Hash computation and TFB

[Luboš Doležel]

Hello,

hash computation with Toll-Free Bridging is a tricky subject. Do it 
wrong and you'll get all sorts of trouble, especially with dictionaries, 
which use hashes a lot.

The code in corebase currently dispatches all CFHash() calls on ObjC 
objects to -hash, which is bad. The following expectation breaks due to 
this dispatch:

CFHash(@"string") == CFHash(CFSTR("string"))

because NSString uses a different hashing algorithm than CFString.
My suggestion is to do away with the ObjC dispatch in CFHash() and 
alter all the CF*Hash() functions to support ObjC types.

While looking at CFStringHash(), I've also noticed that either 8-bit or 
16-bit raw character data is used for hashing based on what is 
available. I believe this breaks the following case:

===
CFStringRef str1 = CFSTR("str");
CFStringRef str2 = CFStringCreateWithCharacters(NULL, (UniChar*) 
"s\0t\0r\0", 3); // "str" in UTF-16

CFHash(str1) == CFHash(str2);
===

While the two strings are obviously identical, different bytes are used 
to generate the hash in both cases.

This problem can by solved by converting the character data to Unicode 
first, which has a performance impact, but only once for every CFString.

The situation with CFHash() calls on NSStrings is worse, since corebase 
has nowhere to save the calculated hash, so it must be recalculated 
every time. But I think it's better to be slow than to be wrong. Please 
review the attached patch and let me know if you have any observations.

--
Luboš Doležel

corebase-hash-fix.patch
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