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    //! Operations related to UTF-8 validation.

    use crate::mem;

    use super::Utf8Error;

    /// Returns the initial codepoint accumulator for the first byte.
    /// The first byte is special, only want bottom 5 bits for width 2, 4 bits
    /// for width 3, and 3 bits for width 4.
    #[inline]
    fn utf8_first_byte(byte: u8, width: u32) -> u32 {
        (byte & (0x7F >> width)) as u32
    }

    /// Returns the value of `ch` updated with continuation byte `byte`.
    #[inline]
    fn utf8_acc_cont_byte(ch: u32, byte: u8) -> u32 {
        (ch << 6) | (byte & CONT_MASK) as u32
    }

    /// Checks whether the byte is a UTF-8 continuation byte (i.e., starts with the
    /// bits `10`).
    #[inline]
    pub(super) fn utf8_is_cont_byte(byte: u8) -> bool {
        (byte & !CONT_MASK) == TAG_CONT_U8
    }

    #[inline]
    fn unwrap_or_0(opt: Option<&u8>) -> u8 {
        match opt {
            Some(&byte) => byte,
            None => 0,
        }
    }

    /// Reads the next code point out of a byte iterator (assuming a
    /// UTF-8-like encoding).
    #[unstable(feature = "str_internals", issue = "none")]
    #[inline]
    pub fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) -> Option<u32> {
        // Decode UTF-8
        let x = *bytes.next()?;
        if x < 128 {
            return Some(x as u32);
        }

        // Multibyte case follows
        // Decode from a byte combination out of: [[[x y] z] w]
        // NOTE: Performance is sensitive to the exact formulation here
        let init = utf8_first_byte(x, 2);
        let y = unwrap_or_0(bytes.next());
        let mut ch = utf8_acc_cont_byte(init, y);
        if x >= 0xE0 {
            // [[x y z] w] case
            // 5th bit in 0xE0 .. 0xEF is always clear, so `init` is still valid
            let z = unwrap_or_0(bytes.next());
            let y_z = utf8_acc_cont_byte((y & CONT_MASK) as u32, z);
            ch = init << 12 | y_z;
            if x >= 0xF0 {
                // [x y z w] case
                // use only the lower 3 bits of `init`
                let w = unwrap_or_0(bytes.next());
                ch = (init & 7) << 18 | utf8_acc_cont_byte(y_z, w);
            }
        }

        Some(ch)
    }

    /// Reads the last code point out of a byte iterator (assuming a
    /// UTF-8-like encoding).
    #[inline]
    pub(super) fn next_code_point_reverse<'a, I>(bytes: &mut I) -> Option<u32>
    where
        I: DoubleEndedIterator<Item = &'a u8>,
    {
        // Decode UTF-8
        let w = match *bytes.next_back()? {
            next_byte if next_byte < 128 => return Some(next_byte as u32),
            back_byte => back_byte,
        };

        // Multibyte case follows
        // Decode from a byte combination out of: [x [y [z w]]]
        let mut ch;
        let z = unwrap_or_0(bytes.next_back());
        ch = utf8_first_byte(z, 2);
        if utf8_is_cont_byte(z) {
            let y = unwrap_or_0(bytes.next_back());
            ch = utf8_first_byte(y, 3);
            if utf8_is_cont_byte(y) {
                let x = unwrap_or_0(bytes.next_back());
                ch = utf8_first_byte(x, 4);
                ch = utf8_acc_cont_byte(ch, y);
            }
            ch = utf8_acc_cont_byte(ch, z);
        }
        ch = utf8_acc_cont_byte(ch, w);

        Some(ch)
    }

    // use truncation to fit u64 into usize
    const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;

    /// Returns `true` if any byte in the word `x` is nonascii (>= 128).
    #[inline]
    fn contains_nonascii(x: usize) -> bool {
        (x & NONASCII_MASK) != 0
    }

    /// Walks through `v` checking that it's a valid UTF-8 sequence,
    /// returning `Ok(())` in that case, or, if it is invalid, `Err(err)`.
    #[inline(always)]
    pub(super) fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
        let mut index = 0;
        let len = v.len();

        let usize_bytes = mem::size_of::<usize>();
        let ascii_block_size = 2 * usize_bytes;
        let blocks_end = if len >= ascii_block_size { len - ascii_block_size + 1 } else { 0 };
        let align = v.as_ptr().align_offset(usize_bytes);

        while index < len {
            let old_offset = index;
            macro_rules! err {
                ($error_len: expr) => {
                    return Err(Utf8Error { valid_up_to: old_offset, error_len: $error_len })
                };
            }

