advent-of-code/2023/day01/src/branchless.rs

pub unsafe fn part2(input: &str) -> u64 {
    let sum = input
        .lines()
        .map(|line| {
            let bytes = line.as_bytes();

            let mut digits = [0_u8; 128];

            assert!(bytes.len() <= digits.len());

            let mut i = 0;

            while i < bytes.len() {
                let mut insert = |b| digits[i] |= b;

                // in memory:
                // o n e X X X X X
                // in the integer bytes:
                // X X X X X e n o
                // this out of bounds read is UB under SB, but fine under models that don't do provenance narrowing with slices. i dont care enough to fix it.
                let block = bytes.as_ptr().add(i).cast::<u64>().read_unaligned().to_le();

                let one = (block & ((1 << (8)) - 1)) as u8;
                let three = block & ((1 << (8 * 3)) - 1);
                let four = block & ((1 << (8 * 4)) - 1);
                let five = block & ((1 << (8 * 5)) - 1);

                const fn gorble(s: &[u8]) -> u64 {
                    let mut bytes = [0; 8];
                    let mut i = 0;
                    while i < s.len() {
                        bytes[7 - i] = s[i];
                        i += 1;
                    }
                    // like: u64::from_be_bytes([0, 0, 0, b't', b'h', b'g', b'i', b'e'])
                    u64::from_be_bytes(bytes)
                }
                macro_rules! check {
                    ($const:ident $len:ident == $str:expr => $value:expr) => {
                        const $const: u64 = gorble($str);
                        insert(if $len == $const { $value } else { 0 });
                    };
                }

                insert(if one.is_ascii_digit() { one } else { 0 });

                check!(EIGHT five == b"eight" => b'8');
                check!(SEVEN five == b"seven" => b'7');
                check!(THREE five == b"three" => b'3');

                check!(FIVE four == b"five" => b'5');
                check!(FOUR four == b"four" => b'4');
                check!(NINE four == b"nine" => b'9');

                check!(SIX three == b"six" => b'6');
                check!(TWO three == b"two" => b'2');
                check!(ONE three == b"one" => b'1');

                i += 1;
            }

            let first = digits[..bytes.len()].iter().find(|&&d| d > b'0').unwrap();
            let last = digits[..bytes.len()]
                .iter()
                .rev()
                .find(|&&d| d > b'0')
                .unwrap();

            let first = (first - b'0') as u64;
            let last = (last - b'0') as u64;

            first * 10 + last
        })
        .sum::<u64>();

    sum
}