On Saturday, 25 May 2013 at 19:02:43 UTC, Joakim wrote:
On Saturday, 25 May 2013 at 18:09:26 UTC, Diggory wrote:
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...
No, that directly _contradicts_ what you said about Unicode
having "nothing to do with code pages." All UCS did is take a
bunch of existing code pages and standardize them into one
massive character set. For example, ISCII was a pre-existing
single-byte encoding and Unicode "largely preserves the ISCII
layout within each block."
http://en.wikipedia.org/wiki/ISCII
All a code page is is a table of mappings, UCS is just a much
larger, standardized table of such mappings.
UCS does have nothing to do with code pages, it was designed as a
replacement for them. A codepage is a strict subset of the
possible characters, UCS is the entire set of possible characters.
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.
I see you've abandoned without note your claim that phobos
doesn't convert UTF-8 to UTF-32 internally. Perhaps converting
to UTF-32 is "as fast as any variable width encoding is going
to get" but my claim is that single-byte encodings will be
faster.
I haven't "abandoned my claim". It's a simple fact that phobos
does not convert UTF-8 string to UTF-32 strings before it uses
them.
ie. the difference between this:
string mystr = ...;
dstring temp = mystr.to!dstring;
for (int i = 0; i < temp.length; ++i)
process(temp[i]);
and this:
string mystr = ...;
size_t i = 0;
while (i < mystr.length) {
dchar current = decode(mystr, i);
process(current);
}
And if you can't see why the latter example is far more efficient
I give up...
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.
I wouldn't be so fast to ditch this. There is a real argument
to be made that strings of different languages are sufficiently
different that there should be no multi-language strings. Is
this the best route? I'm not sure, but I certainly wouldn't
dismiss it out of hand.
- 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.
I disagree, this would still be far faster than UTF-8,
particularly if you designed your header right.
The cache misses alone caused by simply accessing the separate
headers would be a larger overhead than decoding UTF-8 which
takes a few assembly instructions and has perfect locality and
can be efficiently pipelined by the CPU.
Then there's all the extra processing involved combining the
headers when you concatenate strings. Plus you lose the one
benefit a fixed width encoding has because random access is no
longer possible without first finding out which header controls
the location you want to access.
- 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.
I didn't think of this possibility, but you may be right that
it's sub-optimal.
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...
No, it's not the same at all. The contents of an
arbitrary-length file cannot be compressed to a single byte,
you would have collisions galore. But since most non-english
alphabets are less than 256 characters, they can all be
uniquely encoded in a single byte per character, with the
header determining what language's code page to use. I don't
understand your analogy whatsoever.
It's very simple - the more information about the type of data
you are compressing you have at the time of writing the algorithm
the better compression ration you can get, to the point that if
you know exactly what the file is going to contain you can
compress it to nothing. This is why you have specialised
compression algorithms for images, video, audio, etc.
It doesn't matter how few characters non-english alphabets have -
unless you know WHICH alphabet it is before-hand you can't store
it in a single byte. Since any given character could be in any
alphabet the best you can do is look at the probabilities of
different characters appearing and use shorter representations
for more common ones. (This is the basis for all lossless
compression) The english alphabet plus 0-9 and basic punctuation
are by far the most common characters used on computers so it
makes sense to use one byte for those and multiple bytes for
rarer characters.
- 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
You haven't shown this.
If you had thought through your suggestion of multiple code pages
per string you would see that I had.
- Given the frequency distribution of unicode characters,
UTF-8 does a pretty good job at encoding higher frequency
characters in fewer bytes.
No, it does a very bad job of this. Every non-ASCII character
takes at least two bytes to encode, whereas my single-byte
encoding scheme would encode every alphabet with less than 256
characters in a single byte.
And strings with mixed characters would use lots of memory and be
extremely slow. Common when using proper names, quotes, inline
translations, graphical characters, etc. etc. Not to mention the
added complexity to actually implement the algorithms.
