#15367: Empty lists while creating parents
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Reporter: roed | Owner:
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.13
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: Nils Bruin | Reviewers: Simon King
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/nbruin/ticket/15367 | b95fa73c3fca342851dde858028a12629c6147ee
Dependencies: | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:33 nbruin]:
> Replying to [comment:32 SimonKing]:
> > Can you elaborate: What is `PyCapsule`? I've never heard of it.
>
> It's a python object wrapping a `void *`:
http://docs.python.org/2/c-api/capsule.html , so it does exactly what we
need.
Thank you for the pointer!
> > Concerning the hash function: Didn't we have the current hash function
`<size_t>key1 + 13*(<size_t>key2) ^ 503*(<size_t>key3)` in the original
implementation? If I recall correctly, I chose it because it was
recommended as a good hash function for tuples.
>
> It is indeed a good hash function, particularly for moduli that are a
power of 2. Where did you get it from? or was it already in the code?
It has been in the code (as I have just checked by looking at the patch
from #715).
When we tried to create a generalisation of `TripleDict` towards a
dictionary with fixed key length at #12313, I did some google search
concerning hash functions for tuples. I don't recall the web page, but it
resulted in this code, see attachment "idkey_dict" at #12313:
{{{
cdef list find_bucket(self, tuple keytuple):
# Determine the appropriate bucket
cdef size_t h = 0x345678
cdef size_t i
for i from 0<=i<self._keylen:
h *= 1000003
h ^= <size_t>PyTuple_GET_ITEM(keytuple,i)
cdef list all_buckets = self.buckets
return <object>PyList_GET_ITEM(all_buckets, h %
PyList_GET_SIZE(all_buckets))
}}}
--
Ticket URL: <http://trac.sagemath.org/ticket/15367#comment:36>
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