter.next(), Some(&mut -1));
            assert_eq!(iter.next_back(), None);
        }

        #[test]
        fn test_iter_mut_empty() {
            let mut list: VecList<i32> = VecList::new();
            let mut iter = list.iter_mut();
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_iter_mut_fused() {
            let mut list: VecList<i32> = VecList::new();
            list.push_back(0);
            let mut iter = list.iter_mut();
            assert_eq!(iter.next(), Some(&mut 0));
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_iter_mut_size_hint() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            let mut iter = list.iter_mut();

            assert_eq!(iter.size_hint(), (5, Some(5)));
            iter.next();
            assert_eq!(iter.size_hint(), (4, Some(4)));
            iter.next();
            assert_eq!(iter.size_hint(), (3, Some(3)));
            iter.next();
            assert_eq!(iter.size_hint(), (2, Some(2)));
            iter.next();
            assert_eq!(iter.size_hint(), (1, Some(1)));
            iter.next();
            assert_eq!(iter.size_hint(), (0, Some(0)));
        }

        #[test]
        fn test_vec_list_back() {
            let mut list = VecList::new();
            assert_eq!(list.back(), None);

            let index_1 = list.push_back(0);
            assert_eq!(list.back(), Some(&0));

            let index_2 = list.push_back(1);
            assert_eq!(list.back(), Some(&1));

            list.remove(index_2);
            assert_eq!(list.back(), Some(&0));

            list.remove(index_1);
            assert_eq!(list.back(), None);
        }

        #[test]
        fn test_vec_list_back_mut() {
            let mut list = VecList::new();
            assert_eq!(list.back_mut(), None);

            let index_1 = list.push_back(0);
            assert_eq!(list.back_mut(), Some(&mut 0));

            let index_2 = list.push_back(1);
            assert_eq!(list.back_mut(), Some(&mut 1));

            list.remove(index_2);
            assert_eq!(list.back_mut(), Some(&mut 0));

            list.remove(index_1);
            assert_eq!(list.back_mut(), None);
        }

        #[test]
        fn test_vec_list_capacity() {
            let list: VecList<i32> = VecList::new();
            assert_eq!(list.capacity(), 0);
        }

        #[test]
        fn test_vec_list_clear() {
            let mut list = VecList::new();
            let index = list.push_back(0);
            list.clear();
            assert!(list.is_empty());
            assert_eq!(list.get(index), None);
        }

        #[test]
        fn test_vec_list_contains() {
            let mut list = VecList::new();
            assert!(!list.contains(&0));

            let index = list.push_back(0);
            assert!(list.contains(&0));

            list.remove(index);
            assert!(!list.contains(&0));
        }

        #[test]
        fn test_vec_list_drain() {
            let mut list = VecList::new();
            list.drain();
            assert!(list.is_empty());

            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.drain();
            assert!(list.is_empty());
        }

        #[test]
        fn test_vec_list_debug() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            assert_eq!(format!("{:?}", list), "[0, 1, -1, 2, -2]");
        }

        #[test]
        fn test_vec_list_equality() {
            let mut list_1 = VecList::new();
            list_1.push_back(0);
            list_1.push_back(1);
            list_1.push_back(-1);
            list_1.push_back(2);
            list_1.push_back(-2);

            let mut list_2 = list_1.clone();
            list_2.pop_back();
            assert_ne!(list_1, list_2);

            list_2.push_back(-2);
            assert_eq!(list_1, list_2);
        }

        #[test]
        fn test_vec_list_extend() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.extend([-1, 2, -2].iter());

            assert_eq!(list, &[0, 1, -1, 2, -2][..]);
        }

        #[test]
        fn test_vec_list_from_iterator() {
            let list = VecList::from_iter([0, 1, -1, 2, -2].iter().cloned());
            assert_eq!(list, &[0, 1, -1, 2, -2][..]);
        }

