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    //! Useful traits for manipulating sequences of data stored in `GenericArray`s

    use super::*;
    use core::{mem, ptr};
    use core::ops::{Add, Sub};
    use typenum::operator_aliases::*;

    /// Defines some sequence with an associated length and iteration capabilities.
    ///
    /// This is useful for passing N-length generic arrays as generics.
    pub unsafe trait GenericSequence<T>: Sized + IntoIterator {
        /// `GenericArray` associated length
        type Length: ArrayLength<T>;

        /// Concrete sequence type used in conjuction with reference implementations of `GenericSequence`
        type Sequence: GenericSequence<T, Length = Self::Length> + FromIterator<T>;

        /// Initializes a new sequence instance using the given function.
        ///
        /// If the generator function panics while initializing the sequence,
        /// any already initialized elements will be dropped.
        fn generate<F>(f: F) -> Self::Sequence
        where
            F: FnMut(usize) -> T;

        #[doc(hidden)]
        fn inverted_zip<B, U, F>(
            self,
            lhs: GenericArray<B, Self::Length>,
            mut f: F,
        ) -> MappedSequence<GenericArray<B, Self::Length>, B, U>
        where
            GenericArray<B, Self::Length>: GenericSequence<B, Length = Self::Length>
                + MappedGenericSequence<B, U>,
            Self: MappedGenericSequence<T, U>,
            Self::Length: ArrayLength<B> + ArrayLength<U>,
            F: FnMut(B, Self::Item) -> U,
        {
            unsafe {
                let mut left = ArrayConsumer::new(lhs);

                let (left_array_iter, left_position) = left.iter_position();

                FromIterator::from_iter(
                    left_array_iter
                        .zip(self.into_iter())
                        .map(|(l, right_value)| {
                            let left_value = ptr::read(l);

                            *left_position += 1;

                            f(left_value, right_value)
                        })
                )
            }
        }

        #[doc(hidden)]
        fn inverted_zip2<B, Lhs, U, F>(self, lhs: Lhs, mut f: F) -> MappedSequence<Lhs, B, U>
        where
            Lhs: GenericSequence<B, Length = Self::Length> + MappedGenericSequence<B, U>,
            Self: MappedGenericSequence<T, U>,
            Self::Length: ArrayLength<B> + ArrayLength<U>,
            F: FnMut(Lhs::Item, Self::Item) -> U,
        {
            FromIterator::from_iter(lhs.into_iter().zip(self.into_iter()).map(|(l, r)| f(l, r)))
        }
    }

    /// Accessor for `GenericSequence` item type, which is really `IntoIterator::Item`
    ///
    /// For deeply nested generic mapped sequence types, like shown in `tests/generics.rs`,
    /// this can be useful for keeping things organized.
    pub type SequenceItem<T> = <T as IntoIterator>::Item;

    unsafe impl<'a, T: 'a, S: GenericSequence<T>> GenericSequence<T> for &'a S
    where
        &'a S: IntoIterator,
    {
        type Length = S::Length;
        type Sequence = S::Sequence;

        #[inline]
        fn generate<F>(f: F) -> Self::Sequence
        where
            F: FnMut(usize) -> T,
        {
            S::generate(f)
        }
    }

    unsafe impl<'a, T: 'a, S: GenericSequence<T>> GenericSequence<T> for &'a mut S
    where
        &'a mut S: IntoIterator,
    {
        type Length = S::Length;
        type Sequence = S::Sequence;

        #[inline]
        fn generate<F>(f: F) -> Self::Sequence
        where
            F: FnMut(usize) -> T,
        {
            S::generate(f)
        }
    }

    /// Defines any `GenericSequence` which can be lengthened or extended by appending
    /// or prepending an element to it.
    ///
    /// Any lengthened sequence can be shortened back to the original using `pop_front` or `pop_back`
    pub unsafe trait Lengthen<T>: Sized + GenericSequence<T> {
        /// `GenericSequence` that has one more element than `Self`
        type Longer: Shorten<T, Shorter = Self>;

        /// Returns a new array with the given element appended to the end of it.
        ///
        /// Example:
        ///
        /// ```ignore
        /// let a = arr![i32; 1, 2, 3];
        ///
        /// let b = a.append(4);
        ///
        /// assert_eq!(b, arr![i32; 1, 2, 3, 4]);
        /// ```
        fn append(self, last: T) -> Self::Longer;

        /// Returns a new array with the given element prepended to the front of it.
        ///
        /// Example:
        ///
        /// ```ignore
        /// let a = arr![i32; 1, 2, 3];
        ///
        /// let b = a.prepend(4);
        ///
        /// assert_eq!(b, arr![i32; 4, 1, 2, 3]);
        /// ```
        fn prepend(self, first: T) -> Self::Longer;
    }

    /// Defines a `GenericSequence` which can be shortened by removing the first or last element from it.
    ///
    /// Additionally, any shortened sequence can be lengthened by
    /// appending or prepending an element to it.
    pub unsafe trait Shorten<T>: Sized + GenericSequence<T> {
        /// `GenericSequence` that has one less element than `Self`
        type Shorter: Lengthen<T, Longer = Self>;

