adventofcode/2020/Day09.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 {

  public class Day09 {

    static bool Valid(List<long> longs, int preambleSize, int target) {
      for (int i = target - preambleSize; i < target - 1; i++) {
        for (int j = i + 1; j < target; j++) {
          if (i == j) {
            continue;
          }
          if (longs[i] + longs[j] == longs[target]) {
            return true;
          }
        }
      }
      return false;
    }

    static long FindInvalid(List<long> longs, int preambleSize) {
      for (int target = preambleSize; target < longs.Count(); target++) {
        if (!Valid(longs, preambleSize, target)) {
          return longs[target];
        }
      }
      throw new Exception("didn't find an invalid int");
    }

    static long CalculateWeakness(List<long> longs, int minIndex, int maxIndex) {
      long largest = 0;
      long smallest = long.MaxValue;
      for (int i = minIndex; i <= maxIndex; i++) {
        if (longs[i] > largest) {
          largest = longs[i];
        }
        if (longs[i] < smallest) {
          smallest = longs[i];
        }
      }
      return smallest + largest;
    }

    static long FindWeakness(List<long> longs, long target) {
      for (int i = 0; i < longs.Count() - 1; i++) {
        long total = 0;
        for (int j = i; j < longs.Count() && total < target; j++) {
          total += longs[j];
          if (total == target) {
            return CalculateWeakness(longs, i, j);
          }
        }
      }
      throw new Exception("didn't find a weakness");
    }

    static long Part1() {
      string[] input = File.ReadAllLines(Util.RootDir + "day09.txt");
      List<long> longs = input.ToList().Select(long.Parse).ToList();
      return FindInvalid(longs, 25);
    }

    static long Part2(long target) {
      string[] input = File.ReadAllLines(Util.RootDir + "day09.txt");
      List<long> longs = input.ToList().Select(long.Parse).ToList();
      return FindWeakness(longs, target);
    }

    [Fact]
    public static void Test() {
      string[] example =
@"35
20
15
25
47
40
62
55
65
95
102
117
150
182
127
219
299
277
309
576".Split('\n');
      List<long> longs = example.ToList().Select(long.Parse).ToList();
      Assert.Equal(127, FindInvalid(longs, 5));

      long invalidNumber = Part1();
      Assert.Equal(50047984, invalidNumber);
      Assert.Equal(5407707, Part2(invalidNumber));
    }
  }
}