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    /// A few elementary UTF-8 encoding and decoding functions used by the matching
    /// engines.
    ///
    /// In an ideal world, the matching engines operate on `&str` and we can just
    /// lean on the standard library for all our UTF-8 needs. However, to support
    /// byte based regexes (that can match on arbitrary bytes which may contain
    /// UTF-8), we need to be capable of searching and decoding UTF-8 on a `&[u8]`.
    /// The standard library doesn't really recognize this use case, so we have
    /// to build it out ourselves.
    ///
    /// Should this be factored out into a separate crate? It seems independently
    /// useful. There are other crates that already exist (e.g., `utf-8`) that have
    /// overlapping use cases. Not sure what to do.

    use std::char;

    const TAG_CONT: u8 = 0b1000_0000;
    const TAG_TWO: u8 = 0b1100_0000;
    const TAG_THREE: u8 = 0b1110_0000;
    const TAG_FOUR: u8 = 0b1111_0000;

    /// Returns the smallest possible index of the next valid UTF-8 sequence
    /// starting after `i`.
    pub fn next_utf8(text: &[u8], i: usize) -> usize {
        let b = match text.get(i) {
            None => return i + 1,
            Some(&b) => b,
        };
        let inc = if b <= 0x7F {
            1
        } else if b <= 0b110_11111 {
            2
        } else if b <= 0b1110_1111 {
            3
        } else {
            4
        };
        i + inc
    }

    /// Encode the given Unicode character to `dst` as a single UTF-8 sequence.
    ///
    /// If `dst` is not long enough, then `None` is returned. Otherwise, the number
    /// of bytes written is returned.
    #[allow(dead_code)]
    #[inline]
    pub fn encode_utf8(character: char, dst: &mut [u8]) -> Option<usize> {
        let code = character as u32;
        if code <= 0x7F && !dst.is_empty() {
            dst[0] = code as u8;
            Some(1)
        } else if code <= 0x7FF && dst.len() >= 2 {
            dst[0] = (code >> 6 & 0x1F) as u8 | TAG_TWO;
            dst[1] = (code & 0x3F) as u8 | TAG_CONT;
            Some(2)
        } else if code <= 0xFFFF && dst.len() >= 3  {
            dst[0] = (code >> 12 & 0x0F) as u8 | TAG_THREE;
            dst[1] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
            dst[2] = (code & 0x3F) as u8 | TAG_CONT;
            Some(3)
        } else if dst.len() >= 4 {
            dst[0] = (code >> 18 & 0x07) as u8 | TAG_FOUR;
            dst[1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
            dst[2] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
            dst[3] = (code & 0x3F) as u8 | TAG_CONT;
            Some(4)
        } else {
            None
        }
    }

    /// Decode a single UTF-8 sequence into a single Unicode codepoint from `src`.
    ///
    /// If no valid UTF-8 sequence could be found, then `None` is returned.
    /// Otherwise, the decoded codepoint and the number of bytes read is returned.
    /// The number of bytes read (for a valid UTF-8 sequence) is guaranteed to be
    /// 1, 2, 3 or 4.
    ///
    /// Note that a UTF-8 sequence is invalid if it is incorrect UTF-8, encodes a
    /// codepoint that is out of range (surrogate codepoints are out of range) or
    /// is not the shortest possible UTF-8 sequence for that codepoint.
    #[inline]
    pub fn decode_utf8(src: &[u8]) -> Option<(char, usize)> {
        let b0 = match src.get(0) {
            None => return None,
            Some(&b) if b <= 0x7F => return Some((b as char, 1)),
            Some(&b) => b,
        };
        match b0 {
            0b110_00000 ... 0b110_11111 => {
                if src.len() < 2 {
                    return None;
                }
                let b1 = src[1];
                let cp = ((b0 & !TAG_TWO) as u32) << 6
                         | ((b1 & !TAG_CONT) as u32);
                match cp {
                    0x80 ... 0x7FF => char::from_u32(cp).map(|cp| (cp, 2)),
                    _ => None,
                }
            }
            0b1110_0000 ... 0b1110_1111 => {
                if src.len() < 3 {
                    return None;
                }
                let (b1, b2) = (src[1], src[2]);
                let cp = ((b0 & !TAG_THREE) as u32) << 12
                         | ((b1 & !TAG_CONT) as u32) << 6
                         | ((b2 & !TAG_CONT) as u32);
                match cp {
                    // char::from_u32 will disallow surrogate codepoints.
                    0x800 ... 0xFFFF => char::from_u32(cp).map(|cp| (cp, 3)),
                    _ => None,
                }
            }
            0b11110_000 ... 0b11110_111 => {
                if src.len() < 4 {
                    return None;
                }
                let (b1, b2, b3) = (src[1], src[2], src[3]);
                let cp = ((b0 & !TAG_FOUR) as u32) << 18
                         | ((b1 & !TAG_CONT) as u32) << 12
                         | ((b2 & !TAG_CONT) as u32) << 6
                         | ((b3 & !TAG_CONT) as u32);
                match cp {
                    0x10000 ... 0x10FFFF => char::from_u32(cp).map(|cp| (cp, 4)),
                    _ => None,
                }
            }
            _ => None,
        }
    }

