form-net 0.3.0

Sans-IO form parsing for Rust: application/x-www-form-urlencoded and streaming multipart/form-data, with budgets checked before allocation, safe upload spilling, and typed extraction. No third-party dependencies.
Documentation 
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//! Byte scanning shared by both formats: eight bytes per step with plain
//! `u64` arithmetic (SWAR), safe Rust.

const LO: u64 = 0x0101_0101_0101_0101;
const HI: u64 = 0x8080_8080_8080_8080;

/// A word with the high bit set in every byte of `word` equal to `byte`.
///
/// The classic "has zero byte" test on `word ^ splat(byte)`. It can set
/// false bits only above a true match (borrow propagation), so the lowest set
/// bit always marks the first match.
#[inline]
const fn matches(word: u64, byte: u8) -> u64 {
    let x = word ^ LO.wrapping_mul(byte as u64);
    x.wrapping_sub(LO) & !x & HI
}

#[inline]
fn word_at(chunk: &[u8]) -> u64 {
    let mut bytes = [0u8; 8];
    bytes.copy_from_slice(chunk);
    u64::from_le_bytes(bytes)
}

/// The index of the first byte of `hay` at or after `from` that is any of
/// the bytes in `set` (at most four), or `hay.len()`.
#[inline]
pub(crate) fn find_any<const N: usize>(hay: &[u8], from: usize, set: [u8; N]) -> usize {
    let start = from.min(hay.len());
    let rest = hay.get(start..).unwrap_or(&[]);
    let mut chunks = rest.chunks_exact(8);
    let mut base = start;
    for chunk in &mut chunks {
        let word = word_at(chunk);
        let mut hits = 0u64;
        for &b in &set {
            hits |= matches(word, b);
        }
        if hits != 0 {
            return base.wrapping_add((hits.trailing_zeros() / 8) as usize);
        }
        base = base.wrapping_add(8);
    }
    for (i, &b) in chunks.remainder().iter().enumerate() {
        if set.contains(&b) {
            return base.wrapping_add(i);
        }
    }
    hay.len()
}

/// The index of the first `byte` in `hay` at or after `from`, or
/// `hay.len()`.
#[inline]
pub(crate) fn find_byte(hay: &[u8], from: usize, byte: u8) -> usize {
    find_any(hay, from, [byte])
}

/// How many times `byte` occurs in `hay`.
#[cfg(feature = "alloc")]
pub(crate) fn count_byte(hay: &[u8], byte: u8) -> usize {
    let mut n = 0usize;
    let mut i = 0usize;
    loop {
        i = find_byte(hay, i, byte);
        if i >= hay.len() {
            return n;
        }
        n = n.wrapping_add(1);
        i = i.wrapping_add(1);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn naive(hay: &[u8], from: usize, set: &[u8]) -> usize {
        (from.min(hay.len())..hay.len())
            .find(|&i| set.contains(&hay[i]))
            .unwrap_or(hay.len())
    }

    #[test]
    fn test_find_any_matches_naive_every_position() {
        let hay = b"abcdefgh&ijklmnop=qrstu%vwxyz+0123456789abcdefghij&";
        for from in 0..=hay.len() + 2 {
            assert_eq!(
                find_any(hay, from, *b"&=%+"),
                naive(hay, from, b"&=%+"),
                "from {from}"
            );
        }
    }

    #[test]
    fn test_find_byte_high_bytes_and_none() {
        let hay = [0x80u8, 0xff, 0x0d, 0x01, 0x0d, 0, 0, 0, 0, 0x0d];
        assert_eq!(find_byte(&hay, 0, 0x0d), 2);
        assert_eq!(find_byte(&hay, 3, 0x0d), 4);
        assert_eq!(find_byte(&hay, 5, 0x0d), 9);
        assert_eq!(find_byte(&hay, 0, b'x'), hay.len());
        assert_eq!(find_byte(&[], 0, b'x'), 0);
    }

    #[test]
    fn test_matches_borrow_never_hides_first_match() {
        // 0x01 next to the target byte is the borrow case.
        for target in [0u8, 1, 0x0d, 0x80, 0xff] {
            for pos in 0..8 {
                let mut bytes = [target.wrapping_add(1); 8];
                bytes[pos] = target;
                if pos > 0 {
                    bytes[pos - 1] = target ^ 1;
                }
                let w = u64::from_le_bytes(bytes);
                let first = naive(&bytes, 0, &[target]);
                assert_eq!((matches(w, target).trailing_zeros() / 8) as usize, first);
            }
        }
    }
}