day-09.rs

use std::collections::VecDeque;

struct Circle {
    deque: VecDeque<u32>,
}

impl Circle {
    fn with_marbles(last_marble: u32) -> Self {
        let mut deque =
            VecDeque::with_capacity((last_marble + 1 - (last_marble / 23) * 2) as usize);
        deque.push_back(0);
        Circle { deque }
    }

    fn place_marble(&mut self, marble: u32) -> Option<u32> {
        if marble % 23 != 0 {
            self.move_cursor_clockwise();
            self.deque.push_back(marble);
            None
        } else {
            for _ in 0..7 {
                self.move_cursor_counter_clockwise();
            }
            let removed_marble = self.deque.pop_back().unwrap();
            self.move_cursor_clockwise();
            Some(removed_marble + marble)
        }
    }

    fn move_cursor_clockwise(&mut self) {
        let popped = self.deque.pop_front().unwrap();
        self.deque.push_back(popped);
    }

    fn move_cursor_counter_clockwise(&mut self) {
        let popped = self.deque.pop_back().unwrap();
        self.deque.push_front(popped);
    }
}

pub fn part1(players: usize, last_marble: u32) -> u32 {
    let mut scores = vec![0; players];

    let mut circle = Circle::with_marbles(last_marble);

    for (marble, player) in (1..=last_marble).zip((0..players).cycle()) {
        if let Some(score) = circle.place_marble(marble) {
            scores[player] += score;
        }
    }

    scores.into_iter().max().unwrap()
}

pub fn part2(players: usize, last_marble: u32) -> u32 {
    part1(players, 100 * last_marble)
}