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POHow do I reverse a UTF-8 string in place?
 

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  1. POHow do I reverse a UTF-8 string in place?
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    <p>Recently, someone asked about an <a href="https://stackoverflow.com/questions/198199/how-do-you-reverse-a-string-in-place-in-c-or-c">algorithm for reversing a string in place in C</a>. Most of the proposed solutions had troubles when dealing with non single-byte strings. So, I was wondering what could be a good algorithm for dealing specifically with utf-8 strings.</p> <p>I came up with some code, which I'm posting as an answer, but I'd be glad to see other people's ideas or suggestions. I preferred to use actual code, so I've chosen C#, as it seems to be one of the most popular language in this site, but I don't mind if your code is in another language, as long as it could be reasonably understood by anyone who is familiar with an imperative language. And, as this is intended to see how such an algorithm could be implemented at a low-level (by low-level I just mean dealing with bytes), the idea is to avoid using libraries for the core code.</p> <p><strong>Notes:</strong></p> <p>I'm interested in the algorithm itself, its performance and how could it be optimized (I mean algorithm-level optimization, not replacing i++ with ++i and such; I'm not really interested in actual benchmarks either).</p> <p>I don't mean to actually use it in production code or "reinventing the wheel". This is just out of curiosity and as an exercise.</p> <p>I'm using C# byte arrays so I'm assuming you can get the length of the string without running though the string until you find a NUL. That is, I'm not accounting for the complexity of finding the length of the string. But if you're using C, for instance, you could factor that out by using strlen() before calling the core code. </p> <p><strong>Edit:</strong></p> <p>As Mike F points out, my code (and other people's code posted here) is not dealing with composite characters. Some info about those <a href="http://www.unicode.org/faq/char_combmark.html" rel="nofollow noreferrer">here</a>. I'm not familiar with the concept, but if that means that there are "combining characters", i.e., characters / code points that are only valid in combination with other "base" characters / code points, a look-up table of such characters could be used to preserve the order of the "global" character ("base" + "combining" characters) when reversing.</p>
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    1. USJuan Pablo Califano
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    1. COThis is a fun question, but in order to *usefully* reverse a Unicode string (UTF8 or otherwise) you have to worry about preserving the order of composite characters as well as juggling bytes.
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    2. COThanks for the heads up. I was not aware of composite characters. I'll look that up first.
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