On 2/11/2001 17:05:25, Phil Endecott wrote:
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 discussion was about 'abstract characters,' not codepoints. I think a codepoint iterator is likely useful too, but for most applications the character view is required.
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
Yes, I think that's the behavior desired for most applications.
character plus a combining accent. I would suggest that it would be better to first normalise your strings, so that this doesn't arise.
...but I think that's a little too glib. If you have the string "résumé" in your data store, and a user enters "resume" as a search key, then you either have to search independently for each plausible variant for "resume," or you have to maintain a parallel, accent-free copy of the original text and the means to translate indices into the accent-free version into indices into the rich version, or you have to have some notion of 'abstract character' comparison and iteration. I think the last solution is by far the most preferable, particularly for a low-level library. This is how people actually use strings in practice, after all.
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.
It clearly won't; you need to ::advance at most O(n) times, and Boyer-Moore is already a linear complexity algorithm (and I'd be amazed to see a sublinear one for the general case.) But since the whole algorithm is linear, those extra O(n) iterator advancements past code units that you already know to be irrelevant could be a measurable contributor to the total run time.
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
Yes, that's what I was thinking.
months ago? Not as a string but as a "vector whose iterators are not invalided" or "list with random access" or something?
Good to know; I'll take a look at that.