On Wed, Nov 14, 2007 at 02:25:34PM +0000, James Shaw wrote:
I see this was discussed a couple of years back in the thread "equals
metamethod and hash functions"
http://lua-users.org/lists/lua-l/2005-06/msg00108.html. Some
alternatives were suggested, mostly involving string building.
I'm using tables as keys for another table, and I'm re-generating the
tables when I need the keys (I don't think there's a simple way
for me
to retain references to the original keys).
Coincidence ... I am using some userdata objects as table keys, and
just
today stumbled over the problem that the hashing algorithm seems to
use
the pointer to the actual userdata, however in some cases I
actually need
value semantics, i.e. I want two userdata objects to be equivalent
wrt.
table lookups whenever the pointees are equal...
I would guess that one of the reasons of the missing __hash meta
event is
that it's not that easy to design -- how do you make sure that the
__hash
meta event is "compatible" with the Lua hash function and the other
table
internals?
One spontaneous idea: Make __hash return a lightuserdata and a
number, i.e.
a pointer and a size, and then let Lua use the same algorithm as it
uses for
strings on the "returned" data.
This would hopefully be still faster than all the other
workarounds... (And
probably saner and more "compatible" than returning an "equally
distributed"
size_t that is then mapped down to an actual index.)
However I don't see how to implement such a __hash meta method in
Lua; this
idea is geared towards my currently problematic use case: C __hash
methods
for userdata objects.
Regards, Colin
