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> ints, 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 (ints[i] + ints[j] == ints[target]) {
            return true;
          }
        }
      }
      return false;
    }

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

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

    static int Part2() {
      return -1;
    }

    [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> ints = example.ToList().Select(long.Parse).ToList();
      Assert.Equal(127, FindInvalid(ints, 5));

      Assert.Equal(50047984, Part1());
      Assert.Equal(-1, Part2());
    }
  }
}