advent-of-code/source/year_2020/day_05/mod.rs

use crate::prelude::*;

pub fn solution() -> Solution {
  Solution::new(Day::new(5, 2020), part_1, part_2).with_expected(850, 599)
}

const MAX_ROW_RANGE: i32 = 128;
const MAX_COLUMN_RANGE: i32 = 8;

fn calculate_row_and_column(input: &str) -> (i32, i32) {
  let mut row_range = 0..MAX_ROW_RANGE;
  let mut column_range = 0..MAX_COLUMN_RANGE;

  for character in input.chars() {
    let row_min = row_range.start;
    let row_max = row_range.end;
    let row_difference = (row_max - row_min) / 2;

    let column_min = column_range.start;
    let column_max = column_range.end;
    let column_difference = (column_max - column_min) / 2;

    match character {
      'F' => {
        row_range = row_min..(row_max - row_difference);
      }
      'B' => {
        row_range = (row_min + row_difference)..row_max;
      }
      'L' => {
        column_range = column_min..(column_max - column_difference);
      }
      'R' => {
        column_range = (column_min + column_difference)..column_max;
      }
      _ => unreachable!("{character}"),
    }
  }

  (row_range.start, column_range.start)
}

fn calculate_seat_id((row, column): (i32, i32)) -> i32 {
  (row * 8) + column
}

fn part_1(input: &str) -> Result<String> {
  Ok(
    input
      .lines()
      .map(calculate_row_and_column)
      .map(calculate_seat_id)
      .max()
      .unwrap()
      .to_string(),
  )
}

fn part_2(input: &str) -> Result<String> {
  let seat_ids = input
    .lines()
    .map(calculate_row_and_column)
    .map(calculate_seat_id)
    .collect::<Vec<i32>>();

  let max_seat_id = seat_ids.iter().max().unwrap();
  let min_seat_id = seat_ids.iter().min().unwrap();

  Ok(
    (*min_seat_id..*max_seat_id)
      .find(|index| !seat_ids.contains(index))
      .map(|id| id.to_string())
      .unwrap(),
  )
}