//! Extra functions for slices

pub mod unalign;
pub mod iter;

use {get_unchecked, get_unchecked_mut};
use IndexRange;
use {slice_unchecked, slice_unchecked_mut};

use std::ptr;
use std::cmp::min;
use std::mem::transmute;
use std::mem::{self, align_of, size_of};
use std::hash::{Hasher, Hash};
use std::slice::from_raw_parts;
use std::iter::Rev;
use std::slice::{Iter, IterMut};

use std::ops::{Index, IndexMut};

/// Unaligned load of a u64 at index `i` in `buf`
unsafe fn load_u64(buf: &[u8], i: usize) -> u64 {
    debug_assert!(i + 8 <= buf.len());
    let mut data = 0u64;
    ptr::copy_nonoverlapping(buf.get_unchecked(i), &mut data as *mut _ as *mut u8, 8);
    data
}

/// Return the end index of the longest shared (equal) prefix of `a` and `b`.
pub fn shared_prefix(a: &[u8], b: &[u8]) -> usize {
    let len = min(a.len(), b.len());
    let mut a = &a[..len];
    let mut b = &b[..len];
    let mut offset = 0;
    while a.len() >= 16 {
        unsafe {
            let a0 = load_u64(a, 0);
            let a1 = load_u64(a, 8);
            let b0 = load_u64(b, 0);
            let b1 = load_u64(b, 8);
            let d0 = a0 ^ b0;
            let d1 = a1 ^ b1;
            if d0 ^ d1 != 0 {
                break;
            }
        }
        offset += 16;
        a = &a[16..];
        b = &b[16..];
    }
    for i in 0..a.len() {
        if a[i] != b[i] {
            return offset + i;
        }
    }
    len
}

/// Rotate `steps` towards lower indices.
///
/// The steps to rotate is computed modulo the length of `data`,
/// so any step value is acceptable. This function does not panic.
///
/// ```
/// use odds::slice::rotate_left;
///
/// let mut data = [1, 2, 3, 4];
/// rotate_left(&mut data, 1);
/// assert_eq!(&data, &[2, 3, 4, 1]);
/// rotate_left(&mut data, 2);
/// assert_eq!(&data, &[4, 1, 2, 3]);
/// ```
pub fn rotate_left<T>(data: &mut [T], steps: usize) {
    //return rotate_alt(data, steps);
    // no bounds checks in this method in this version
    if data.len() == 0 {
        return;
    }
    let steps = steps % data.len();

    data[..steps].reverse();
    data[steps..].reverse();
    data.reverse();
}

#[test]
fn correct() {
    let mut a = [0xff; 1024];
    let b = [0xff; 1024];
    for byte in 0..255 { // don't test byte 255
        for i in 0..a.len() {
            a[i] = byte;
            let ans = shared_prefix(&a, &b);
            assert!(ans == i, "failed for index {} and byte {:x} (got ans={})",
                    i, byte, ans);
            a[i] = 0xff;
        }
    }
}

/// Element-finding methods for slices
pub trait SliceFind {
    type Item;
    /// Linear search for the first occurrence  `elt` in the slice.
    ///
    /// Return its index if it is found, or None.
    fn find<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>;

    /// Linear search for the last occurrence  `elt` in the slice.
    ///
    /// Return its index if it is found, or None.
    fn rfind<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>;
}

macro_rules! foreach {
    ($i:pat in $($e:expr),* => $b:expr) => {{
        $(
            let $i = $e;
            $b;
        )*
    }}
}

impl<T> SliceFind for [T] { 
    type Item = T;
    fn find<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>
    {
        let mut xs = self;
        let mut i = 0;
        const C: usize = 8;
        while xs.len() >= C {
            foreach!(j in 0, 1, 2, 3, 4, 5, 6, 7 => {
                if xs[j] == *elt { return Some(i + j); }
            });
            i += C;
            xs = &xs[C..];
        }
        for j in 0..xs.len() {
            if xs[j] == *elt {
                return Some(i + j);
            }
        }
        None
    }