        #[test]
        fn test_vec_list_front() {
            let mut list = VecList::new();
            assert_eq!(list.front(), None);

            let index_1 = list.push_front(0);
            assert_eq!(list.front(), Some(&0));

            let index_2 = list.push_front(1);
            assert_eq!(list.front(), Some(&1));

            list.remove(index_2);
            assert_eq!(list.front(), Some(&0));

            list.remove(index_1);
            assert_eq!(list.front(), None);
        }

        #[test]
        fn test_vec_list_front_mut() {
            let mut list = VecList::new();
            assert_eq!(list.front_mut(), None);

            let index_1 = list.push_front(0);
            assert_eq!(list.front_mut(), Some(&mut 0));

            let index_2 = list.push_front(1);
            assert_eq!(list.front_mut(), Some(&mut 1));

            list.remove(index_2);
            assert_eq!(list.front_mut(), Some(&mut 0));

            list.remove(index_1);
            assert_eq!(list.front_mut(), None);
        }

        #[test]
        fn test_vec_list_get() {
            let mut list = VecList::new();
            let index = list.push_back(0);
            assert_eq!(list.get(index), Some(&0));
            list.remove(index);
            assert_eq!(list.get(index), None);

            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.push_back(1);
            let index_3 = list.push_back(2);

            list.remove(index_1);
            list.pack_to_fit();
            assert_eq!(list.get(index_1), None);
            assert_eq!(list.get(index_2), None);
            assert_eq!(list.get(index_3), None);
        }

        #[test]
        fn test_vec_list_get_mut() {
            let mut list = VecList::new();
            let index = list.push_back(0);
            assert_eq!(list.get_mut(index), Some(&mut 0));
            list.remove(index);
            assert_eq!(list.get_mut(index), None);

            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.push_back(1);
            let index_3 = list.push_back(2);

            list.remove(index_1);
            list.pack_to_fit();
            assert_eq!(list.get_mut(index_1), None);
            assert_eq!(list.get_mut(index_2), None);
            assert_eq!(list.get_mut(index_3), None);
        }

        #[test]
        fn test_vec_list_get_next_index() {
            let mut list = VecList::new();

            let index = list.push_back(0);
            assert_eq!(list.get_next_index(index), None);

            list.push_back(1);
            assert_eq!(list.get_next_index(index).unwrap().index, 1);
        }

        #[test]
        fn test_vec_list_get_previous_index() {
            let mut list = VecList::new();

            let index = list.push_front(0);
            assert_eq!(list.get_previous_index(index), None);

            list.push_front(1);
            assert_eq!(list.get_previous_index(index).unwrap().index, 1);
        }

        #[test]
        fn test_vec_list_index() {
            let mut list = VecList::new();

            let index = list.push_back(5);
            assert_eq!(list[index], 5);

            list[index] = 10;
            assert_eq!(list[index], 10);
        }

        #[should_panic]
        #[test]
        fn test_vec_list_index_panic() {
            let mut list = VecList::new();
            let index = list.push_back(0);
            list.pop_back();
            list[index];
        }

        #[test]
        fn test_vec_list_indices() {
            let mut list = VecList::new();
            let mut iter = list.indices();
            assert_eq!(iter.next(), None);

            list.push_back(0);
            let index = list.push_back(1);
            list.push_back(-1);
            list.remove(index);

            let mut iter = list.indices();
            assert_eq!(iter.next().unwrap().index, 0);
            assert_eq!(iter.next().unwrap().index, 2);
            assert_eq!(iter.next(), None);

            list.pack_to_fit();

            let mut iter = list.indices();
            assert_eq!(iter.next().unwrap().index, 0);
            assert_eq!(iter.next().unwrap().index, 1);
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_vec_list_insert_after() {
            let mut list = VecList::new();
            let index_1 = list.push_front(0);
            let index_2 = list.insert_after(index_1, 1);

            assert_eq!(list.back(), Some(&1));
            assert_eq!(list.get_previous_index(index_2), Some(index_1));
            assert_eq!(list.get_next_index(index_1), Some(index_2));

            let index_3 = list.insert_after(index_1, 2);

            assert_eq!(list.get_previous_index(index_3), Some(index_1));
            assert_eq!(list.get_next_index(index_1), Some(index_3));
            assert_eq!(list.get_next_index(index_3), Some(index_2));
        }

