Fast (scalar) u64 parse function from ASCII byte slice

atou64.rs

// SPDX-License-Identifier: MIT OR Apache-2.0
//
// Copyright (c) 2026 Giuseppe Eletto <giuseppe@eletto.me>
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/// Parses a byte slice into a `u64`.
///
/// Returns `Some(u64)` if the entire slice consists of ASCII digits and the resulting
/// value fits within a 64-bit unsigned integer. Returns `None` if the slice is empty,
/// contains non-digit characters, or would overflow `u64`.
///
/// # Performance
///
/// This function uses a fast path for slices with 1 to 19 digits. Since $2^{64} - 1$
/// is a 20-digit number (`18,446,744,073,709,551,615`), any sequence of 19 digits
/// or fewer is guaranteed to fit in a `u64`, allowing for optimized wrapping arithmetic.
pub fn atou64(buf: &[u8]) -> Option<u64> {
    /// Handles the 20-digit case where overflow is possible.
    ///
    /// This is marked `#[cold]` to keep it out of the primary instruction cache
    /// path, as most inputs are expected to be shorter than 20 digits.
    #[cold]
    fn slow_path(buf: &[u8]) -> Option<u64> {
        let mut val = 0u64;

        for &b in buf {
            let d = match b.wrapping_sub(b'0').into() {
                d @ 0..10 => d,
                _ => return None,
            };

            val = val.checked_mul(10).and_then(|val| val.checked_add(d))?;
        }

        Some(val)
    }

    // fn atou64(..)
    match buf.len() {
        1..20 => {
            // fast path (inline)
            let mut val = 0u64;

            for &b in buf {
                let d = match b.wrapping_sub(b'0').into() {
                    d @ 0..10 => d,
                    _ => return None,
                };

                val = val.wrapping_mul(10).wrapping_add(d);
            }

            Some(val)
        }
        20 => slow_path(buf),
        _ => None,
    }
}