On Saturday, 25 May 2013 at 08:07:42 UTC, Joakim wrote:
On Saturday, 25 May 2013 at 07:48:05 UTC, Diggory wrote:
I think you are a little confused about what unicode actually
is... Unicode has nothing to do with code pages and nobody
uses code pages any more except for compatibility with legacy
applications (with good reason!).
Incorrect.
"Unicode is an effort to include all characters from previous
code pages into a single character enumeration that can be used
with a number of encoding schemes... In practice the various
Unicode character set encodings have simply been assigned their
own code page numbers, and all the other code pages have been
technically redefined as encodings for various subsets of
Unicode."
http://en.wikipedia.org/wiki/Code_page#Relationship_to_Unicode
That confirms exactly what I just said...
You said that phobos converts UTF-8 strings to UTF-32 before
operating on them but that's not true. As it iterates over
UTF-8 strings it iterates over dchars rather than chars, but
that's not in any way inefficient so I don't really see the
problem.
And what's a dchar? Let's check:
dchar : unsigned 32 bit UTF-32
http://dlang.org/type.html
Of course that's inefficient, you are translating your whole
encoding over to a 32-bit encoding every time you need to
process it. Walter as much as said so up above.
Given that all the machine registers are at least 32-bits already
it doesn't make the slightest difference. The only additional
operations on top of ascii are when it's a multi-byte character,
and even then it's some simple bit manipulation which is as fast
as any variable width encoding is going to get.
The only alternatives to a variable width encoding I can see are:
- Single code page per string
This is completely useless because now you can't concatenate
strings of different code pages.
- Multiple code pages per string
This just makes everything overly complicated and is far slower
to decode what the actual character is than UTF-8.
- String with escape sequences to change code page
Can no longer access characters in the middle or end of the
string, you have to parse the entire string every time which
completely negates the benefit of a fixed width encoding.
- An encoding wide enough to store every character
This is just UTF-32.
Also your complaint that UTF-8 reserves the short characters
for the english alphabet is not really relevant - the
characters with longer encodings tend to be rarer (such as
special symbols) or carry more information (such as chinese
characters where the same sentence takes only about 1/3 the
number of characters).
The vast majority of non-english alphabets in UCS can be
encoded in a single byte. It is your exceptions that are not
relevant.
Well obviously... That's like saying "if you know what the exact
contents of a file are going to be anyway you can compress it to
a single byte!"
ie. It's possible to devise an encoding which will encode any
given string to an arbitrarily small size. It's still completely
useless because you'd have to know the string in advance...
- A useful encoding has to be able to handle every unicode
character
- As I've shown the only space-efficient way to do this is using
a variable length encoding like UTF-8
- Given the frequency distribution of unicode characters, UTF-8
does a pretty good job at encoding higher frequency characters in
fewer bytes.
- Yes you COULD encode non-english alphabets in a single byte but
doing so would be inefficient because it would mean the more
frequently used characters take more bytes to encode.
