day15.rs

//! # Timing is Everything
//!
//! Part one is the [Chinese Remainder Theorem](https://en.wikipedia.org/wiki/Chinese_remainder_theorem).
//! The integers n₁, n₂, ... nₖ map to the disc sizes which happen to be prime. This satisfies the
//! requirement that the integers are [pairwise coprime](https://en.wikipedia.org/wiki/Coprime_integers#Coprimality_in_sets).
//!
//! For simplicity we use the "search by sieving" method. We start at zero with a step the size of
//! the first integer. Then we search for each subsequent integer located at the correct offset of
//! minutes and multiply the step by the new integer. This preserve the relative offset at each step
//! in the next search.
use crate::util::iter::*;
use crate::util::parse::*;

type Disc = [usize; 2];

pub fn parse(input: &str) -> Vec<Disc> {
    input.iter_unsigned().skip(1).step_by(2).chunk::<2>().collect()
}

pub fn part1(input: &[Disc]) -> usize {
    solve(input)
}

pub fn part2(input: &[Disc]) -> usize {
    let mut modified = input.to_vec();
    modified.push([11, 0]);
    solve(&modified)
}

fn solve(discs: &[Disc]) -> usize {
    let mut time = 0;
    let mut step = 1;

    for (offset, &[size, position]) in discs.iter().enumerate() {
        while (time + offset + 1 + position) % size != 0 {
            time += step;
        }

        step *= size;
    }

    time
}