fn main() {
    // a string of ascii characters
    const PUZZLE: &str = "AaBCDd";
    println!("Part 1: {}", part_1(PUZZLE));
}

fn part_1(input: &str) -> usize {
    // The idea is to drop characters from input into v one by one. If
    // the new character c and the last character d in v react then
    // they both vanish. If they don't react then v gets a bit bigger
    // and we go for the next drop.

    // For the example input "AaBCDd" I would expect a vector
    // v = 'B', 'D' (because A and a react and D and d react)
    // so the returned length would be 2.
    let mut v = Vec::new();
    for c in input.chars() {
        match v.last().cloned() {
            None => {
                v.push(c);
            }
            Some(d) => if react(&c, &d) {
                v.pop();
            } else {
                v.push(c)
            },
        }
    }
    return v.len();
}

/// characters react if they are the same ascii character of different
/// capitalisation.
/// A a -> True
/// a A -> True
/// A A -> False
/// a a -> False
/// A b -> False
fn react(c: &char, d: &char) -> bool {
    return (c != d) && c.eq_ignore_ascii_case(d);
}