    fn rfind<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>
    {
        let mut xs = self;
        const C: usize = 8;
        while xs.len() >= C {
            let l = xs.len();
            foreach!(j in 0, 1, 2, 3, 4, 5, 6, 7 => {
                if xs[l - 1 - j] == *elt { return Some(l - 1 - j); }
            });
            xs = &xs[..l - C];
        }
        xs.iter().rposition(|x| *x == *elt)
    }
}

impl<T> SliceFind for RevSlice<T> {
    type Item = T;
    fn find<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>
    {
        self.0.rfind(elt).map(move |i| self.raw_index_no_wrap(i))
    }

    fn rfind<U: ?Sized>(&self, elt: &U) -> Option<usize>
        where Self::Item: PartialEq<U>
    {
        self.0.find(elt).map(move |i| self.raw_index_no_wrap(i))
    }
}


/// Element-finding methods for slices
pub trait SliceFindSplit {
    type Item;
    /// Linear search for the first occurrence  `elt` in the slice.
    ///
    /// Return the part before and the part including and after the element.
    /// If the element is not found, the second half is empty.
    fn find_split<U: ?Sized>(&self, elt: &U) -> (&Self, &Self)
        where Self::Item: PartialEq<U>;

    /// Linear search for the last occurrence  `elt` in the slice.
    ///
    /// Return the part before and the part including and after the element.
    /// If the element is not found, the first half is empty.
    fn rfind_split<U: ?Sized>(&self, elt: &U) -> (&Self, &Self)
        where Self::Item: PartialEq<U>;

    /// Linear search for the first occurrence  `elt` in the slice.
    ///
    /// Return the part before and the part including and after the element.
    /// If the element is not found, the second half is empty.
    fn find_split_mut<U: ?Sized>(&mut self, elt: &U) -> (&mut Self, &mut Self)
        where Self::Item: PartialEq<U>;

    /// Linear search for the last occurrence  `elt` in the slice.
    ///
    /// Return the part before and the part including and after the element.
    /// If the element is not found, the first half is empty.
    fn rfind_split_mut<U: ?Sized>(&mut self, elt: &U) -> (&mut Self, &mut Self)
        where Self::Item: PartialEq<U>;
}


/// Unchecked version of `xs.split_at(i)`.
unsafe fn split_at_unchecked<T>(xs: &[T], i: usize) -> (&[T], &[T]) {
    (slice_unchecked(xs, 0, i),
     slice_unchecked(xs, i, xs.len()))
}

impl<T> SliceFindSplit for [T] { 
    type Item = T;
    fn find_split<U: ?Sized>(&self, elt: &U) -> (&Self, &Self)
        where Self::Item: PartialEq<U>
    {
        let i = self.find(elt).unwrap_or(self.len());
        unsafe {
            split_at_unchecked(self, i)
        }
    }

    fn find_split_mut<U: ?Sized>(&mut self, elt: &U) -> (&mut Self, &mut Self)
        where Self::Item: PartialEq<U>
    {
        let i = self.find(elt).unwrap_or(self.len());
        self.split_at_mut(i)
    }

    fn rfind_split<U: ?Sized>(&self, elt: &U) -> (&Self, &Self)
        where Self::Item: PartialEq<U>
    {
        let i = self.rfind(elt).unwrap_or(0);
        unsafe {
            split_at_unchecked(self, i)
        }
    }

    fn rfind_split_mut<U: ?Sized>(&mut self, elt: &U) -> (&mut Self, &mut Self)
        where Self::Item: PartialEq<U>
    {
        let i = self.rfind(elt).unwrap_or(0);
        self.split_at_mut(i)
    }
}


/// Extra iterator adaptors for iterators of slice elements.
pub trait SliceIterExt : Iterator {
    /// Return an iterator adaptor that joins together adjacent slices if possible.
    ///
    /// Only implemented for iterators with slice or string slice elements.
    /// Only slices that are contiguous together can be joined.
    ///
    /// ```
    /// use odds::slice::SliceIterExt;
    ///
    /// // Split a string into a slice per letter, filter out whitespace,
    /// // and join into words again by mending adjacent slices.
    /// let text = String::from("Warning:  γ-radiation (ionizing)");
    /// let char_slices = text.char_indices()
    ///                       .map(|(index, ch)| &text[index..index + ch.len_utf8()]);
    /// let words = char_slices.filter(|s| !s.chars().any(char::is_whitespace))
    ///                        .mend_slices();
    ///
    /// assert!(words.eq(vec!["Warning:", "γ-radiation", "(ionizing)"]));
    /// ```
    fn mend_slices(self) -> MendSlices<Self>
        where Self: Sized,
              Self::Item: MendSlice
    {
        MendSlices::new(self)
    }
}

