advent-of-code/source/year_2021/day_07/mod.rs

use crate::prelude::*;

pub fn solution() -> Solution {
  Solution::new(Day::new(7, 2021), part_1, part_2)
    .with_expected(328318, 89791146)
}

fn parse_crabs(input: &str) -> Result<(Vec<isize>, isize)> {
  let crabs = input
    .split(',')
    .map(str::parse)
    .collect::<Result<Vec<_>, _>>()?;

  let highest_crab = *crabs
    .iter()
    .max()
    .ok_or_else(|| eyre!("Unable to find highest crab"))?;

  Ok((crabs, highest_crab))
}

fn part_1(input: &str) -> Result<String> {
  let (crabs, highest_crab) = parse_crabs(input)?;
  (0..=highest_crab)
    .map(|target_position| {
      crabs
        .iter()
        .map(|crab| (target_position - crab).abs())
        .sum::<isize>()
    })
    .min()
    .map(|result| result.to_string())
    .ok_or_else(|| eyre!("Unable to find lowest fuel"))
}

fn part_2(input: &str) -> Result<String> {
  let (crabs, highest_crab) = parse_crabs(input)?;
  (0..=highest_crab)
    .map(|target_position| {
      crabs
        .iter()
        .map(|crab| (target_position - crab).abs())
        .map(|steps| (steps * (steps + 1)) / 2)
        .sum::<isize>()
    })
    .min()
    .map(|result| result.to_string())
    .ok_or_else(|| eyre!("Unable to find lowest fuel"))
}