            macro_rules! next {
                () => {{
                    index += 1;
                    // we needed data, but there was none: error!
                    if index >= len {
                        err!(None)
                    }
                    v[index]
                }};
            }

            let first = v[index];
            if first >= 128 {
                let w = UTF8_CHAR_WIDTH[first as usize];
                // 2-byte encoding is for codepoints  \u{0080} to  \u{07ff}
                //        first  C2 80        last DF BF
                // 3-byte encoding is for codepoints  \u{0800} to  \u{ffff}
                //        first  E0 A0 80     last EF BF BF
                //   excluding surrogates codepoints  \u{d800} to  \u{dfff}
                //               ED A0 80 to       ED BF BF
                // 4-byte encoding is for codepoints \u{1000}0 to \u{10ff}ff
                //        first  F0 90 80 80  last F4 8F BF BF
                //
                // Use the UTF-8 syntax from the RFC
                //
                // https://tools.ietf.org/html/rfc3629
                // UTF8-1      = %x00-7F
                // UTF8-2      = %xC2-DF UTF8-tail
                // UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
                //               %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
                // UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
                //               %xF4 %x80-8F 2( UTF8-tail )
                match w {
                    2 => {
                        if next!() & !CONT_MASK != TAG_CONT_U8 {
                            err!(Some(1))
                        }
                    }
                    3 => {
                        match (first, next!()) {
                            (0xE0, 0xA0..=0xBF)
                            | (0xE1..=0xEC, 0x80..=0xBF)
                            | (0xED, 0x80..=0x9F)
                            | (0xEE..=0xEF, 0x80..=0xBF) => {}
                            _ => err!(Some(1)),
                        }
                        if next!() & !CONT_MASK != TAG_CONT_U8 {
                            err!(Some(2))
                        }
                    }
                    4 => {
                        match (first, next!()) {
                            (0xF0, 0x90..=0xBF) | (0xF1..=0xF3, 0x80..=0xBF) | (0xF4, 0x80..=0x8F) => {}
                            _ => err!(Some(1)),
                        }
                        if next!() & !CONT_MASK != TAG_CONT_U8 {
                            err!(Some(2))
                        }
                        if next!() & !CONT_MASK != TAG_CONT_U8 {
                            err!(Some(3))
                        }
                    }
                    _ => err!(Some(1)),
                }
                index += 1;
            } else {
                // Ascii case, try to skip forward quickly.
                // When the pointer is aligned, read 2 words of data per iteration
                // until we find a word containing a non-ascii byte.
                if align != usize::MAX && align.wrapping_sub(index) % usize_bytes == 0 {
                    let ptr = v.as_ptr();
                    while index < blocks_end {
                        // SAFETY: since `align - index` and `ascii_block_size` are
                        // multiples of `usize_bytes`, `block = ptr.add(index)` is
                        // always aligned with a `usize` so it's safe to dereference
                        // both `block` and `block.offset(1)`.
                        unsafe {
                            let block = ptr.add(index) as *const usize;
                            // break if there is a nonascii byte
                            let zu = contains_nonascii(*block);
                            let zv = contains_nonascii(*block.offset(1));
                            if zu | zv {
                                break;
                            }
                        }
                        index += ascii_block_size;
                    }
                    // step from the point where the wordwise loop stopped
                    while index < len && v[index] < 128 {
                        index += 1;
                    }
                } else {
                    index += 1;
                }
            }
        }

        Ok(())
    }

    // https://tools.ietf.org/html/rfc3629
    static UTF8_CHAR_WIDTH: [u8; 256] = [
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, // 0x1F
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, // 0x3F
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, // 0x5F
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, // 0x7F
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, // 0x9F
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, // 0xBF
        0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
        2, // 0xDF
        3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // 0xEF
        4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xFF
    ];

    /// Given a first byte, determines how many bytes are in this UTF-8 character.
    #[unstable(feature = "str_internals", issue = "none")]
    #[inline]
    pub fn utf8_char_width(b: u8) -> usize {
        UTF8_CHAR_WIDTH[b as usize] as usize
    }

    /// Mask of the value bits of a continuation byte.
    const CONT_MASK: u8 = 0b0011_1111;
    /// Value of the tag bits (tag mask is !CONT_MASK) of a continuation byte.
    const TAG_CONT_U8: u8 = 0b1000_0000;

    // truncate `&str` to length at most equal to `max`
    // return `true` if it were truncated, and the new str.
    pub(super) fn truncate_to_char_boundary(s: &str, mut max: usize) -> (bool, &str) {
        if max >= s.len() {
            (false, s)
        } else {
            while !s.is_char_boundary(max) {
                max -= 1;
            }
            (true, &s[..max])
        }
    }