impl<I: ?Sized> SliceIterExt for I where I: Iterator { }

/// An iterator adaptor that glues together adjacent contiguous slices.
///
/// See [`.mend_slices()`](../trait.Itertools.html#method.mend_slices) for more information.
pub struct MendSlices<I>
    where I: Iterator
{
    last: Option<I::Item>,
    iter: I,
}

impl<I: Clone> Clone for MendSlices<I>
    where I: Iterator,
          I::Item: Clone
{
    fn clone(&self) -> Self {
        MendSlices {
            last: self.last.clone(),
            iter: self.iter.clone(),
        }
    }
}

impl<I> MendSlices<I>
    where I: Iterator
{
    /// Create a new `MendSlices`.
    pub fn new(mut iter: I) -> Self {
        MendSlices {
            last: iter.next(),
            iter: iter,
        }
    }
}

impl<I> Iterator for MendSlices<I>
    where I: Iterator,
          I::Item: MendSlice
{
    type Item = I::Item;

    fn next(&mut self) -> Option<I::Item> {
        // this fuses the iterator
        let mut last = match self.last.take() {
            None => return None,
            Some(x) => x,
        };
        for next in &mut self.iter {
            match MendSlice::mend(last, next) {
                Ok(joined) => last = joined,
                Err((last_, next_)) => {
                    self.last = Some(next_);
                    return Some(last_);
                }
            }
        }

        Some(last)
    }

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

/// A trait for items that can *maybe* be joined together.
pub trait MendSlice
{
    #[doc(hidden)]
    /// If the slices are contiguous, return them joined into one.
    fn mend(Self, Self) -> Result<Self, (Self, Self)>
        where Self: Sized;
}

impl<'a, T> MendSlice for &'a [T] {
    #[inline]
    fn mend(a: Self, b: Self) -> Result<Self, (Self, Self)> {
        unsafe {
            let a_end = a.as_ptr().offset(a.len() as isize);
            if a_end == b.as_ptr() {
                Ok(::std::slice::from_raw_parts(a.as_ptr(), a.len() + b.len()))
            } else {
                Err((a, b))
            }
        }
    }
}

impl<'a, T> MendSlice for &'a mut [T] {
    #[inline]
    fn mend(a: Self, b: Self) -> Result<Self, (Self, Self)> {
        unsafe {
            let a_end = a.as_ptr().offset(a.len() as isize);
            if a_end == b.as_ptr() {
                Ok(::std::slice::from_raw_parts_mut(a.as_mut_ptr(), a.len() + b.len()))
            } else {
                Err((a, b))
            }
        }
    }
}

impl<'a> MendSlice for &'a str {
    #[inline]
    fn mend(a: Self, b: Self) -> Result<Self, (Self, Self)> {
        unsafe { mem::transmute(MendSlice::mend(a.as_bytes(), b.as_bytes())) }
    }
}


/// "plain old data": Types that we can stick arbitrary bit patterns into,
/// and thus use them as blocks in `split_aligned_for` or in `UnalignedIter`.
pub unsafe trait Pod : Copy { }
macro_rules! impl_pod {
    (@array $($e:expr),+) => {
        $(
        unsafe impl<T> Pod for [T; $e] where T: Pod { }
        )+
    };
    ($($t:ty)+) => {
        $(
        unsafe impl Pod for $t { }
        )+
    };
}
impl_pod!{u8 u16 u32 u64 usize i8 i16 i32 i64 isize}
impl_pod!{@array 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}


