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    use std::{error, fmt, mem, slice};

    fn naive(x: &[u8], y: &[u8]) -> u64 {
        assert_eq!(x.len(), y.len());
        x.iter().zip(y).fold(0, |a, (b, c)| a + (*b ^ *c).count_ones() as u64)
    }

    #[derive(PartialEq, Eq, Ord, PartialOrd, Hash, Clone)]
    pub struct DistanceError {
        _x: ()
    }
    impl fmt::Debug for DistanceError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "hamming::distance_fast: called with slices with different 8-byte alignments")
        }
    }
    impl fmt::Display for DistanceError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(self, f)
        }
    }
    impl error::Error for DistanceError {
        fn description(&self) -> &str {
            "hamming::distance_fast: called with slices with different 8-byte alignments"
        }
    }

    /// Computes the bitwise [Hamming
    /// distance](https://en.wikipedia.org/wiki/Hamming_distance) between
    /// `x` and `y`, that is, the number of bits where `x` and `y` differ,
    /// or, the number of set bits in the xor of `x` and `y`.
    ///
    /// This is a highly optimised version of the following naive version:
    ///
    /// ```rust
    /// fn naive(x: &[u8], y: &[u8]) -> u64 {
    ///     x.iter().zip(y).fold(0, |a, (b, c)| a + (*b ^ *c).count_ones() as u64)
    /// }
    /// ```
    ///
    /// This function requires that `x` and `y` have the same 8-byte
    /// alignment (that is, `x.as_ptr() as usize % 8 == y.as_ptr() as
    /// usize % 8`). If not, `Err` is returned. If sub-optimal performance
    /// can be tolerated, consider using `distance` which incorporates a
    /// fallback to a slower but less restrictive algorithm.
    ///
    /// It is essentially guaranteed that `x` and `y` will have the same
    /// 8-byte alignment if they are both just `Vec<u8>`s of non-trivial
    /// length (e.g. larger than 8) as in the example below.
    ///
    /// This is implemented using the same tree-merging approach as
    /// `weight`, see there for details.
    ///
    /// # Panics
    ///
    /// `x` and `y` must have the same length, or else `distance_fast` panics.
    ///
    /// # Performance Comparison
    ///
    /// | length | `naive` (ns) | `distance_fast` (ns) | `naive`/`distance_fast` |
    /// |--:|--:|--:|--:|
    /// | 1 | 5  | 6  | 0.83 |
    /// | 10 | 44  | 45  | 0.97 |
    /// | 100 | 461  | 473  | 0.97 |
    /// | 1,000 | 4,510  | 397  | 11 |
    /// | 10,000 | 46,700  | 2,740  | 17 |
    /// | 100,000 | 45,600  | 20,400  | 22 |
    /// | 1,000,000 | 4,590,000  | 196,000  | 23 |
    ///
    /// # Examples
    ///
    /// ```rust
    /// let x = vec![0xFF; 1000];
    /// let y = vec![0; 1000];
    /// assert_eq!(hamming::distance_fast(&x, &y), Ok(8 * 1000));
    ///
    /// // same alignment, but moderately complicated
    /// assert_eq!(hamming::distance_fast(&x[1..1000 - 8], &y[8 + 1..]), Ok(8 * (1000 - 8 - 1)));
    ///
    /// // differing alignments
    /// assert!(hamming::distance_fast(&x[1..], &y[..999]).is_err());
    /// ```
    pub fn distance_fast(x: &[u8], y: &[u8]) -> Result<u64, DistanceError> {
        assert_eq!(x.len(), y.len());

        const M1: u64 = 0x5555555555555555;
        const M2: u64 = 0x3333333333333333;
        const M4: u64 = 0x0F0F0F0F0F0F0F0F;
        const M8: u64 = 0x00FF00FF00FF00FF;

        type T30 = [u64; 30];
        let size = mem::size_of::<T30>();
        let alignment = mem::align_of::<T30>();

        let ptr1 = x.as_ptr() as usize;
        let ptr2 = y.as_ptr() as usize;

