Reddit Daily Programmer 321 Intermediate

Intermediate.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace Reddit
{
    internal static class Intermediate
    {
        internal static void Run(string input)
        {
            var items = new List<Result>();
            int size = Alphabet.Length;
            for (int i = 0; i < size * size; i++)
            {
                int a = i / size;
                int b = i % size;
                if (HighestCommonFactor(a, size) == 1)
                {
                    var item = new Result()
                    {
                        A = a,
                        B = b
                    };
                    items.Add(item);
                }
            }
            foreach (var item in items)
            {
                item.Message = Decrypt(item.A, item.B, input);
            }
            foreach (var item in items)
            {
                var words = Split(item.Message);
                var comparer = StringComparer.OrdinalIgnoreCase;
                int score = 0;
                foreach (var word in words)
                {
                    if (Words.Contains(word, comparer))
                    {
                        score = score + 1;
                    }
                }
                item.Score = score;
            }
            int max = items.Max(e => e.Score);
            foreach (var item in items
                .Where(e => e.Score == max))
            {
                Console.WriteLine(item.Message);
            }
        }

        internal class Result
        {
            internal int A;
            internal int B;
            internal string Message;
            internal int Score;
        }

        internal static char[] Alphabet = "abcdefghijklmnopqrstuvwxyz".ToArray();

        internal static int HighestCommonFactor(int first, int second)
        {
            int a = Math.Abs(first);
            int b = Math.Abs(second);
            int c;
            while (b != 0)
            {
                c = b;
                b = a % b;
                a = c;
            }
            return a;
        }

        internal static string Decrypt(int a, int b, string message)
        {
            char[] array = Affine(a, b);
            return Substitute(array, Alphabet, message);
        }

        internal static char[] Affine(int a, int b)
        {
            int size = Alphabet.Length;
            char[] array = new char[size];
            for (int i = 0; i < size; i++)
            {
                int index = ((a * i) + b) % size;
                array[i] = Alphabet[index];
            }
            return array;
        }

        internal static string Substitute(char[] first, char[] second, string message)
        {
            var builder = new StringBuilder();
            foreach (char c in message)
            {
                int i = Array.IndexOf(first, char.ToLower(c));
                if (i == -1)
                {
                    builder.Append(c);
                }
                else
                {
                    if (char.IsUpper(c))
                    {
                        builder.Append(char.ToUpper(second[i]));
                    }
                    else
                    {
                        builder.Append(second[i]);
                    }
                }
            }
            return builder.ToString();
        }

        internal static List<string> Split(string message)
        {
            var items = new List<string>();
            char[] separator = new char[] { ' ', '.' };
            using (var reader = new StringReader(message))
            {
                string value;
                while ((value = reader.ReadLine()) != null)
                {
                    items.AddRange(value.Split(separator,
                        StringSplitOptions.RemoveEmptyEntries));
                }
            }
            return items;
        }

        internal static List<string> Words = new List<string>();

        internal static void Load(string path)
        {
            Words.AddRange(File.ReadAllLines(path));
        }
    }
}