/// Split the input slice into three chunks,
/// so that the middle chunk is a slice of a larger "block size"
/// (for example T could be u64) that is correctly aligned for `T`.
///
/// The first and last returned slices are the remaining head and tail
/// parts that could not be baked into `&[T]`
///
/// # Examples
///
/// ```
/// extern crate odds;
/// use odds::slice::split_aligned_for;
///
/// fn count_ones(data: &[u8]) -> u32 {
///     let mut total = 0;
///     let (head, mid, tail) = split_aligned_for::<[u64; 2]>(data);
///     total += head.iter().map(|x| x.count_ones()).sum();
///     total += mid.iter().map(|x| x[0].count_ones() + x[1].count_ones()).sum();
///     total += tail.iter().map(|x| x.count_ones()).sum();
///     total
/// }
///
/// fn main() {
///     assert_eq!(count_ones(&vec![3u8; 127]), 127 * 2);
/// }
/// ```
pub fn split_aligned_for<T: Pod>(data: &[u8]) -> (&[u8], &[T], &[u8]) {
    let ptr = data.as_ptr();
    let align_t = align_of::<T>();
    let size_t = size_of::<T>();
    let align_ptr = ptr as usize & (align_t - 1);
    let prefix = if align_ptr == 0 { 0 } else { align_t - align_ptr };
    let t_len;

    if prefix > data.len() {
        t_len = 0;
    } else {
        t_len = (data.len() - prefix) / size_t;
    }
    unsafe {
        (from_raw_parts(ptr, prefix),
         from_raw_parts(ptr.offset(prefix as isize) as *const T, t_len),
         from_raw_parts(ptr.offset((prefix + t_len * size_t) as isize),
                        data.len() - t_len * size_t - prefix))
    }
}

#[test]
fn test_split_aligned() {
    let data = vec![0; 1024];
    assert_eq!(data.as_ptr() as usize & 7, 0);
    let (a, b, c) = split_aligned_for::<u8>(&data);
    assert_eq!(a.len(), 0);
    assert_eq!(b.len(), data.len());
    assert_eq!(c.len(), 0);

    let (a, b, c) = split_aligned_for::<u64>(&data);
    assert_eq!(a.len(), 0);
    assert_eq!(b.len(), data.len() / 8);
    assert_eq!(c.len(), 0);

    let offset1 = &data[1..data.len() - 2];
    let (a, b, c) = split_aligned_for::<u64>(offset1);
    assert_eq!(a.len(), 7);
    assert_eq!(b.len(), data.len() / 8 - 2);
    assert_eq!(c.len(), 6);

    let data = [0; 7];
    let (a, b, c) = split_aligned_for::<u64>(&data);
    assert_eq!(a.len() + c.len(), 7);
    assert_eq!(b.len(), 0);
}


/* All of these use this trick:
 *
    for i in 0..4 {
        if i < data.len() {
            f(&data[i]);
        }
    }
 * The intention is that the range makes sure the compiler
 * sees that the loop is not autovectorized or something that generates
 * a lot of code in vain that does not pay off when it's only 3 elements or less.
 */

#[cfg(test)]
pub fn unroll_2<'a, T, F>(data: &'a [T], mut f: F)
    where F: FnMut(&'a T)
{
    let mut data = data;
    while data.len() >= 2 {
        f(&data[0]);
        f(&data[1]);
        data = &data[2..];
    }
    // tail
    if 0 < data.len() {
        f(&data[0]);
    }
}
#[cfg(test)]
pub fn unroll_4<'a, T, F>(data: &'a [T], mut f: F)
    where F: FnMut(&'a T)
{
    let mut data = data;
    while data.len() >= 4 {
        f(&data[0]);
        f(&data[1]);
        f(&data[2]);
        f(&data[3]);
        data = &data[4..];
    }
    // tail
    for i in 0..3 {
        if i < data.len() {
            f(&data[i]);
        }
    }
}

#[cfg(test)]
pub fn unroll_8<'a, T, F>(data: &'a [T], mut f: F)
    where F: FnMut(&'a T)
{
    let mut data = data;
    while data.len() >= 8 {
        f(&data[0]);
        f(&data[1]);
        f(&data[2]);
        f(&data[3]);
        f(&data[4]);
        f(&data[5]);
        f(&data[6]);
        f(&data[7]);
        data = &data[8..];
    }
    // tail
    for i in 0..7 {
        if i < data.len() {
            f(&data[i]);
        }
    }
}

