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

//! [`Display`] implementation.

use std::collections::{HashMap, HashSet};

use color_eyre::Result;

/// Type alias for [`HashSet`] with [`char`]s.
pub type CharSet = HashSet<char>;

/// The display with a given size and set of characters.
#[derive(Debug)]
pub struct Display(pub isize, pub CharSet);

impl Display {
  /// Parse a [`Display`] from a string.
  pub fn parse(s: &str) -> Self {
    Self(s.len() as isize, CharSet::from_iter(s.chars()))
  }

  /// Calculate the remaining displays based on the existing other ones.
  pub fn figure_out_from_others(
    others: Vec<Self>,
  ) -> Result<HashMap<isize, CharSet>> {
    let mut displays = HashMap::<isize, CharSet>::new();

    // Loop over all displays infinitely and stop when we've figured out all 10.
    for display in others.iter().cycle() {
      if displays.len() == 10 {
        break;
      }

      let length = display.0;
      let charset = display.1.clone();

      // First check for the known unique segment lengths and insert them.
      if let Some(key) = match length {
        2 => Some(1),
        4 => Some(4),
        3 => Some(7),
        7 => Some(8),
        _ => None,
      } {
        displays.insert(key, charset);
        continue;
      }

      // Without the 1 and 4 segments we can't do anything else so grab those
      // or continue the loop.
      let (one, four) = match (displays.get(&1), displays.get(&4)) {
        (Some(one), Some(four)) => (one, four),
        _ => continue,
      };

      // Create the subset we'll use to figure out 0 and 5 using the difference
      // between 4 and 1.
      let five_zero_subset = CharSet::from_iter(four.difference(one).copied());

      // The segments for 2, 3 and 5 all have a length of 5.
      if length == 5 {
        // Only 3 has 1 as its subset.
        if one.is_subset(&charset) {
          displays.insert(3, charset);
          continue;
        }

        // Then only 5 will have the subset created earlier.
        if five_zero_subset.is_subset(&charset) {
          displays.insert(5, charset);
          continue;
        }

        // Then to figure out 2 we need to know both 3 and 5 so continue if we
        // haven't gotten them yet.
        let (three, five) = match (displays.get(&3), displays.get(&5)) {
          (Some(three), Some(five)) => (three, five),
          _ => continue,
        };

        // Then 2 will simply be the remaining set of length 5 as long as it
        // doesn't equal 3 or 5.
        if ![three, five].contains(&&charset) {
          displays.insert(2, charset);
          continue;
        }
      }

      // The segments for 0, 6 and 9 all have length 6.
      if length == 6 {
        // Only 6 *does not* have the subset for 1.
        if !one.is_subset(&charset) {
          displays.insert(6, charset);
          continue;
        }

        // Then 0 *does not* have this subset from earlier.
        if !five_zero_subset.is_subset(&charset) {
          displays.insert(0, charset);
          continue;
        }

        let (zero, six) = match (displays.get(&0), displays.get(&6)) {
          (Some(zero), Some(six)) => (zero, six),
          _ => continue,
        };

        // And again to figure out the remaining segment we use the other 2
        // we know now.
        if ![zero, six].contains(&&charset) {
          displays.insert(9, charset);
          continue;
        }
      }
    }

    Ok(displays)
  }
}