SOExample, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            Console.WriteLine(ShuffleSubstrings("abcdabcdabcd", 4, 3));
            Console.WriteLine(ShuffleSubstrings("abcdefghijkl", 4, 3));
        }
        
        private static Random _random = new Random();

public static string ShuffleSubstrings(string input, int n, int k)
        {
            if (input.Length != (n * k))
            {
                throw new ArgumentException("Length of input is not equal to kn");
            }

var characters = input.ToCharArray();
            for (int g = 0; g < input.Length; g += n)
            {
                ShuffleSubarray(characters, g, n);
            }
            return new string(characters);
        }

private static void ShuffleSubarray<T>(T[] array, int startPosition, int length)
        {
            // For loop to handle individual group
            for (int i = startPosition; i < startPosition + length; ++i)
            {
                // shuffle taken from taken from https://www.dotnetperls.com/fisher-yates-shuffle, modified to work with groups)
                int r = i + _random.Next(length - (i % length));
                T tmp = array[r];
                array[r] = array[i];
                array[i] = tmp;
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,06 sec, average memory usage: 12 Mb, average nr of threads: 2, absolute service time: 0,27 sec