#[cfg(test)]
pub fn zip_unroll_4<'a, 'b, A, B, F>(a: &'a [A], b: &'b [B], mut f: F)
    where F: FnMut(usize, &'a A, &'b B)
{
    let len = min(a.len(), b.len());
    let mut a = &a[..len];
    let mut b = &b[..len];
    while a.len() >= 4 {
        f(0, &a[0], &b[0]);
        f(1, &a[1], &b[1]);
        f(2, &a[2], &b[2]);
        f(3, &a[3], &b[3]);
        a = &a[4..];
        b = &b[4..];
    }
    // tail
    for i in 0..3 {
        if i < a.len() {
            f(0, &a[i], &b[i]);
        }
    }
}

#[cfg(test)]
pub fn zip_unroll_8<'a, 'b, A, B, F>(a: &'a [A], b: &'b [B], mut f: F)
    where F: FnMut(usize, &'a A, &'b B)
{
    let len = min(a.len(), b.len());
    let mut a = &a[..len];
    let mut b = &b[..len];
    while a.len() >= 8 {
        f(0, &a[0], &b[0]);
        f(1, &a[1], &b[1]);
        f(2, &a[2], &b[2]);
        f(3, &a[3], &b[3]);
        f(4, &a[4], &b[4]);
        f(5, &a[5], &b[5]);
        f(6, &a[6], &b[6]);
        f(7, &a[7], &b[7]);
        a = &a[8..];
        b = &b[8..];
    }

    // tail
    for i in 0..7 {
        if i < a.len() {
            f(0, &a[i], &b[i]);
        }
    }
}

#[cfg(test)]
pub fn f64_dot(xs: &[f64], ys: &[f64]) -> f64 {
    let mut sum = [0.; 8];
    zip_unroll_8(xs, ys, |i, x, y| sum[i] += x * y);
    sum[0] += sum[4];
    sum[1] += sum[5];
    sum[2] += sum[6];
    sum[3] += sum[7];
    sum[0] += sum[2];
    sum[1] += sum[3];
    sum[0] + sum[1]
}

#[test]
fn test_find() {
    let v = [0, 1, 7, 0, 0, 2, 3, 5, 1, 5, 3, 1, 2, 1];
    assert_eq!(v.find_split(&7), v.split_at(2));
    assert_eq!(v.rfind_split(&7), v.split_at(2));
    assert_eq!(v.rfind_split(&2), v.split_at(v.len() - 2));
}

/// A reversed view of a slice.
///
/// The `RevSlice` is a random accessible range of elements;
/// it wraps a regular slice but presents the underlying elements in
/// reverse order.
///
/// # Example
/// ```
/// use odds::slice::RevSlice;
///
/// let mut data = [0; 8];
///
/// {
///     let mut rev = <&mut RevSlice<_>>::from(&mut data);
///     for (i, elt) in rev.iter_mut().enumerate() {
///         *elt = i;
///     }
///
///     assert_eq!(&rev[..4], &[0, 1, 2, 3][..]);
/// }
/// assert_eq!(&data, &[7, 6, 5, 4, 3, 2, 1, 0]);
/// ```
///
/// Not visible in rustdoc:
///
/// - A boxed slice can be reversed too:
///   `impl<T> From<Box<[T]>> for Box<RevSlice<T>>`.
#[derive(Debug, Eq)]
#[repr(C)]
pub struct RevSlice<T>([T]);

impl<T> RevSlice<T> {
    /// Return the length of the slice.
    pub fn len(&self) -> usize {
        self.0.len()
    }

    // arithmetic overflow checked in debug builds
    #[inline]
    fn raw_index_no_wrap(&self, i: usize) -> usize {
        self.len() - (1 + i)
    }

    /// Return the index into the underlying slice, if it's in bounds
    fn raw_index(&self, i: usize) -> Option<usize> {
        if i < self.len() {
            Some(self.raw_index_no_wrap(i))
        } else {
            None
        }
    }

    /// Get element at index `i`.
    ///
    /// See also indexing notation: `&foo[i]`.
    pub fn get(&self, i: usize) -> Option<&T> {
        unsafe {
            self.raw_index(i).map(move |ri| get_unchecked(&self.0, ri))
        }
    }

