Day 9 Advent of Code 2020

aoc2020-09.cs

using System;
using System.Collections.Generic;
using System.Linq;
using AdventOfCode.Library;

namespace AdventOfCode
{
    // Day 9
    internal static class Program
    {
        private static long _xmasKey;
        public static void Main()
        {
            var data = TextUtility.ReadLines(removeBlank: true);
            data.Process(true, 1, Solver1);
            data.Process(true, 2, Solver2);
        }

        private static string Solver1(List<string> arg1)
        {
            var numbers = arg1.Select(a => Convert.ToInt64(a)).ToList();
            for (int i = 25; i < arg1.Count; i++)
            {
                var slice = numbers.Skip(i - 25).Take(25).ToList();
                var valid = GetKeys(slice);

                if (!valid.Contains(numbers[i]))
                {
                    _xmasKey = numbers[i];
                    return numbers[i].ToString();
                }
            }
	        
            return "-S1";
        }

        private static string Solver2(List<string> arg1)
        {
            var numbers = arg1.Select(a => Convert.ToInt64(a)).ToList();

            for (int i = 0; i < numbers.Count; i++)
            {
                for (int k = 0; k < numbers.Count - i-2; k++)
                {
                    var subset = numbers.Skip(k).Take(i).ToList();
                    if (_xmasKey == subset.Sum() && subset.Count()>1)
                    {
                        var result = subset.Min() + subset.Max();
                        return $"{result}";
                    }
                }
            }

            return "-S2";
        }

        private static HashSet<long> GetKeys(IEnumerable<long> keys)
        {
            var values = keys.ToList();
            var result = new HashSet<long>();

            for (int i = 0; i < values.Count; i++)
            {
                for (int k = 0; k < values.Count; k++)
                {
                    if (i != k)
                    {
                        result.Add(values[i] + values[k]);
                    }
                }
            }

            return result;
        }
    }
}