y2023-d1-original.rs

fn compute_result(input: &str) -> u32 {
    input
        .replace(char::is_alphabetic, "")
        .lines()
        .map(|l| {
            let mut iter = l.chars().flat_map(|c| char::to_digit(c, 10)).peekable();

            let first = *iter.peek().expect("no first numerical character");
            let last = iter.last().expect("no last numerical character");

            first * 10 + last
        })
        .sum()
}

fn solve_part1(input: &str) -> Box<dyn std::fmt::Display> {
    let res = compute_result(input.trim_end());

    Box::new(res)
}

fn replace_spelled_out_digits(input: &str) -> String {
    let mut ret = String::new();
    let mut slice = input;

    let numerals = vec![
        ("one", '1'),
        ("two", '2'),
        ("three", '3'),
        ("four", '4'),
        ("five", '5'),
        ("six", '6'),
        ("seven", '7'),
        ("eight", '8'),
        ("nine", '9'),
    ];

    loop {
        let len = slice.len();

        if len < 3 {
            ret.push_str(slice);

            break;
        }

        let mut found = false;
        for (s, c) in numerals.iter() {
            if slice.starts_with(s) {
                ret.push(*c);
                found = true;
                break;
            }
        }

        if !found {
            // For simplicity sake, discard anything that we don't want.
            let next_char = slice.chars().next().expect("no next char");

            if next_char.is_numeric() || next_char == '\n' {
                ret.push(next_char);
            }
        }

        // Advance the slice char by char to make sure we don't miss cases where
        // the last letters of a numeral form the first letters of another one.
        slice = &slice[1..];
    }

    ret
}

fn solve_part2(input: &str) -> Box<dyn std::fmt::Display> {
    let replaced = replace_spelled_out_digits(input.trim_end());
    let res = compute_result(&replaced);

    Box::new(res)
}