using System;

using System.Linq;

using System.Threading.Tasks.Dataflow;

using System.Collections.Generic;

using System.Net.Http;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Fetching The Iliad of Homer to find reversed words");

        // Downloads the requested resource as a string.

        var downloadString = new TransformBlock<string, string>(uri =>

        {

            Console.WriteLine($"{DateTime.Now}: Downloading '{0}'...", uri);

            var data = new HttpClient().GetStringAsync(uri).ConfigureAwait(false).GetAwaiter().GetResult();

            Console.WriteLine($"{DateTime.Now}: Downloaded '{0}'...", uri);

            return data;

        });

        // Separates the specified text into an array of words.

        var createWordList = new TransformBlock<string, string[]>(text =>

        {

           Console.WriteLine("Creating word list...");

           // Remove common punctuation by replacing all non-letter characters 

           // with a space character.

           char[] tokens = text.Select(c => char.IsLetter(c) ? c : ' ').ToArray();

           text = new string(tokens);

           // Separate the text into an array of words.

           return text.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);

        });

        // Removes short words and duplicates.

        var filterWordList = new TransformBlock<string[], string[]>(words =>

        {

           Console.WriteLine("Filtering word list...");

           return words

              .Where(word => word.Length > 3)

              .Distinct()

              .ToArray();

        });

        // Finds all words in the specified collection whose reverse also 

        // exists in the collection.

        var findReversedWords = new TransformManyBlock<string[], string>(words =>

        {

           Console.WriteLine("Finding reversed words...");

           var wordsSet = new HashSet<string>(words);

           return from word in words.AsParallel()

                  let reverse = new string(word.Reverse().ToArray())

                  where word != reverse && wordsSet.Contains(reverse)

                  select word;

        });

        // Prints the provided reversed words to the console.    

        var printReversedWords = new ActionBlock<string>(reversedWord =>

        {

           Console.WriteLine("Found reversed words {0}/{1}",

              reversedWord, new string(reversedWord.Reverse().ToArray()));

        });

        var linkOptions = new DataflowLinkOptions { PropagateCompletion = true };

​

        downloadString.LinkTo(createWordList, linkOptions);

        createWordList.LinkTo(filterWordList, linkOptions);

        filterWordList.LinkTo(findReversedWords, linkOptions);

        findReversedWords.LinkTo(printReversedWords, linkOptions);

​

        // downloads The Iliad of Homer 

        downloadString.Post("http://www.gutenberg.org/files/6130/6130-0.txt");

        downloadString.Complete();

        printReversedWords.Completion.Wait();

    }

}