adventofcode/2020/Day07.cs

using System;
using static System.Console;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
using Xunit;

namespace AdventOfCode {

  struct Contains {
    public Contains(string outer, string inner, int count) {
      Outer = outer;
      Inner = inner;
      Count = count;
    }
    public string Outer;
    public string Inner;
    public int Count;

    public override string ToString() {
      return $"{Outer} contains {Count} {Inner}";
    }
  }

  public class Day07 {

    static void AddRules(string rule, List<Contains> contains) {
      string[] tokens = rule.Split(" bags contain ");
      string color = tokens[0];
      string[] otherBags = tokens[1].Split(", ");
      foreach (var s in otherBags) {
        Match m = Regex.Match(s, @"^(\d+) (\w+ \w+) bag");
        if (!m.Success) {
          continue;
        }
        int count = int.Parse(m.Groups[1].Value);
        Contains c = new Contains(color, m.Groups[2].Value, count);
        contains.Add(c);
      }
    }

    static List<Contains> ParseRules(string[] rules) {
      var contains = new List<Contains>();
      rules.ToList().ForEach(rule => AddRules(rule, contains));
      return contains;
    }

    static int WhatCanContain(List<Contains> contains, string target) {
      var result = new HashSet<string>();
      int setSize = 0;
      result.Add(target);

      // transitive closure: keep adding things, until a round of doing so doesn't add anything new
      while (true) {
        foreach (Contains c in contains) {
          if (result.Contains(c.Inner)) {
            result.Add(c.Outer);
          }
        }
        if (result.Count() == setSize) {  // nothing new to add, we're done
          return result.Count() - 1;  // our target doesn't actually contain itself
        }
        setSize = result.Count();
      }
    }

    static int CountBags(List<Contains> rules, string target) {
      int total = 1;
      foreach (Contains rule in rules) {
        if (rule.Outer == target) {
          total += rule.Count * CountBags(rules, rule.Inner);
        }
      }
      return total;
    }

    static int Part1() {
      List<Contains> contains = ParseRules(File.ReadAllLines(Util.RootDir + "day07.txt"));
      return WhatCanContain(contains, "shiny gold");
    }

    static int Part2() {
      List<Contains> contains = ParseRules(File.ReadAllLines(Util.RootDir + "day07.txt"));
      return CountBags(contains, "shiny gold") - 1;
    }

    [Fact]
    public static void Test() {
      string[] example =
@"light red bags contain 1 bright white bag, 2 muted yellow bags.
dark orange bags contain 3 bright white bags, 4 muted yellow bags.
bright white bags contain 1 shiny gold bag.
muted yellow bags contain 2 shiny gold bags, 9 faded blue bags.
shiny gold bags contain 1 dark olive bag, 2 vibrant plum bags.
dark olive bags contain 3 faded blue bags, 4 dotted black bags.
vibrant plum bags contain 5 faded blue bags, 6 dotted black bags.
faded blue bags contain no other bags.
dotted black bags contain no other bags.".Split('\n');

      Assert.Equal(4, WhatCanContain(ParseRules(example), "shiny gold"));
      Assert.Equal(213, Part1());

      string[] example2 =
@"shiny gold bags contain 2 dark red bags.
dark red bags contain 2 dark orange bags.
dark orange bags contain 2 dark yellow bags.
dark yellow bags contain 2 dark green bags.
dark green bags contain 2 dark blue bags.
dark blue bags contain 2 dark violet bags.
dark violet bags contain no other bags.".Split('\n');
      Assert.Equal(127, CountBags(ParseRules(example2), "shiny gold"));

      Assert.Equal(38426, Part2());
    }
  }
}