Palindromes in C#

Introduction

This code calculates if a string is a palindrome, and was inspired by the Channel9 video by Gary Daniels and Evan Goldring - Mock whiteboard problem.

Background

There are two main approaches to calculating palindromes: byte symmetry, or on the basis of words. Byte symmetry checks to see if a string is a mirror image of itself "helloolleh", whilst the other approach handles white spaces "Was it Eliot's toilet I saw?"

Using the code

I've not inluded the code as an attachment, rather as inline text that can be used directly.

static bool isPalindrome(string subject)
{
  const int STEP_FORWARD = 1;
  const int STEP_BACKWARD = -STEP_FORWARD;
  const int BEFORE_START = -1;
  const int CHAR_AT_A_TIME = 1;
  const int COMPARE_EQUALS = 0;

  // assume its not a palindrome
  bool result = false;

  // how to compare
  CompareInfo ci = CultureInfo.CurrentCulture.CompareInfo;
  CompareOptions co =
    CompareOptions.IgnoreCase |
    CompareOptions.IgnoreKanaType |
    CompareOptions.IgnoreNonSpace |
    CompareOptions.IgnoreSymbols |
    CompareOptions.IgnoreWidth; |

  // null strings are not palindromes
  if (subject != null)
  {
    int AFTER_END = subject.Length;

    // single letter words are palindromes
    result = (AFTER_END == 1 && IsPalindromeChar(subject[0]));

    // start the comparison points at valid characters
    int startOffset = GetNextValidCharacter(subject, BEFORE_START, 
                                         STEP_FORWARD, AFTER_END);
    int endOffset = GetNextValidCharacter(subject, AFTER_END, 
                                STEP_BACKWARD, BEFORE_START);

    while (startOffset < endOffset)
    {
      result = ci.Compare(subject, startOffset, CHAR_AT_A_TIME, 
               subject, endOffset, CHAR_AT_A_TIME, co) == COMPARE_EQUALS;
  
      if (!result)
        break;

      // move the comparison points towards each other
      startOffset = GetNextValidCharacter(subject, startOffset, 
                                      STEP_FORWARD, endOffset);
      endOffset = GetNextValidCharacter(subject, endOffset, 
                               STEP_BACKWARD, startOffset);
    }
  }

  return result;
}

static int GetNextValidCharacter(string subject, int offset, 
                                 int step, int bound)
{
  if (offset != bound)
    offset += step;

  while (offset != bound && !IsPalindromeChar(subject[offset]))
    offset += step;

  return offset;
}   

static bool IsPalindromeChar(char c)
{
  return char.IsLetter(c) || char.IsDigit(c);
}

Points of interest

Reading the Wikipedia entry on palindromes shows that there are more than just alphabetical palindromes, so it is worth checking with your users before assuming that this code is what they mean by a palindrome.

On my blog, I've also included the byte symmetry code approach.

History

• Aug/2008 - First version.