[issue5186] Reduce hash collisions for objects with no __hash__ method
Raymond Hettinger
report at bugs.python.org
Thu Feb 12 03:08:47 CET 2009
Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:
[Antoine]
> Ok, updated patch:
> - uses a 4-bit rotate (not shift)
> - avoids comparing an unsigned long to -1
> - tries to streamline the win64 special path (but I can't test)
pointer_hash4.patch looks fine to me. Still, I think it's worth
considering the simpler and faster: x |= x>>4. The latter doesn't
require any special-casing for various pointer sizes. It just works.
[Adam]
> Adding an arbitrary set of OR, XOR, or add makes me uneasy;
> I know enough to do them wrong (reduce entropy), but not
> enough to do them right.
It's easy enough to prove (just show that the function is reversible)
and easy enough to test:
assert len(set(ids)) == len(set(map(f, set(ids)))) # for any large
group of ids
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