Simple solution to the longest non-repeating substring problem

LongestSubstringSolution.cs

using System.Runtime.CompilerServices;

namespace DSA {
  public class LongestSubstringProblem {
    /// <summary>
    /// Solves the LongestSubstringProblem in O(N) time without any generic data structures or custom data structures.
    /// </summary>
    /// <param name="input"></param>
    /// <returns></returns>
    public static string SimpleBitMaskSolution(string input) {
      if (string.IsNullOrEmpty(input))
        return string.Empty;

      var candidateStart = 0;
      var candidateLength = 1;
      var mapMask = 0U;
      var mapCount = 0;
      int start = 0;
      const char a = 'a';

      foreach (var character in input) {
        var currentCharacterMask = 1U << (character - a);
        while ((mapMask & currentCharacterMask) != 0) {
          var startCharacterMask = 1U << (input[start] - a);
          mapMask &= ~startCharacterMask;
          mapCount--;
          start++;
        }
        // Mark the character as present in the current substring.
        mapMask |= currentCharacterMask;
        mapCount++;
        if (candidateLength < mapCount) {
          candidateStart = start;
          candidateLength = mapCount;
        }
      }
      return input.Substring(candidateStart, candidateLength);
    }
}