advent-of-code/2020/src/bin/day09.rs

#![feature(test)]
extern crate test;
use aoc2020::common::*;

use itertools::{Itertools, MinMaxResult};

fn read_input() -> String {
    read_file(9)
}

fn parse_input(raw: &str) -> Vec<usize> {
    raw.lines().map(|l| l.parse().unwrap()).collect()
}

fn part1(input: &[usize], window_size: usize) -> usize {
    *input
        .windows(window_size + 1)
        .find_map(|xs| {
            xs.last()
                .filter(|last| !xs[..window_size].iter().tuple_combinations().any(|(a, b)| a + b == **last))
        })
        .unwrap()
}

fn part2(input: &[usize], bad_num: usize) -> usize {
    (0..input.len()).find_map(|i| sum_up_to(&input[i..], bad_num)).unwrap()
}

fn sum_up_to(input: &[usize], n: usize) -> Option<usize> {
    let mut acc = input[0];
    for x in 1..input.len() {
        acc += input[x];
        if acc == n {
            return match input[..x].iter().minmax() {
                MinMaxResult::MinMax(min, max) => Some(min + max),
                _ => unreachable!(),
            };
        }
        if acc > n {
            return None;
        }
    }
    None
}

fn main() {
    let input = parse_input(&read_input());
    let p1 = part1(&input, 25);
    println!("Part 1: {}", p1);
    println!("Part 2: {}", part2(&input, p1));
}

#[cfg(test)]
mod tests {
    use super::*;
    use test::black_box;
    use aoc2020::*;
    use paste::paste;

    const TEST_INPUT: &str = "35
20
15
25
47
40
62
55
65
95
102
117
150
182
127
219
299
277
309
576";

    test!(part1(5) == 127);
    test!(part2(127) == 62);
    bench!(part1(25) == 393911906);
    bench!(part2(393911906) == 59341885);
}