        #[should_panic]
        #[test]
        fn test_vec_list_insert_after_panic_index_invalidated() {
            let mut list = VecList::new();
            let index = list.push_front(0);
            list.remove(index);
            list.insert_after(index, 1);
        }

        #[should_panic]
        #[test]
        fn test_vec_list_insert_after_panic_index_out_of_bounds() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            list.push_back(1);
            let index_2 = list.push_back(2);

            list.remove(index_1);
            list.pack_to_fit();
            list.insert_after(index_2, 3);
        }

        #[test]
        fn test_vec_list_insert_before() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.insert_before(index_1, 1);

            assert_eq!(list.front(), Some(&1));
            assert_eq!(list.get_previous_index(index_1), Some(index_2));
            assert_eq!(list.get_next_index(index_2), Some(index_1));

            let index_3 = list.insert_before(index_1, 2);

            assert_eq!(list.get_previous_index(index_1), Some(index_3));
            assert_eq!(list.get_next_index(index_3), Some(index_1));
            assert_eq!(list.get_next_index(index_2), Some(index_3));
        }

        #[should_panic]
        #[test]
        fn test_vec_list_insert_before_panic_index_invalidated() {
            let mut list = VecList::new();
            let index = list.push_front(0);
            list.remove(index);
            list.insert_before(index, 1);
        }

        #[should_panic]
        #[test]
        fn test_vec_list_insert_before_panic_index_out_of_bounds() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            list.push_back(1);
            let index_2 = list.push_back(2);

            list.remove(index_1);
            list.pack_to_fit();
            list.insert_before(index_2, 3);
        }

        #[test]
        fn test_vec_list_into_iterator() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            assert_eq!(list.into_iter().collect::<Vec<_>>(), [0, 1, -1, 2, -2]);
        }

        #[test]
        fn test_vec_list_is_empty() {
            let mut list = VecList::new();
            assert!(list.is_empty());
            list.push_back(0);
            assert!(!list.is_empty());
        }

        #[test]
        fn test_vec_list_iter() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(2);

            let mut iter = list.iter();
            assert_eq!(iter.next(), Some(&0));
            assert_eq!(iter.next(), Some(&1));
            assert_eq!(iter.next(), Some(&2));
            assert_eq!(iter.next(), None);
        }

        #[test]
        fn test_vec_list_iter_mut() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(2);

            let mut iter = list.iter_mut();
            let value = iter.next().unwrap();
            *value = 100;

            assert_eq!(iter.next(), Some(&mut 1));
            assert_eq!(iter.next(), Some(&mut 2));
            assert_eq!(iter.next(), None);
            assert_eq!(list.front(), Some(&100));
        }

        #[test]
        fn test_vec_list_len() {
            let mut list = VecList::new();
            assert_eq!(list.len(), 0);
            let index = list.push_back(0);
            assert_eq!(list.len(), 1);
            list.remove(index);
            assert_eq!(list.len(), 0);
        }

        #[test]
        fn test_vec_list_new() {
            let list: VecList<i32> = VecList::new();
            assert_eq!(list.capacity(), 0);
            assert_eq!(list.len(), 0);
        }

        #[test]
        fn test_vec_list_ordering() {
            let mut list_1 = VecList::new();
            list_1.push_back(0);
            list_1.push_back(1);
            list_1.push_back(-1);
            list_1.push_back(2);
            list_1.push_back(-2);

            let mut list_2 = list_1.clone();

            list_2.push_back(5);
            assert!(list_1 < list_2);

            list_2.pop_back();
            list_2.pop_back();
            assert!(list_1 > list_2);

            list_2.push_back(3);
            assert!(list_1 < list_2);

            list_2.pop_back();
            list_2.push_back(-3);
            assert!(list_1 > list_2);
        }