    /// Get element at index `i`.
    ///
    /// See also indexing notation: `&mut foo[i]`.
    pub fn get_mut(&mut self, i: usize) -> Option<&mut T> {
        unsafe {
            self.raw_index(i).map(move |ri| get_unchecked_mut(&mut self.0, ri))
        }
    }

    pub fn inner_ref(&self) -> &[T] {
        &self.0
    }

    pub fn inner_mut(&mut self) -> &mut [T] {
        &mut self.0
    }

    #[cfg(feature = "std")]
    pub fn into_boxed_slice(self: Box<Self>) -> Box<[T]> {
        unsafe {
            transmute(self)
        }
    }

    /// Return a by-reference iterator
    pub fn iter(&self) -> Rev<Iter<T>> {
        self.into_iter()
    }

    /// Return a by-mutable-reference iterator
    pub fn iter_mut(&mut self) -> Rev<IterMut<T>> {
        self.into_iter()
    }

    pub fn split_at(&self, i: usize) -> (&Self, &Self) {
        assert!(i <= self.len());
        let ri = self.len() - i;
        let (a, b) = self.0.split_at(ri);
        (<_>::from(b), <_>::from(a))
    }

    pub fn split_at_mut(&mut self, i: usize) -> (&mut Self, &mut Self) {
        assert!(i <= self.len());
        let ri = self.len() - i;
        let (a, b) = self.0.split_at_mut(ri);
        (<_>::from(b), <_>::from(a))
    }
}

impl<T, U> PartialEq<RevSlice<U>> for RevSlice<T>
    where T: PartialEq<U>,
{
    fn eq(&self, rhs: &RevSlice<U>) -> bool {
        self.0 == rhs.0
    }
}

/// `RevSlice` compares by logical element sequence.
impl<T, U> PartialEq<[U]> for RevSlice<T>
    where T: PartialEq<U>,
{
    fn eq(&self, rhs: &[U]) -> bool {
        if self.len() != rhs.len() {
            return false;
        }
        for (x, y) in self.into_iter().zip(rhs) {
            if x != y {
                return false;
            }
        }
        true
    }
}

impl<T> Hash for RevSlice<T>
    where T: Hash,
{
    fn hash<H: Hasher>(&self, h: &mut H) {
        // hash like a slice of the same logical sequence
        self.len().hash(h);
        for elt in self {
            elt.hash(h)
        }
    }
}

impl<'a, T, Slice: ?Sized> From<&'a Slice> for &'a RevSlice<T>
    where Slice: AsRef<[T]>
{
    fn from(slc: &'a Slice) -> Self {
        unsafe {
            transmute(slc.as_ref())
        }
    }
}

impl<'a, T, Slice: ?Sized> From<&'a mut Slice> for &'a mut RevSlice<T>
    where Slice: AsMut<[T]>
{
    fn from(slc: &'a mut Slice) -> Self {
        unsafe {
            transmute(slc.as_mut())
        }
    }
}

#[cfg(feature = "std")]
impl<T> From<Box<[T]>> for Box<RevSlice<T>> {
    fn from(slc: Box<[T]>) -> Self {
        unsafe {
            transmute(slc)
        }
    }
}

impl<T> Index<usize> for RevSlice<T> {
    type Output = T;
    fn index(&self, i: usize) -> &T {
        if let Some(x) = self.get(i) {
            x
        } else {
            panic!("Index {} is out of bounds for RevSlice of length {}", i, self.len());
        }
    }
}

impl<T> IndexMut<usize> for RevSlice<T> {
    fn index_mut(&mut self, i: usize) -> &mut T {
        let len = self.len();
        if let Some(x) = self.get_mut(i) {
            return x;
        } else {
            panic!("Index {} is out of bounds for RevSlice of length {}", i, len);
        }
    }
}

impl<'a, T> Default for &'a RevSlice<T> {
    fn default() -> Self {
        Self::from(&[])
    }
}

impl<'a, T> Default for &'a mut RevSlice<T> {
    fn default() -> Self {
        Self::from(&mut [])
    }
}

