hex.rs

pub(crate) fn parse_hex(buffer: &[u8]) -> FeedingResult<usize> {
    let mut output = 0;

    const fn naive_upper(v: u8) -> u8 {
        v & !(0x20 * (v >= b'A') as u8)
    }

    // !{char} = incorrect
    const TABLE: &[usize] = &[
        0xF, 0xE, 0xD, 0xC, 0xB, 0xA, 0, // !@
        0, // !?
        0, // !>
        0, // !=
        0, // !<
        0, // !;
        0, // !:
        9, 8, 7, 6, 5, 4, 3, 2, 1, 0,
    ];

    let buf_len = buffer.len();
    for (idx, chr) in buffer.iter().copied().enumerate() {
        let shl = 4 * (buf_len - idx - 1);

        // branch can be elided if known that input is correct
        if !matches!(chr, b'0'..=b'9' | b'A'..=b'Z' | b'a'..=b'z') {
            return Err(FeedingError::Malformed);
        }

        output += TABLE[(b'F' - naive_upper(chr)) as usize] << shl;
    }

    Ok(output)
}