        #[test]
        fn test_vec_list_pop_back() {
            let mut list = VecList::new();
            assert_eq!(list.pop_back(), None);

            list.push_back(0);
            assert_eq!(list.pop_back(), Some(0));
        }

        #[test]
        fn test_vec_list_pop_front() {
            let mut list = VecList::new();
            assert_eq!(list.pop_front(), None);

            list.push_front(0);
            assert_eq!(list.pop_front(), Some(0));
        }

        #[test]
        fn test_vec_list_push_back() {
            let mut list = VecList::new();
            list.push_back(0);
            assert_eq!(list.back(), Some(&0));
            list.push_back(1);
            assert_eq!(list.back(), Some(&1));
            list.push_back(2);
            assert_eq!(list.back(), Some(&2));
        }

        #[test]
        fn test_vec_list_push_front() {
            let mut list = VecList::new();
            list.push_front(0);
            assert_eq!(list.front(), Some(&0));
            list.push_front(1);
            assert_eq!(list.front(), Some(&1));
            list.push_front(2);
            assert_eq!(list.front(), Some(&2));
        }

        #[test]
        fn test_vec_list_remove() {
            let mut list = VecList::new();
            let index = list.push_back(0);
            assert_eq!(list.remove(index), Some(0));
            assert_eq!(list.remove(index), None);
        }

        #[test]
        fn test_vec_list_reserve() {
            let mut list: VecList<i32> = VecList::new();
            assert_eq!(list.capacity(), 0);

            list.reserve(10);
            let capacity = list.capacity();

            assert!(capacity >= 10);
            list.reserve(5);

            assert_eq!(list.capacity(), capacity);
        }

        #[test]
        fn test_vec_list_retain() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(-1);
            list.push_back(2);
            list.push_back(-2);

            list.retain(|&mut value| value >= 0);
            assert_eq!(list.into_iter().collect::<Vec<_>>(), [0, 1, 2]);
        }

        #[test]
        fn test_vec_list_pack_to() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.push_back(1);
            let index_3 = list.push_back(2);
            assert!(list.capacity() >= 3);

            list.remove(index_1);
            assert!(list.capacity() >= 3);

            let indices = list.indices();
            assert_eq!(indices.map(|index| index.index).collect::<Vec<_>>(), [1, 2]);

            let map = list.pack_to(5);
            assert_eq!(list.capacity(), 5);

            let indices = list.indices();
            assert_eq!(indices.map(|index| index.index).collect::<Vec<_>>(), [0, 1]);

            assert_eq!(map.len(), 2);
            assert_eq!(map.get(&index_2).unwrap().index, 0);
            assert_eq!(map.get(&index_3).unwrap().index, 1);
        }

        #[test]
        fn test_vec_list_pack_to_empty() {
            let mut list: VecList<i32> = VecList::with_capacity(5);
            list.pack_to(0);
            assert_eq!(list.capacity(), 0);
        }

        #[should_panic]
        #[test]
        fn test_vec_list_pack_to_panic() {
            let mut list = VecList::new();
            list.push_back(0);
            list.push_back(1);
            list.push_back(2);
            list.pack_to(2);
        }

        #[test]
        fn test_vec_list_pack_to_fit() {
            let mut list = VecList::new();
            let index_1 = list.push_back(0);
            let index_2 = list.push_back(1);
            let index_3 = list.push_back(2);
            assert!(list.capacity() >= 3);

            list.remove(index_1);
            assert!(list.capacity() >= 3);

            let indices = list.indices();
            assert_eq!(indices.map(|index| index.index).collect::<Vec<_>>(), [1, 2]);

            let map = list.pack_to_fit();
            assert_eq!(list.capacity(), 2);

            let indices = list.indices();
            assert_eq!(indices.map(|index| index.index).collect::<Vec<_>>(), [0, 1]);

            assert_eq!(map.len(), 2);
            assert_eq!(map.get(&index_2).unwrap().index, 0);
            assert_eq!(map.get(&index_3).unwrap().index, 1);
        }

        #[test]
        fn test_vec_list_with_capacity() {
            let list: VecList<i32> = VecList::with_capacity(10);
            assert_eq!(list.capacity(), 10);
        }
    }