advent-of-code/2021/day8/rust/src/v3.rs

use crate::LineResult;

const FREQ_TABLE: [usize; 256] = {
    let mut tab = [0; 256];
    tab[17] = 1;
    tab[25] = 7;
    tab[30] = 4;
    tab[34] = 2;
    tab[37] = 5;
    tab[39] = 3;
    tab[41] = 6;
    tab[42] = 0;
    tab[45] = 9;
    tab[49] = 8;
    tab
};

#[must_use]
#[inline]
pub fn unscramble(line: &str) -> LineResult {
    let mut bytes = line.bytes();

    let mut counts = [0; 7];
    loop {
        match bytes.next().unwrap() {
            c @ b'a'..=b'g' => {
                counts[c as usize - b'a' as usize] += 1;
            }
            b'|' => break,
            _ => {}
        }
    }

    bytes.next();

    let mut freq = 0;
    let mut unique_digits = 0;
    let mut number = 0;
    loop {
        let c = bytes.next();
        match c {
            Some(b' ') | None => {
                let digit = FREQ_TABLE[freq & 0xff];
                if [1, 4, 7, 8].contains(&digit) {
                    unique_digits += 1;
                }
                number = number * 10 + digit;
                freq = 0;
                if c.is_none() {
                    break;
                }
            }
            Some(c @ b'a'..=b'g') => {
                freq += counts[c as usize - b'a' as usize];
            },
            Some(_) => {}
        }
    }

    LineResult {
        unique_digits,
        number,
    }
}