        // round up to the nearest multiple
        let aligned1 = (ptr1 + alignment - 1) / alignment * alignment;
        let aligned2 = (ptr2 + alignment - 1) / alignment * alignment;
        let distance1 = aligned1 - ptr1;
        let distance2 = aligned2 - ptr2;

        // can't fit a single T30 in
        if x.len() < size + distance1 {
            return Ok(naive(x, y))
        }

        if distance1 != distance2 {
            return Err(DistanceError { _x: () });
        }

        let (head1, middle1) = x.split_at(distance1);
        let (head2, middle2) = y.split_at(distance2);

        assert!(middle1.as_ptr() as usize % alignment == 0);
        assert!(middle2.as_ptr() as usize % alignment == 0);
        let (thirty1, thirty2) = unsafe {
            (slice::from_raw_parts(middle1.as_ptr() as *const T30,
                                   middle1.len() / size),
             slice::from_raw_parts(middle2.as_ptr() as *const T30,
                                   middle2.len() / size))
        };
        debug_assert_eq!(thirty1.len(), thirty2.len());

        let tail1 = &middle1[thirty1.len() * size..];
        let tail2 = &middle2[thirty2.len() * size..];
        let mut count = naive(head1, head2) + naive(tail1, tail2);
        for (array1, array2) in thirty1.iter().zip(thirty2) {
            let mut acc = 0;
            for j_ in 0..10 {
                let j = j_ * 3;
                let mut count1 = array1[j] ^ array2[j];
                let mut count2 = array1[j + 1] ^ array2[j + 1];
                let mut half1 = array1[j + 2] ^ array2[j + 2];
                let mut half2 = half1;
                half1 &= M1;
                half2 = (half2 >> 1) & M1;
                count1 -= (count1 >> 1) & M1;
                count2 -= (count2 >> 1) & M1;
                count1 += half1;
                count2 += half2;
                count1 = (count1 & M2) + ((count1 >> 2) & M2);
                count1 += (count2 & M2) + ((count2 >> 2) & M2);
                acc += (count1 & M4) + ((count1 >> 4) & M4);
            }
            acc = (acc & M8) + ((acc >> 8) & M8);
            acc =  acc       +  (acc >> 16);
            acc =  acc       +  (acc >> 32);
            count += acc & 0xFFFF;
        }
        Ok(count)
    }

    /// Computes the bitwise [Hamming
    /// distance](https://en.wikipedia.org/wiki/Hamming_distance) between
    /// `x` and `y`, that is, the number of bits where `x` and `y` differ,
    /// or, the number of set bits in the xor of `x` and `y`.
    ///
    /// When `x` and `y` have the same 8-byte alignment, this uses
    /// `distance_fast`, a highly optimised version of the following naive
    /// version:
    ///
    /// ```rust
    /// fn naive(x: &[u8], y: &[u8]) -> u64 {
    ///     x.iter().zip(y).fold(0, |a, (b, c)| a + (*b ^ *c).count_ones() as u64)
    /// }
    /// ```
    ///
    /// If alignments differ, a slower but less restrictive algorithm is
    /// used.
    ///
    /// It is essentially guaranteed that `x` and `y` will have the same
    /// 8-byte alignment if they are both just `Vec<u8>`s of non-trivial
    /// length (e.g. larger than 8) as in the example below.
    ///
    /// # Panics
    ///
    /// `x` and `y` must have the same length, or else `distance` panics.
    ///
    /// # Performance Comparison
    ///
    /// | length | `naive` (ns) | `distance` (ns) | `naive`/`distance` |
    /// |--:|--:|--:|--:|
    /// | 1 | 5  | 6  | 0.83 |
    /// | 10 | 44  | 45  | 0.97 |
    /// | 100 | 461  | 473  | 0.97 |
    /// | 1,000 | 4,510  | 397  | 11 |
    /// | 10,000 | 46,700  | 2,740  | 17 |
    /// | 100,000 | 45,600  | 20,400  | 22 |
    /// | 1,000,000 | 4,590,000  | 196,000  | 23 |
    ///
    /// The benchmarks ensured that `x` and `y` had the same alignment.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let x = vec![0xFF; 1000];
    /// let y = vec![0; 1000];
    /// assert_eq!(hamming::distance(&x, &y), 8 * 1000);
    /// ```
    pub fn distance(x: &[u8], y: &[u8]) -> u64 {
        distance_fast(x, y)
            .ok()
            .unwrap_or_else(|| naive(x, y))
    }

