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    // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.
    //
    // Original implementation taken from rust-memchr
    // Copyright 2015 Andrew Gallant, bluss and Nicolas Koch

    #[allow(dead_code)]
    pub mod fallback {
        use cmp;
        use mem;

        const LO_U64: u64 = 0x0101010101010101;
        const HI_U64: u64 = 0x8080808080808080;

        // use truncation
        const LO_USIZE: usize = LO_U64 as usize;
        const HI_USIZE: usize = HI_U64 as usize;

        /// Return `true` if `x` contains any zero byte.
        ///
        /// From *Matters Computational*, J. Arndt
        ///
        /// "The idea is to subtract one from each of the bytes and then look for
        /// bytes where the borrow propagated all the way to the most significant
        /// bit."
        #[inline]
        fn contains_zero_byte(x: usize) -> bool {
            x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
        }

        #[cfg(target_pointer_width = "32")]
        #[inline]
        fn repeat_byte(b: u8) -> usize {
            let mut rep = (b as usize) << 8 | b as usize;
            rep = rep << 16 | rep;
            rep
        }

        #[cfg(target_pointer_width = "64")]
        #[inline]
        fn repeat_byte(b: u8) -> usize {
            let mut rep = (b as usize) << 8 | b as usize;
            rep = rep << 16 | rep;
            rep = rep << 32 | rep;
            rep
        }

        /// Return the first index matching the byte `a` in `text`.
        pub fn memchr(x: u8, text: &[u8]) -> Option<usize> {
            // Scan for a single byte value by reading two `usize` words at a time.
            //
            // Split `text` in three parts
            // - unaligned initial part, before the first word aligned address in text
            // - body, scan by 2 words at a time
            // - the last remaining part, < 2 word size
            let len = text.len();
            let ptr = text.as_ptr();
            let usize_bytes = mem::size_of::<usize>();

            // search up to an aligned boundary
            let align = (ptr as usize) & (usize_bytes- 1);
            let mut offset;
            if align > 0 {
                offset = cmp::min(usize_bytes - align, len);
                if let Some(index) = text[..offset].iter().position(|elt| *elt == x) {
                    return Some(index);
                }
            } else {
                offset = 0;
            }

            // search the body of the text
            let repeated_x = repeat_byte(x);

            if len >= 2 * usize_bytes {
                while offset <= len - 2 * usize_bytes {
                    unsafe {
                        let u = *(ptr.offset(offset as isize) as *const usize);
                        let v = *(ptr.offset((offset + usize_bytes) as isize) as *const usize);

                        // break if there is a matching byte
                        let zu = contains_zero_byte(u ^ repeated_x);
                        let zv = contains_zero_byte(v ^ repeated_x);
                        if zu || zv {
                            break;
                        }
                    }
                    offset += usize_bytes * 2;
                }
            }

            // find the byte after the point the body loop stopped
            text[offset..].iter().position(|elt| *elt == x).map(|i| offset + i)
        }

        /// Return the last index matching the byte `a` in `text`.
        pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
            // Scan for a single byte value by reading two `usize` words at a time.
            //
            // Split `text` in three parts
            // - unaligned tail, after the last word aligned address in text
            // - body, scan by 2 words at a time
            // - the first remaining bytes, < 2 word size
            let len = text.len();
            let ptr = text.as_ptr();
            let usize_bytes = mem::size_of::<usize>();

            // search to an aligned boundary
            let end_align = (ptr as usize + len) & (usize_bytes - 1);
            let mut offset;
            if end_align > 0 {
                offset = if end_align >= len { 0 } else { len - end_align };
                if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) {
                    return Some(offset + index);
                }
            } else {
                offset = len;
            }

            // search the body of the text
            let repeated_x = repeat_byte(x);

            while offset >= 2 * usize_bytes {
                unsafe {
                    let u = *(ptr.offset(offset as isize - 2 * usize_bytes as isize) as *const usize);
                    let v = *(ptr.offset(offset as isize - usize_bytes as isize) as *const usize);

                    // break if there is a matching byte
                    let zu = contains_zero_byte(u ^ repeated_x);
                    let zv = contains_zero_byte(v ^ repeated_x);
                    if zu || zv {
                        break;
                    }
                }
                offset -= 2 * usize_bytes;
            }

            // find the byte before the point the body loop stopped
            text[..offset].iter().rposition(|elt| *elt == x)
        }

        // test fallback implementations on all platforms
        #[test]
        fn matches_one() {
            assert_eq!(Some(0), memchr(b'a', b"a"));
        }

        #[test]
        fn matches_begin() {
            assert_eq!(Some(0), memchr(b'a', b"aaaa"));
        }

        #[test]
        fn matches_end() {
            assert_eq!(Some(4), memchr(b'z', b"aaaaz"));
        }

        #[test]
        fn matches_nul() {
            assert_eq!(Some(4), memchr(b'\x00', b"aaaa\x00"));
        }

        #[test]
        fn matches_past_nul() {
            assert_eq!(Some(5), memchr(b'z', b"aaaa\x00z"));
        }

        #[test]
        fn no_match_empty() {
            assert_eq!(None, memchr(b'a', b""));
        }

        #[test]
        fn no_match() {
            assert_eq!(None, memchr(b'a', b"xyz"));
        }

        #[test]
        fn matches_one_reversed() {
            assert_eq!(Some(0), memrchr(b'a', b"a"));
        }

        #[test]
        fn matches_begin_reversed() {
            assert_eq!(Some(3), memrchr(b'a', b"aaaa"));
        }

        #[test]
        fn matches_end_reversed() {
            assert_eq!(Some(0), memrchr(b'z', b"zaaaa"));
        }

        #[test]
        fn matches_nul_reversed() {
            assert_eq!(Some(4), memrchr(b'\x00', b"aaaa\x00"));
        }

        #[test]
        fn matches_past_nul_reversed() {
            assert_eq!(Some(0), memrchr(b'z', b"z\x00aaaa"));
        }

        #[test]
        fn no_match_empty_reversed() {
            assert_eq!(None, memrchr(b'a', b""));
        }

        #[test]
        fn no_match_reversed() {
            assert_eq!(None, memrchr(b'a', b"xyz"));
        }

        #[test]
        fn each_alignment_reversed() {
            let mut data = [1u8; 64];
            let needle = 2;
            let pos = 40;
            data[pos] = needle;
            for start in 0..16 {
                assert_eq!(Some(pos - start), memrchr(needle, &data[start..]));
            }
        }
    }

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