aoc-2020/15.rs

use std::collections::HashMap;
use std::fs;

fn find_nth(numbers: &[usize], end: usize) -> usize {
    let numbers: Vec<usize> = numbers.iter().map(|i| *i).collect();

    let mut last_spoken: HashMap<usize, usize> = HashMap::new();

    let mut last: usize = *numbers.last().unwrap();

    let mut numbers: Vec<usize> = numbers[0..numbers.len() - 1].iter().map(|i| *i).collect();

    for (i, num) in numbers.iter().enumerate() {
        last_spoken.insert(*num, i);
    }

    for i in numbers.len()..end {
        let next = match last_spoken.get(&(last)) {
            Some(index) => i - index,
            None => 0,
        };
        numbers.push(last);
        last_spoken.insert(last, i);
        last = next;
    }

    *numbers.last().unwrap()
}

fn main() {
    let num_str = fs::read_to_string("15.input").unwrap();
    let num_str = num_str.trim();

    let nums: Vec<usize> = num_str
        .split(",")
        .map(|s| s.parse::<usize>().unwrap())
        .collect();

    println!("Answer 1: {}", find_nth(&nums, 2020));
    println!("Answer 2: {}", find_nth(&nums, 30000000));
}