BurrowsWheelerTransform.cs

using System;
using System.Linq;

namespace Algorithms.DataCompression
{
    /// <summary>
    /// The Burrows–Wheeler transform (BWT) rearranges a character string into runs of similar characters.
    /// This is useful for compression, since it tends to be easy to compress a string that has runs of repeated characters.
    /// See <a href="https://en.wikipedia.org/wiki/Burrows%E2%80%93Wheeler_transform">here</a> for more info.
    /// </summary>
    public class BurrowsWheelerTransform
    {
        /// <summary>
        /// Encodes the input string using BWT and returns encoded string and the index of original string in the sorted rotation matrix.
        /// </summary>
        /// <param name="s">Input string.</param>
        public (string encoded, int index) Encode(string s)
        {
            if (s.Length == 0)
            {
                return (string.Empty, 0);
            }

            var rotations = GetRotations(s);
            Array.Sort(rotations, StringComparer.Ordinal);
            var lastColumn = rotations
                .Select(x => x[x.Length - 1])
                .ToArray();
            var encoded = new string(lastColumn);
            return (encoded, Array.IndexOf(rotations, s));
        }

        /// <summary>
        /// Decodes the input string and returns original string.
        /// </summary>
        /// <param name="s">Encoded string.</param>
        /// <param name="index">Index  of original string in the sorted rotation matrix.</param>
        public string Decode(string s, int index)
        {
            if (s.Length == 0)
            {
                return string.Empty;
            }

            var rotations = new string[s.Length];

            for (var i = 0; i < s.Length; i++)
            {
                for (var j = 0; j < s.Length; j++)
                {
                    rotations[j] = s[j] + rotations[j];
                }

                Array.Sort(rotations, StringComparer.Ordinal);
            }

            return rotations[index];
        }

        private string[] GetRotations(string s)
        {
            var result = new string[s.Length];

            for (var i = 0; i < s.Length; i++)
            {
                result[i] = s.Substring(i) + s.Substring(0, i);
            }

            return result;
        }
    }
}