adventofcode/2020/Day01.cs

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

namespace AdventOfCode {

  public class Day01 {

    static int Part1(List<int> numbers) {
      for (var i = 0; i < numbers.Count; i++) {
        for (var j = i + 1; j < numbers.Count; j++) {
          if (numbers[i] + numbers[j] == 2020) {
            return numbers[i] * numbers[j];
          }
        }
      }
      throw new ArgumentException("no pair of numbers summed to 2020");
    }

    static int Part2(List<int> numbers) {
      for (var i = 0; i < numbers.Count; i++) {
        for (var j = i + 1; j < numbers.Count; j++) {
          for (var k = j + 1; k < numbers.Count; k++) {
            if (numbers[i] + numbers[j] + numbers[k] == 2020) {
              return numbers[i] * numbers[j] * numbers[k];
            }
          }
        }
      }
      throw new ArgumentException("no triple of numbers summed to 2020");
    }

    [Fact]
    public static void Test() {
      var emptyInput = new List<int>();
      var bogusInput = new List<int>() {
        111, 222, 333, 444, 555, 666, 777, 888, 999
      };
      var exampleInput = new List<int>() {
        1721, 979, 366, 299, 675, 1456
      };
      var testInput = Util.ReadInts(Util.RootDir + "day01.txt");

      Assert.Throws<ArgumentException>(() => Part1(emptyInput));
      Assert.Throws<ArgumentException>(() => Part1(bogusInput));
      Assert.Equal(514579, Part1(exampleInput));
      Assert.Equal(355875, Part1(testInput));

      Assert.Throws<ArgumentException>(() => Part2(emptyInput));
      Assert.Throws<ArgumentException>(() => Part2(bogusInput));
      Assert.Equal(241861950, Part2(exampleInput));
      Assert.Equal(140379120, Part2(testInput));
    }
  }
}