    #[cfg(all(test, feature = "unstable"))]
    fn distance_(x: &[u8], y: &[u8]) -> u64 {
        distance_fast(x, y).unwrap()
    }

    #[cfg(test)]
    mod tests {
        use quickcheck as qc;
        use rand;
        #[test]
        fn naive_smoke() {
            let tests: &[(&[u8], &[u8], u64)] = &[
                (&[], &[], 0),
                (&[0], &[0], 0),
                (&[0], &[0xFF], 8),
                (&[0b10101010], &[0b01010101], 8),
                (&[0b11111010], &[0b11110101], 4),
                (&[0; 10], &[0; 10], 0),
                (&[0xFF; 10], &[0x0F; 10], 4 * 10),
                (&[0x3B; 10000], &[0x3B; 10000], 0),
                (&[0x77; 10000], &[0x3B; 10000], 3 * 10000),
                ];
            for &(x, y, expected) in tests {
                assert_eq!(super::naive(x, y), expected);
            }
        }
        #[test]
        fn distance_fast_qc() {
            fn prop(v: Vec<u8>, w: Vec<u8>, misalign: u8) -> qc::TestResult {
                let l = ::std::cmp::min(v.len(), w.len());
                if l < misalign as usize {
                    return qc::TestResult::discard()
                }

                let x = &v[misalign as usize..l];
                let y = &w[misalign as usize..l];
                qc::TestResult::from_bool(super::distance_fast(x, y).unwrap() == super::naive(x, y))
            }
            qc::QuickCheck::new()
                .gen(qc::StdGen::new(rand::thread_rng(), 10_000))
                .quickcheck(prop as fn(Vec<u8>,Vec<u8>,u8) -> qc::TestResult)
        }
        #[test]
        fn distance_fast_smoke_huge() {
            let v = vec![0b1001_1101; 10234567];
            let w = vec![0b1111_1111; v.len()];

            assert_eq!(super::distance_fast(&v, &v).unwrap(), 0);
            assert_eq!(super::distance_fast(&v, &w).unwrap(), 3 * w.len() as u64);
        }
        #[test]
        fn distance_smoke() {
            let v = vec![0; 10000];
            let w = vec![0xFF; v.len()];
            for len_ in 0..99 {
                let len = len_ * 10;
                for i in 0..8 {
                    for j in 0..8 {
                        assert_eq!(super::distance(&v[i..i+len], &w[j..j+len]),
                                   len as u64 * 8)
                    }
                }
            }
        }
    }

    #[cfg(all(test, feature = "unstable"))]
    mod benches {
        use test;
        fn bench<F: FnMut(&[u8], &[u8]) -> u64>(b: &mut test::Bencher, n: usize, mut f: F) {
            let data = vec![0xFF; n];
            b.iter(|| {
                let d1 = test::black_box(&data);
                let d2 = test::black_box(&data);
                f(d1, d2)
            })
        }
        macro_rules! test_mod {
            ($name: ident) => {
                mod $name {
                    use test;
                    use super::bench;
                    use super::super::$name;
                    #[bench]
                    fn _0000001(b: &mut test::Bencher) { bench(b, 1, $name) }
                    #[bench]
                    fn _0000010(b: &mut test::Bencher) { bench(b, 10, $name) }
                    #[bench]
                    fn _0000100(b: &mut test::Bencher) { bench(b, 100, $name) }
                    #[bench]
                    fn _0001000(b: &mut test::Bencher) { bench(b, 1000, $name) }
                    #[bench]
                    fn _0010000(b: &mut test::Bencher) { bench(b, 10000, $name) }
                    #[bench]
                    fn _0100000(b: &mut test::Bencher) { bench(b, 100000, $name) }
                    #[bench]
                    fn _1000000(b: &mut test::Bencher) { bench(b, 1000000, $name) }
                }
            }
        }
        test_mod!(naive);
        test_mod!(distance_);
    }

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