advent-of-code/source/year_2021/day_08/mod.rs

//! Day 08 of 2021.

mod display;

use display::{CharSet, Display};

use crate::prelude::*;

/// Get the solution for day 08 of 2021.
pub fn solution() -> Solution {
  Solution::new(Day::new(8, 2021), part_1, part_2).with_expected(521, 1016804)
}

/// The logic to solve part one.
fn part_1(input: &str) -> Result<String> {
  Ok(
    input
      .lines()
      .map(|line| {
        line
          .split(" | ")
          .nth(1)
          .ok_or_else(|| eyre!("Invalid input: {}", line))
      })
      .collect::<Result<Vec<_>>>()?
      .into_iter()
      .flat_map(|line| line.split(' ').map(Display::parse))
      .filter(|display| [2, 4, 3, 7].contains(&display.0))
      .count()
      .to_string(),
  )
}

/// The logic to solve part two.
fn part_2(input: &str) -> Result<String> {
  let mut sum = 0;

  for line in input.lines() {
    let mut split = line.split(" | ");

    // Figure out the displays from the signal side.
    let displays = Display::figure_out_from_others(
      split
        .next()
        .ok_or_else(|| eyre!("Invalid input: {}", line))?
        .split(' ')
        .map(Display::parse)
        .collect::<Vec<_>>(),
    )?;

    // Get all the CharSets from the encoded side.
    let encoded = split
      .next()
      .ok_or_else(|| eyre!("Invalid input: {}", line))?
      .split(' ')
      .map(str::chars)
      .map(CharSet::from_iter);

    // Loop through the encoded numbers backwards so we can use the loop index
    // to multiply it by 10 to the power of the index.
    // So 123 would be (3 * 1) + (2 * 10) + (1 * 100).
    for (index, set) in encoded.rev().enumerate() {
      let decoded_number = displays
        .iter()
        .find(|display| display.1 == &set)
        .map(|display| display.0)
        .ok_or_else(|| eyre!("Impossible to decode {:?}", set))?;

      sum += 10_isize.pow(index as u32) * decoded_number;
    }
  }

  Ok(sum.to_string())
}