« Detecting Bindings that should be OneTime | Main | Concurrency Pre-Con Highlights »
October 22, 2008
How to Reverse a Unicode String in C#
Perhaps due to the lack of a built-in String.Reverse method in the .NET Framework, it's very common (1, 2, 3, 4, 5, 6, 7) for implementations of such a method to be posted.
Unfortunately, most of these implementations do not handle characters outside Unicode's Basic Multilingual Plane correctly. These supplementary characters have code points between U+10000 and U+10FFFF and so cannot be represented with one 16-bit char. In UTF-16 (which is how .NET strings are encoded), these Unicode characters are represented as two C# chars, a high surrogate followed by a low surrogate. When the string is reversed, the order of these two chars has to be preserved.
Here's our method that reverses a string while handling surrogate code units correctly:
/// <summary>
/// Reverses the specified string.
/// </summary>
/// <param name="input">The string to reverse.</param>
/// <returns>The input string, reversed.</returns>
/// <remarks>This method correctly reverses strings containing supplementary characters
/// (which are encoded with two surrogate code units).</remarks>
public static string Reverse(this string input)
{
if (input == null)
throw new ArgumentNullException("input");
// allocate a buffer to hold the output
char[] output = new char[input.Length];
for (int outputIndex = 0, inputIndex = input.Length - 1; outputIndex < input.Length; outputIndex++, inputIndex--)
{
// check for surrogate pair
if (input[inputIndex] >= 0xDC00 && input[inputIndex] <= 0xDFFF &&
inputIndex > 0 && input[inputIndex - 1] >= 0xD800 && input[inputIndex - 1] <= 0xDBFF)
{
// preserve the order of the surrogate pair code units
output[outputIndex + 1] = input[inputIndex];
output[outputIndex] = input[inputIndex - 1];
outputIndex++;
inputIndex--;
}
else
{
output[outputIndex] = input[inputIndex];
}
}
return new string(output);
}
Posted by Bradley Grainger at October 22, 2008 9:07 PM
Trackback Pings
TrackBack URL for this entry:
http://blog.logos.com/mt-cgi/mt-tb.cgi/261
Comments
Thanks Bradley - how does the speed of it compare to the others? Are there ways to optimize this?
Posted by: Guy Ellis at October 22, 2008 9:35 PM
I haven't profiled this function for performance--mostly because I've never been reversing strings in a performance-critical section of my code.
I suspect that there would be performance penalties due to the repeated array index checks (that are automatically inserted by the CLR) each time through the loop; it's possible that unsafe code could avoid these checks and run faster.
When writing utility libraries, however, I prefer to stick to safe code (because it may not always be used in a Full Trust application).
Posted by: Bradley Grainger 
at October 22, 2008 10:19 PM
Hey don't u think you need to increment outputIndex and decrement inputIndex twice if you get a surrogate pair?
This code
output[outputIndex + 1] = input[inputIndex];
output[outputIndex] = input[inputIndex - 1];
outputIndex++;
inputIndex--;
Posted by: Sid at July 17, 2009 10:31 PM
Yes: the variables are incremented (decremented) once in the if statement and then again in the for statement's loop expression.
Posted by: Bradley Grainger at July 18, 2009 7:49 AM