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Subject: Re: [boost] [UTF String] Feedback on UTF String library, please
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2011-02-12 12:05:26

Brent Spillner wrote:
>>Just when is it really valid to want to jump the 278th "abstract
>>character" in a string?
>>Seriously, how often do these situations arise?
> Well, Boyer-Moore string searching, obviously.

If you're doing Boyer-Moore on a UTF-8 string you should normally do it
on the bytes, not on the codepoints, so there is no need for random
access to the codepoints. This works because of the particular
properties of UTF-8, and will be faster because you don't have the
overhead of the UTF-8 decoding.

The exception would be if you want to do comparison at the "grapheme
cluster" level, i.e. if you want to consider a single-codepoint,
multi-byte representation of e.g. an accented character as equal to the
multiple-codepoint, multi-byte representation comprising a base
character plus a combining accent. I would suggest that it would be
better to first normalise your strings, so that this doesn't arise.

Even if you did want to do Boyer-Moore on the codepoint-sequence, I
think that using std::advance to move a bidirectional iterator would
not change its complexity order.

I believe this is also true for UTF-16, but I have less experience with that.

> If strings for you are just handles that you pass to other people's
> APIs, then no, you don't need random access to the contents. But if
> you're seriously aiming to create a Unicode-aware drop-in replacement
> for existing string classes, then it ought to support all of the
> existing code that assumes efficient random access. And if you can
> provide measurable speedup for at least the common case, even at the
> cost of a few extra bits per data chunk, that's worth pursuing.

Are you suggesting some sort of indexed string? I guess you could use
an augmented tree to provide O(log N) access to the N'th character in a
UTF-8 string. In fact didn't someone propose something like that a few
months ago? Not as a string but as a "vector whose iterators are not
invalided" or "list with random access" or something?

Regards, Phil.

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