using System;

using System.Collections.Generic;

using System.Collections.Immutable;

using System.Linq;

public class Program

{

    public static void Main()

    {

      const string input = "aaaabbbcca";

​

      var imperativeCounts = GetLettersCount(input);

      var functionalCounts = GetLettersCountFunctional(input);

​

      Console.WriteLine($"Imperative: [{string.Join(", ", imperativeCounts)}]");

      Console.WriteLine($"Functional: [{string.Join(", ", functionalCounts)}]");

    }

    private static ICollection<(char letter, int count)> GetLettersCount(string input)

    {

      var output = new List<LetterCount>();

​

      foreach (var @char in input)

      {

        var lastOutput = output.LastOrDefault();

        if (

          lastOutput is null ||

          lastOutput.Letter != @char)

        {

          lastOutput = new(@char);

          output.Add(lastOutput);

        }

​

        lastOutput.Count++;

      }

​

      return output

        .Select(letterCount => (letterCount.Letter, letterCount.Count))

        .ToArray();

    }

​

    private sealed class LetterCount

    {

      public LetterCount(char letter)

      {

        Letter = letter;

      }

​

      public char Letter { get; }

​

      public int Count { get; set; }

    }

​

    private static ICollection<(char letter, int count)> GetLettersCountFunctional(string input)

    {

      return input

        .SliceRepeatedLetters()

        .Select(slice => (slice[0], slice.Length))

        .ToArray();

    }

}

​

public static class StringCountStringExtensions

{

    public static int IndexOf(this string input, Func<char, bool> predicate)

    {

      try

      {

        return input

          .Select((@char, index) => (@char, index))

          .First(pair => predicate(pair.@char))

          .index;

      }

      catch (InvalidOperationException)

      {

        return -1;

      }

    }

​

    public static (string repeatedBlock, string remaining) GetRepeatedBlock(this string input)

    {

      var nextCharIndex = input

        .IndexOf(@char => @char != input[0]);

​

      var sliceLength = nextCharIndex == -1

        ? input.Length

        : nextCharIndex;

​

      return (

        input.Substring(0, sliceLength),

        input.Substring(sliceLength));

    }

​

    public static ICollection<string> SliceRepeatedLetters(this string input)

    {

      ICollection<string> DoSlice(string remainingInput, ImmutableList<string> results)

      {

        if (remainingInput.Length == 0) return results;

​

        var (block, remaining) = remainingInput.GetRepeatedBlock();

​

        return DoSlice(remaining, results.Add(block));

      }

​

      return DoSlice(input, ImmutableList<string>.Empty);

    }

}