day15.rs

#[aoc_generator(day15)]
fn input_generator(input: &str) -> Vec<usize> {
    input.split(',').map(|w| w.parse().unwrap()).collect()
}

#[aoc(day15, part1)]
fn part1(numbers: &[usize]) -> usize {
    make_turns(numbers, 2020)
}

#[aoc(day15, part2)]
fn part2(numbers: &[usize]) -> usize {
    make_turns(numbers, 30000000)
}

fn make_turns(numbers: &[usize], turns: usize) -> usize {
    let mut said = vec![std::usize::MAX; turns];
    for (turn, &n) in numbers.iter().enumerate() {
        said[n] = turn;
    }
    let mut last = numbers[numbers.len() - 1];
    for turn in numbers.len()..turns {
        let mut previous = turn - 1;
        std::mem::swap(&mut previous, &mut said[last]);
        last = (turn - 1).saturating_sub(previous);
    }
    last
}