Newbie question about text encoding

[random832 at fastmail.us]

On Fri, Mar 6, 2015, at 08:39, Chris Angelico wrote:
> Number of code points is the most logical way to length-limit
> something. If you want to allow users to set their display names but
> not to make arbitrarily long ones, limiting them to X code points is
> the safest way (and preferably do an NFC or NFD normalization before
> counting, for consistency);

Why are you length-limiting it? Storage space? Limit it in whatever
encoding they're stored in. Why are combining marks "pathological" but
surrogate characters not? Display space? Limit it by columns. If you're
going to allow a Japanese user's name to be twice as wide, you've got a
problem when you go to display it.

> this means you disallow pathological cases
> where every base character has innumerable combining marks added.

No it doesn't. If you limit it to, say, fifty, someone can still post
two base characters with twenty combining marks each. If you actually
want to disallow this, you've got to do more work. You've disallowed
some of the pathological cases, some of the time, by coincidence. And
limiting the number of UTF-8 bytes, or the number of UTF-16 code points,
will accomplish this just as well.

Now, if you intend to _silently truncate_ it to the desired length, you
certainly don't want to leave half a character in, of course. But who's
to say the base character plus first few combining marks aren't also
"half a character"? If you're _splitting_ a string, rather than merely
truncating it, you probably don't want those combining marks at the
beginning of part two.