advent-of-code/source/year_2020/day_15/mod.rs

//! Day 15 of 2020.

use crate::prelude::*;

/// Get the solution for day 15 of 2020.
pub fn solution() -> Solution {
  Solution::new(Day::new(15, 2020), part_1, part_2).with_expected(441, 10613991)
}

/// Solve the puzzle for a given target.
fn solve(input: &str, target: usize) -> isize {
  let mut numbers = HashMap::new();
  let mut previous_number = (0, (0, 0));

  for (index, number) in input
    .trim()
    .split(',')
    .map(str::parse::<isize>)
    .map(Result::unwrap)
    .enumerate()
  {
    numbers.insert(number, (index, index));
    previous_number = (number, (index, index));
  }

  for index in numbers.len()..target {
    let next_num = previous_number.1 .1 - previous_number.1 .0;
    let num = numbers
      .entry(next_num as isize)
      .or_insert_with(|| (index, index));
    num.0 = num.1;
    num.1 = index;

    previous_number = (next_num as isize, *num);
  }

  *numbers.iter().max_by(|a, b| a.1 .1.cmp(&b.1 .1)).unwrap().0
}

/// The logic to solve part one.
fn part_1(input: &str) -> Result<String> {
  Ok(solve(input, 2020).to_string())
}

/// The logic to solve part two.
fn part_2(input: &str) -> Result<String> {
  Ok(solve(input, 30000000).to_string())
}