Rotating a string

Code snippet to rotate a string:

static void Main(string[] args)
{
 string subject = "abcdefg";

 // test harness, check it handles multiples of size and negatives and zero
 for (int amount = -subject.Length * 3; amount < subject.Length * 3; amount++)
 {
  string result = RotateStringTest(subject, amount);

  char[] inplace = subject.ToCharArray();

  RotateString(inplace, amount);

  if (amount % subject.Length == 0)
   Debug.Assert(result.CompareTo(subject) == 0);
  Debug.Assert(result.CompareTo(new String(inplace)) == 0);
 }
}

/// <summary>
/// Rotates a string
/// </summary>
/// <param name="subject">string to rotate</param>
/// <param name="amount">&lt; 0 left, 0 none, &gt; 0 right</param>
/// <returns></returns>
private static void RotateString(char[] subject, int amount)
{
 int length = subject.Length;

 // negate rotations can be re-expressed as positive amounts
 if (amount < 0)
  amount = length - (-amount % length);

 // 0 rotation mean return unchanged
 if (amount % length == 0)
  return;

 int dest = amount;
 char prev = subject[0];

 do
 {
  dest %= length;

  Swap(ref subject[dest], ref prev);

  dest += amount;
 } while (dest != amount);
}

/// <summary>
/// swaps two chars wihout using temp variable
/// </summary>
private static void Swap(ref char a, ref char b)
{
 a ^= b;
 b ^= a;
 a ^= b;
}

/// <summary>
/// Rotates a string test (slow)
/// </summary>
/// <param name="subject">string to rotate</param>
/// <param name="amount">&lt; 0 left, 0 none, &gt; 0 right</param>
/// <returns></returns>
private static string RotateStringTest(string subject, int amount)
{
 char[] result = subject.ToCharArray();
 int length = subject.Length;

 // negate rotations can be re-expressed as positive amounts
 if (amount < 0)
  amount = length - (-amount % length);

 // 0 rotation mean return unchanged
 if (amount % length != 0)
 {
  for (int pos = 0; pos < length; pos++)
  {
   int dest = (pos + amount) % length;

   result[dest] = subject[pos];
  }
 }

 return new String(result);
}

ps this post was really just to test this javascript code hightlighter.. It seems to work very well for both Xaml and C# - cool!