advent-of-code/source/year_2020/day_09/mod.rs

use crate::prelude::*;

pub fn solution() -> Solution {
  Solution::new(Day::new(9, 2020), part_1, part_2)
    .with_expected(22477624, 2980044)
}

const PREAMBLE_LENGTH: usize = 25;

fn parse_numbers(input: &str) -> Vec<usize> {
  input
    .lines()
    .map(|number| number.parse().unwrap())
    .collect()
}

fn part_1(input: &str) -> Result<String> {
  let numbers = parse_numbers(input);
  let mut result_one = 0;
  for (index, number) in numbers.iter().enumerate() {
    if index < PREAMBLE_LENGTH {
      continue;
    }

    let preamble = {
      let mut set = HashSet::new();
      let _numbers = numbers
        .iter()
        .skip(index - PREAMBLE_LENGTH)
        .take(PREAMBLE_LENGTH);

      for a in _numbers.clone() {
        for b in _numbers.clone() {
          set.insert(a + b);
        }
      }

      set
    };

    if !preamble.contains(number) {
      result_one = *number;
      break;
    }
  }

  Ok(result_one.to_string())
}

fn part_2(input: &str) -> Result<String> {
  let numbers = parse_numbers(input);
  let result_one = part_1(input)?;
  let mut result_two = 0;
  'outer: for preamble_length in 2..numbers.len() {
    for (index, _) in numbers.iter().enumerate() {
      if index < preamble_length {
        continue;
      }

      let preamble = numbers
        .iter()
        .skip(index - preamble_length)
        .take(preamble_length);

      let preamble_total = preamble.clone().sum::<usize>();

      if preamble_total.to_string() == result_one {
        let mut preamble = preamble.collect::<Vec<&usize>>();
        preamble.sort_unstable();
        result_two = *preamble.first().unwrap() + *preamble.last().unwrap();
        break 'outer;
      }
    }
  }

  Ok(result_two.to_string())
}