Case-insensitive UTF-8 string collation for SQLite (C/C++)

Question

I am looking for a method to compare and sort UTF-8 strings in C++ in a case-insensitive manner to use it in a custom collation function in SQLite.

1. The method shou

Answer 1

I believe you will need to roll your own or use an third party library. I recommend a third party library because there are a lot of rules that need to be followed to get true international support - best to let someone who is an expert deal with them.

Answer 2

What you really want is logically impossible. There is no locale-independent, case-insensitive way of sorting strings. The simple counter-example is "i" <> "I" ? The naive answer is no, but in Turkish these strings are unequal. "i" is uppercased to "İ" (U+130 Latin Capital I with dot above)

UTF-8 strings add extra complexity to the question. They're perfectly valid multi-byte char* strings, if you have an appropriate locale. But neither the C nor the C++ standard defines such a locale; check with your vendor (too many embedded vendors, sorry, no genearl answer here). So you HAVE to pick a locale whose multi-byte encoding is UTF-8, for the mbscmp function to work. This of course influences the sort order, which is locale dependent. And if you have NO locale in which const char* is UTF-8, you can't use this trick at all. (As I understand it, Microsoft's CRT suffers from this. Their multi-byte code only handles characters up to 2 bytes; UTF-8 needs 3)

wchar_t is not the standard solution either. It supposedly is so wide that you don't have to deal with multi-byte encodings, but your collation will still depend on locale (LC_COLLATE) . However, using wchar_t means you now choose locales that do not use UTF-8 for const char*.

With this done, you can basically write your own ordering by converting strings to lowercase and comparing them. It's not perfect. Do you expect L"ß" == L"ss" ? They're not even the same length. Yet, for a German you have to consider them equal. Can you live with that?

Answer 3

I don't think there's a standard C/C++ library function you can use. You'll have to roll your own or use a 3rd-party library. The full Unicode specification for locale-specific collation can be found here: http://www.unicode.org/reports/tr10/ (warning: this is a long document).

Answer 4

I have no definitive answer in the form of example code, but I should point out that an UTF-8 bytestream contains, in fact, Unicode characters and you have to use the wchar_t versions of the C/C++ runtime library.

You have to convert those UTF-8 bytes into wchar_t strings first, though. This is not very hard, as the UTF-8 encoding standard is very well documented. I know this, because I've done it, but I can't share that code with you.

Answer 5

If you are using it to do searching and sorting for your locale only, I suggest your function to call a simple replace function that convert both multi-byte strings into one byte per char ones using a table like:

A -> a
à -> a
á -> a
ß -> ss
Ç -> c
and so on

Then simply call strcmp and return the results.

Answer 6

On Windows you can call fall back on the OS function CompareStringW and use the NORM_IGNORECASE flag. You'll have to convert your UTF-8 strings to UTF-16 first. Otherwise, take a look at IBM's International Components for Unicode.