impl<T, R> Index<R> for RevSlice<T>
    where R: IndexRange,
{
    type Output = RevSlice<T>;
    fn index(&self, index: R) -> &RevSlice<T> {
        // [0 1 2 3 4]
        //  4 3 2 1 0
        // [       ] <- rev 1..5  is 0..4
        let start = index.start().unwrap_or(0);
        let end = index.end().unwrap_or(self.len());
        assert!(start <= end && end <= self.len());
        let end_r = self.len() - start;
        let start_r = self.len() - end;
        unsafe {
            <&RevSlice<_>>::from(slice_unchecked(&self.0, start_r, end_r))
        }
    }
}

impl<T, R> IndexMut<R> for RevSlice<T>
    where R: IndexRange,
{
    fn index_mut(&mut self, index: R) -> &mut RevSlice<T> {
        // [0 1 2 3 4]
        //  4 3 2 1 0
        // [       ] <- rev 1..5  is 0..4
        let start = index.start().unwrap_or(0);
        let end = index.end().unwrap_or(self.len());
        assert!(start <= end && end <= self.len());
        let end_r = self.len() - start;
        let start_r = self.len() - end;
        unsafe {
            <&mut RevSlice<_>>::from(slice_unchecked_mut(&mut self.0, start_r, end_r))
        }
    }
}

impl<'a, T> IntoIterator for &'a RevSlice<T> {
    type Item = &'a T;
    type IntoIter = Rev<Iter<'a, T>>;
    fn into_iter(self) -> Self::IntoIter {
        self.0.iter().rev()
    }
}

impl<'a, T> IntoIterator for &'a mut RevSlice<T> {
    type Item = &'a mut T;
    type IntoIter = Rev<IterMut<'a, T>>;
    fn into_iter(self) -> Self::IntoIter {
        self.0.iter_mut().rev()
    }
}

#[test]
fn test_rev_slice_1() {
    let data = [1, 2, 3, 4];
    let rev = [4, 3, 2, 1];

    assert_eq!(<&RevSlice<_>>::from(&data[..]), &rev[..]);
    assert!(<&RevSlice<_>>::from(&data[..]) != &data[..]);
    let r = <&RevSlice<_>>::from(&data[..]);
    assert_eq!(r[0], rev[0]);
    assert_eq!(r[3], rev[3]);
}

#[should_panic]
#[test]
fn test_rev_slice_2() {
    let data = [1, 2, 3, 4];

    let r = <&RevSlice<_>>::from(&data[..]);
    r[4];
}

#[should_panic]
#[test]
fn test_rev_slice_3() {
    let data = [1, 2, 3, 4];

    let r = <&RevSlice<_>>::from(&data[..]);
    r[!0];
}

#[test]
fn test_rev_slice_slice() {
    let data = [1, 2, 3, 4];
    let rev = [4, 3, 2, 1];

    let r = <&RevSlice<_>>::from(&data[..]);
    
    for i in 0..r.len() {
        for j in i..r.len() {
            println!("{:?}, {:?}", &r[i..j], &rev[i..j]);
            assert_eq!(&r[i..j], &rev[i..j]);
        }
    }
}

#[test]
fn test_rev_slice_find() {
    let data = [1, 2, 3, 4];

    let r = <&RevSlice<_>>::from(&data[..]);
    
    for (i, elt) in r.into_iter().enumerate() {
        assert_eq!(r.find(elt), Some(i));
    }
    for (i, elt) in r.into_iter().enumerate() {
        assert_eq!(r.rfind(elt), Some(i));
    }
}

#[test]
fn test_rev_slice_split() {
    let data = [1, 2, 3, 4];

    let r = <&RevSlice<_>>::from(&data[..]);

    for i in 0..r.len() {
        let (a, b) = r.split_at(i);
        assert_eq!(a, &r[..i]);
        assert_eq!(b, &r[i..]);
    }
}

#[test]
fn test_rev_slice_hash() {
    let data = [1, 2, 3, 4];
    let rev = [4, 3, 2, 1];

    let r = <&RevSlice<_>>::from(&data[..]);

    #[allow(deprecated)]
    fn hash<T: ?Sized + Hash>(value: &T) -> u64 {
        use std::hash::SipHasher;
        let mut h = SipHasher::new();
        value.hash(&mut h);
        h.finish()
    }
    for i in 0..r.len() {
        for j in i..r.len() {
            assert_eq!(hash(&r[i..j]), hash(&rev[i..j]));
        }
    }
}