AOC/4 (C#)

Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

namespace cs_day_4
{
    class Program
    {
        static void Main(string[] args)
        {
            // Note: 'Resource' is a class which contains the input for the puzzle and the 
            // integer value of 'a'.
        
            var time = Stopwatch.StartNew();
            Console.WriteLine(CountValidPassphrases(Resource.Input));
            time.Stop();
            Console.WriteLine(time.ElapsedMilliseconds);
        }

        static int CountValidPassphrases(string input)
        {
            var lines = input.Split(Environment.NewLine, StringSplitOptions.RemoveEmptyEntries);
            return lines.AsParallel().Count(IsValid);
        }

        static bool IsValid(string input)
        {
            var words = input.Split(' ');
            var set = new HashSet<WordMap>();

            for (var i = 0; i < words.Length; i++)
            {
                var map = new WordMap(words[i]);
                if (!set.Add(map))
                {
                    return false;
                }
            }
            return true;
        }

        struct WordMap
        {
            public byte[] Counts;

            public WordMap(string word)
            {
                Counts = new byte[26];
                for (var i = 0; i < word.Length; i++)
                {
                    var idx = word[i] - Resource.AOffset;
                    ++Counts[idx];
                }
            }

            // This one isn't black magic, but it sure as hell is slow.
            override public bool Equals(object obj)
            {
                return obj is WordMap && ((WordMap)obj).Counts.SequenceEqual(Counts);
            }

            // Black magic from:
            // https://stackoverflow.com/questions/3404715/c-sharp-hashcode-for-array-of-ints
            override public int GetHashCode()
            {
                var hc = Counts.Length;
                for (var i = 0; i < Counts.Length; i++)
                {
                    hc = unchecked(hc * 17 + Counts[i]);
                }
                return hc;
            }
        }
    }
}