String extension Diff

gistfile1.cs

public static class Extensions
{        
  public static string Diff(this string first, string second)
  {
      var s1 = first.ToCharArray();
      var s2 = second.ToCharArray();
      int s1P = s1.Length, s2P = s2.Length;
      var num = new int[s1P + 1, s2P + 1];
      // fill array
      for (var i = 1; i <= s1P; i++)
          for (var j = 1; j <= s2P; j++)
              num[i, j] = (s1[i - 1] == (s2[j - 1]) ? 1 + num[i - 1, j - 1]
                      : Math.Max(num[i - 1, j], num[i, j - 1]));
      // trace
      var c2 = new List<char>();
      var c3 = new List<char>();
      int eq = 0, eqc = 0;

      while (s1P != 0 && s2P != 0)
      {
          if (s1[s1P - 1] == (s2[s2P - 1]))
          {
              s1P--;
              s2P--;
              eq++;
              c2.Insert(0, '=');
              c3.Insert(0, s1[s1P]);
          }
          else if (num[s1P, s2P - 1] >= num[s1P - 1, s2P])
          {
              s2P--;
              c2.Insert(0, '+');
              c3.Insert(0, s2[s2P]);
          }
          else
          {
              eq++;
              s1P--;
              c2.Insert(0, '-');
              c3.Insert(0, s1[s1P]);
          }
      }

      while (s2P-- > 0)
      {
          c2.Insert(0, '+');
          c3.Insert(0, s2[s2P]);
      }
      while (eq++ < s1.Length)
      {
          c2.Insert(0, '-');
          c3.Insert(eqc, s1[eqc++]);
      }

      var sb = new StringBuilder();
      foreach (var t in c2)
          sb.Append(t);
      sb.Append('\n');

      foreach (var t in c3)
          sb.Append(t);
      sb.Append('\n');
      return sb.ToString();
  }      
}