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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Can anyone improve this ?
Date: Sun, 16 Jan 2022 12:12:45 -0800
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Bonita Montero <Bonita.Montero@gmail.com> writes:
>
>> sizes instead of size;  corrected:
>>
>> size_t utf8Strlen( char const *str )
>> {
>>   static size_t const sizes[8] = { 1, 0, 2, 3, 4, 0, 0, 0 };
>>   size_t len = 0;
>>   for( unsigned char c; (c = *str); )
>>   {
>>     size_t size = sizes[countl_zero<unsigned char>( ~c )];
>>     if( !size ) [[unlikely]]
>>       return -1;
>>     ++len;
>>     for( char const *cpEnd = str + size; ++str != cpEnd; )
>>       if( ((unsigned char)*str & 0x0C0) != 0x080 ) [[unlikely]]
>>         return -1;
>>   }
>>   return len;
>> }
>
> You reject simpler solutions because they don't detect the one error
> you have decided to look for.  There are other encoding errors that
> this code won't catch.  Seems a bit arbitrary to me.  [...]

(Sorry, accidental send)

I wouldn't say arbitrary.  The function verifies that its input is
syntactically well-formed while ignoring the question of whether it
is semantically well-formed.  That choice may not be ideal but I
don't think it's totally unreasonable.