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

namespace AdventOfCode {

  public class Program {

    static string RootDir =
        Environment.GetEnvironmentVariable("HOME") + "/src/adventofcode/2020/";

    static List<int> ReadInts(string filename) {
      string[] lines = File.ReadAllLines(filename);
      return lines.Select(s => int.Parse(s)).ToList();
    }

    static int Day1Part1(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 Day1Part2(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]
    static void Day1Test() {
      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 = ReadInts(RootDir + "day1.txt");

      Assert.Throws<ArgumentException>(() => Day1Part1(emptyInput));
      Assert.Throws<ArgumentException>(() => Day1Part1(bogusInput));
      Assert.Equal(514579, Day1Part1(exampleInput));
      Assert.Equal(355875, Day1Part1(testInput));

      Assert.Throws<ArgumentException>(() => Day1Part2(emptyInput));
      Assert.Throws<ArgumentException>(() => Day1Part2(bogusInput));
      Assert.Equal(241861950, Day1Part2(exampleInput));
      Assert.Equal(140379120, Day1Part2(testInput));
    }

    static void Main(string[] args) {
      Day1Test();
    }
  }
}