    /// Like decode_utf8, but decodes the last UTF-8 sequence in `src` instead of
    /// the first.
    pub fn decode_last_utf8(src: &[u8]) -> Option<(char, usize)> {
        if src.is_empty() {
            return None;
        }
        let mut start = src.len() - 1;
        if src[start] <= 0x7F {
            return Some((src[start] as char, 1));
        }
        while start > src.len().saturating_sub(4) {
            start -= 1;
            if is_start_byte(src[start]) {
                break;
            }
        }
        match decode_utf8(&src[start..]) {
            None => None,
            Some((_, n)) if n < src.len() - start => None,
            Some((cp, n)) => Some((cp, n)),
        }
    }

    fn is_start_byte(b: u8) -> bool {
        b & 0b11_000000 != 0b1_0000000
    }

    #[cfg(test)]
    mod tests {
        use std::str;

        use quickcheck::quickcheck;

        use super::{
            TAG_CONT, TAG_TWO, TAG_THREE, TAG_FOUR,
            decode_utf8, decode_last_utf8, encode_utf8,
        };

        #[test]
        fn prop_roundtrip() {
            fn p(given_cp: char) -> bool {
                let mut tmp = [0; 4];
                let encoded_len = encode_utf8(given_cp, &mut tmp).unwrap();
                let (got_cp, got_len) = decode_utf8(&tmp[..encoded_len]).unwrap();
                encoded_len == got_len && given_cp == got_cp
            }
            quickcheck(p as fn(char) -> bool)
        }

        #[test]
        fn prop_roundtrip_last() {
            fn p(given_cp: char) -> bool {
                let mut tmp = [0; 4];
                let encoded_len = encode_utf8(given_cp, &mut tmp).unwrap();
                let (got_cp, got_len) =
                    decode_last_utf8(&tmp[..encoded_len]).unwrap();
                encoded_len == got_len && given_cp == got_cp
            }
            quickcheck(p as fn(char) -> bool)
        }

        #[test]
        fn prop_encode_matches_std() {
            fn p(cp: char) -> bool {
                let mut got = [0; 4];
                let n = encode_utf8(cp, &mut got).unwrap();
                let expected = cp.to_string();
                &got[..n] == expected.as_bytes()
            }
            quickcheck(p as fn(char) -> bool)
        }

        #[test]
        fn prop_decode_matches_std() {
            fn p(given_cp: char) -> bool {
                let mut tmp = [0; 4];
                let n = encode_utf8(given_cp, &mut tmp).unwrap();
                let (got_cp, _) = decode_utf8(&tmp[..n]).unwrap();
                let expected_cp =
                    str::from_utf8(&tmp[..n]).unwrap().chars().next().unwrap();
                got_cp == expected_cp
            }
            quickcheck(p as fn(char) -> bool)
        }

        #[test]
        fn prop_decode_last_matches_std() {
            fn p(given_cp: char) -> bool {
                let mut tmp = [0; 4];
                let n = encode_utf8(given_cp, &mut tmp).unwrap();
                let (got_cp, _) = decode_last_utf8(&tmp[..n]).unwrap();
                let expected_cp =
                    str::from_utf8(&tmp[..n]).unwrap()
                        .chars().rev().next().unwrap();
                got_cp == expected_cp
            }
            quickcheck(p as fn(char) -> bool)
        }

        #[test]
        fn reject_invalid() {
            // Invalid start byte
            assert_eq!(decode_utf8(&[0xFF]), None);
            // Surrogate pair
            assert_eq!(decode_utf8(&[0xED, 0xA0, 0x81]), None);
            // Bad lengths
            assert_eq!(decode_utf8(&[0xC3]), None); // 2 bytes
            assert_eq!(decode_utf8(&[0xEF, 0xBF]), None); // 3 bytes
            assert_eq!(decode_utf8(&[0xF4, 0x8F, 0xBF]), None); // 4 bytes
            // Not a minimal UTF-8 sequence
            assert_eq!(decode_utf8(&[TAG_TWO, TAG_CONT | b'a']), None);
            assert_eq!(decode_utf8(&[TAG_THREE, TAG_CONT, TAG_CONT | b'a']), None);
            assert_eq!(decode_utf8(&[
                TAG_FOUR, TAG_CONT, TAG_CONT, TAG_CONT | b'a',
            ]), None);
        }

        #[test]
        fn reject_invalid_last() {
            // Invalid start byte
            assert_eq!(decode_last_utf8(&[0xFF]), None);
            // Surrogate pair
            assert_eq!(decode_last_utf8(&[0xED, 0xA0, 0x81]), None);
            // Bad lengths
            assert_eq!(decode_last_utf8(&[0xC3]), None); // 2 bytes
            assert_eq!(decode_last_utf8(&[0xEF, 0xBF]), None); // 3 bytes
            assert_eq!(decode_last_utf8(&[0xF4, 0x8F, 0xBF]), None); // 4 bytes
            // Not a minimal UTF-8 sequence
            assert_eq!(decode_last_utf8(&[TAG_TWO, TAG_CONT | b'a']), None);
            assert_eq!(decode_last_utf8(&[
                TAG_THREE, TAG_CONT, TAG_CONT | b'a',
            ]), None);
            assert_eq!(decode_last_utf8(&[
                TAG_FOUR, TAG_CONT, TAG_CONT, TAG_CONT | b'a',
            ]), None);
        }
    }

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