On Wed, Aug 06, 2008 at 08:25:14AM +0200, Stefan Behnel wrote:
> Hmm, was that in my patch? Out of the top of my head, shouldn't the last line
> read
>
> xmlDictComputeBigKey(prefix, -1, xmlDictComputeBigKey(name, len, len))))
>
> or something in that line? This looks like a copy&paste error to me...
err, yes, actually we have
#define xmlDictComputeKey(dict, name, len) \
(((dict)->size == MIN_DICT_SIZE) ? \
xmlDictComputeFastKey(name, len) : \
xmlDictComputeBigKey(name, len, len)) \
#define xmlDictComputeQKey(dict, prefix, name, len) \
(((prefix) == NULL) ? \
(xmlDictComputeKey(dict, name, len)) : \
(((dict)->size == MIN_DICT_SIZE) ? \
xmlDictComputeFastQKey(prefix, name, len) : \
xmlDictComputeBigKey(prefix, xmlStrlen(prefix), \
xmlDictComputeBigKey(name, len, len))))
> Anyway:
>
> > the problem is that basically if you compute the key for a QName
> > as "a:b" you can get 2 different answers, one if you accessed it using
> > "a:b" directly and hence xmlDictComputeKey() or if using "a" prefix and
> > "b" name, given the algorithm the key are not the same, and it is a key
> > property of the dictionary to always return the same exact pointer for
> > the same string. This breaks that property.
>
> True, I didn't know about this property. And the 4-byte-at-once property will
> really make this very hard to achieve.
yes, that's the core of the problem
> A way I see to fix this is to search the string for the first ':' and always
> calculate the hash separately for the part before and after the ':', not
> including the ':' itself. That should not break hashing namespace URIs either
> (AFAIR, at the least the XML namespace gets hashed at some point). Something
> like
>
> int len = strlen(s)
> char* prefix_end = strchr(s, ':')
> if (prefix_end)
> h = xmlDictComputeBigKey(s, prefix_end-s,
> xmlDictComputeBigKey(prefix_end+1, len-(prefix_end-s-1),
> len-(prefix_end-s-1)))
> else
> h = xmlDictComputeBigKey(s, len, len)
Another option I looked at is the 'One-at-a-Time Hash' from
http://burtleburtle.net/bob/hash/doobs.html , looking at the criterias
and the results it looks like a good hash too, not too expensive and
should work well. I will try to make a patch using this this morning,
if you have a bit of time then, maybe you can rerun your initial tests
with that one, is that possible ?
Daniel
--
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard | virtualization library http://libvirt.org/
[EMAIL PROTECTED] | libxml GNOME XML XSLT toolkit http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/
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