        /// Returns a new array without the last element, and the last element.
        ///
        /// Example:
        ///
        /// ```ignore
        /// let a = arr![i32; 1, 2, 3, 4];
        ///
        /// let (init, last) = a.pop_back();
        ///
        /// assert_eq!(init, arr![i32; 1, 2, 3]);
        /// assert_eq!(last, 4);
        /// ```
        fn pop_back(self) -> (Self::Shorter, T);

        /// Returns a new array without the first element, and the first element.
        /// Example:
        ///
        /// ```ignore
        /// let a = arr![i32; 1, 2, 3, 4];
        ///
        /// let (head, tail) = a.pop_front();
        ///
        /// assert_eq!(head, 1);
        /// assert_eq!(tail, arr![i32; 2, 3, 4]);
        /// ```
        fn pop_front(self) -> (T, Self::Shorter);
    }

    unsafe impl<T, N: ArrayLength<T>> Lengthen<T> for GenericArray<T, N>
    where
        N: Add<B1>,
        Add1<N>: ArrayLength<T>,
        Add1<N>: Sub<B1, Output = N>,
        Sub1<Add1<N>>: ArrayLength<T>,
    {
        type Longer = GenericArray<T, Add1<N>>;

        fn append(self, last: T) -> Self::Longer {
            let mut longer: Self::Longer = unsafe { mem::uninitialized() };

            unsafe {
                ptr::write(longer.as_mut_ptr() as *mut _, self);
                ptr::write(&mut longer[N::to_usize()], last);
            }

            longer
        }

        fn prepend(self, first: T) -> Self::Longer {
            let mut longer: Self::Longer = unsafe { mem::uninitialized() };

            let longer_ptr = longer.as_mut_ptr();

            unsafe {
                ptr::write(longer_ptr as *mut _, first);
                ptr::write(longer_ptr.offset(1) as *mut _, self);
            }

            longer
        }
    }

    unsafe impl<T, N: ArrayLength<T>> Shorten<T> for GenericArray<T, N>
    where
        N: Sub<B1>,
        Sub1<N>: ArrayLength<T>,
        Sub1<N>: Add<B1, Output = N>,
        Add1<Sub1<N>>: ArrayLength<T>,
    {
        type Shorter = GenericArray<T, Sub1<N>>;

        fn pop_back(self) -> (Self::Shorter, T) {
            let init_ptr = self.as_ptr();
            let last_ptr = unsafe { init_ptr.offset(Sub1::<N>::to_usize() as isize) };

            let init = unsafe { ptr::read(init_ptr as _) };
            let last = unsafe { ptr::read(last_ptr as _) };

            mem::forget(self);

            (init, last)
        }

        fn pop_front(self) -> (T, Self::Shorter) {
            let head_ptr = self.as_ptr();
            let tail_ptr = unsafe { head_ptr.offset(1) };

            let head = unsafe { ptr::read(head_ptr as _) };
            let tail = unsafe { ptr::read(tail_ptr as _) };

            mem::forget(self);

            (head, tail)
        }
    }

    /// Defines a `GenericSequence` that can be split into two parts at a given pivot index.
    pub unsafe trait Split<T, K>: GenericSequence<T>
    where
        K: ArrayLength<T>,
    {
        /// First part of the resulting split array
        type First: GenericSequence<T>;
        /// Second part of the resulting split array
        type Second: GenericSequence<T>;

        /// Splits an array at the given index, returning the separate parts of the array.
        fn split(self) -> (Self::First, Self::Second);
    }

    unsafe impl<T, N, K> Split<T, K> for GenericArray<T, N>
    where
        N: ArrayLength<T>,
        K: ArrayLength<T>,
        N: Sub<K>,
        Diff<N, K>: ArrayLength<T>,
    {
        type First = GenericArray<T, K>;
        type Second = GenericArray<T, Diff<N, K>>;

        fn split(self) -> (Self::First, Self::Second) {
            let head_ptr = self.as_ptr();
            let tail_ptr = unsafe { head_ptr.offset(K::to_usize() as isize) };

            let head = unsafe { ptr::read(head_ptr as _) };
            let tail = unsafe { ptr::read(tail_ptr as _) };

            mem::forget(self);

            (head, tail)
        }
    }

    /// Defines `GenericSequence`s which can be joined together, forming a larger array.
    pub unsafe trait Concat<T, M>: GenericSequence<T>
    where
        M: ArrayLength<T>,
    {
        /// Sequence to be concatenated with `self`
        type Rest: GenericSequence<T, Length = M>;

        /// Resulting sequence formed by the concatenation.
        type Output: GenericSequence<T>;

        /// Concatenate, or join, two sequences.
        fn concat(self, rest: Self::Rest) -> Self::Output;
    }

    unsafe impl<T, N, M> Concat<T, M> for GenericArray<T, N>
    where
        N: ArrayLength<T> + Add<M>,
        M: ArrayLength<T>,
        Sum<N, M>: ArrayLength<T>,
    {
        type Rest = GenericArray<T, M>;
        type Output = GenericArray<T, Sum<N, M>>;

        fn concat(self, rest: Self::Rest) -> Self::Output {
            let mut output: Self::Output = unsafe { mem::uninitialized() };

            let output_ptr = output.as_mut_ptr();

            unsafe {
                ptr::write(output_ptr as *mut _, self);
                ptr::write(output_ptr.offset(N::to_usize() as isize) as *mut _, rest);
            }

            output
        }
    }

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