using System;

namespace ShuffleCharacters
{
    public static class StringExtension
    {
        /// <summary>
        /// Shuffles characters in source string according some rule.
        /// </summary>
        /// <param name="source">The source string.</param>
        /// <param name="count">The count of iterations.</param>
        /// <returns>Result string.</returns>
        /// <exception cref="ArgumentException">Source string is null or empty or white spaces.</exception>
        /// <exception cref="ArgumentException">Count of iterations is less than 0.</exception>
        public static string ShuffleChars(string source, int count)
        {
            if (string.IsNullOrWhiteSpace(source))
            {
                throw new ArgumentException("Source string is null or empty or white spaces.", nameof(source));
            }

            if (count < 0)
            {
                throw new ArgumentException("Count of iterations is less than 0", nameof(count));
            }

            bool SequenceEqual(char[] a, char[] b)
            {
                for (int i = 0; i < a.Length; i++)
                {
                    if (a[i] != b[i])
                    {
                        return false;
                    }
                }

                return true;
            }

            char[] result = source.ToCharArray();
            char[] array = new char[source.Length];
            char[] start = new char[source.Length];
            Array.Copy(result, start, result.Length);
            int t = 0;
            int middle = (source.Length / 2) + (source.Length % 2);

            for (int i = 0; i < count; i++)
            {
                (array, result) = (result, array);

                for (int j = 0; j < middle; j++)
                {
                    result[j] = array[j * 2];
                }

                for (int j = middle, k = 1; j < source.Length; j++, k += 2)
                {
                    result[j] = array[k];
                }

                t++;

                if (SequenceEqual(result, start))
                {
                    while (count > i + t)
                    {
                        count -= t;
                    }
                }
            }

            return new string(result);
        }
    }
}