Re:UCS vs. UTF-8 as Internal String Encoding
by lisper on Thursday May 28, 2015 @07:30PM
(#9621850)
> But you don't need constant time access to code points for that, you just need constant time access to code units
Well, sort of. What you really want is constant-time access to (semantically meaningful) substrings. Not all code points are semantically meaningful, but the situation is even worse for code units. A pointer to a random code point may or may not designate the boundary of a semantically meaningful substring. But whatever problems that causes, the situation for code units is even worse, because a pointer to a random code unit may or may not designate the start of a code point, and even if it does then you still have the previous problem that a random code point may or may not designate yada yada yada. (And yes, I know that UTF-8 is self-synchronizing. But that doesn't help with the problem I'm talking about, and if you think it does then you haven't understood the problem.)
What you really want is constant-time dereferencing of designators for semantically meaningful substrings. But no language AFAIK actually has that. The fundamental problem is that most languages have painted themselves into a corner by carving into stone the fact that strings can be dereferenced by integers. Once you've done that, you're pretty much screwed. It's not that you can't make it work, it's just that it requires an awful lot of machinery. You basically need to build an index for every string you construct, and that can get very expensive. Fixed-width representations are a less-than-perfect-but-still-not-entirely-unreasonable engineering solution to this problem.
Well, sort of. What you really want is constant-time access to (semantically meaningful) substrings. Not all code points are semantically meaningful, but the situation is even worse for code units. A pointer to a random code point may or may not designate the boundary of a semantically meaningful substring. But whatever problems that causes, the situation for code units is even worse, because a pointer to a random code unit may or may not designate the start of a code point, and even if it does then you still have the previous problem that a random code point may or may not designate yada yada yada. (And yes, I know that UTF-8 is self-synchronizing. But that doesn't help with the problem I'm talking about, and if you think it does then you haven't understood the problem.)
What you really want is constant-time dereferencing of designators for semantically meaningful substrings. But no language AFAIK actually has that. The fundamental problem is that most languages have painted themselves into a corner by carving into stone the fact that strings can be dereferenced by integers. Once you've done that, you're pretty much screwed. It's not that you can't make it work, it's just that it requires an awful lot of machinery. You basically need to build an index for every string you construct, and that can get very expensive. Fixed-width representations are a less-than-perfect-but-still-not-entirely-unreasonable engineering solution to this problem.