Dmitry Rubanovich added the comment:
Tim,
I am not testing randomly. The scripts calculate secondary hash for **each**
value in a ring to see how often this results in duplicate (or triplicate,
etc.) values. And they do it for each (mod 2**i) ring for i between 8 and 20.
Then (mostly as an afterthought) the scripts calculate how often each scheme
results in a collision if more than 1 secondary hash index is calculated. I
used 3 secondary indexes as a demonstration of the "afterthought" part.
I do understand that the logic relies on being able to reach each value in
0..-1+2**i in the worst case. Isn't that though accomplished by my latest
proposal? It was this:
"...unroll the 1st secondary hash key to use j = 2*j + P + 1. So try to test
for it before the loop. But leave 5*j + P + 1 in the loop as is."
In the code that would mean changing of the loop in, for example,
lookdict_index() from
for (size_t perturb = hash;;) {
perturb >>= PERTURB_SHIFT;
i = mask & ((i << 2) + i + perturb + 1);
ix = dk_get_index(k, i);
if (ix == index) {
return i;
}
if (ix == DKIX_EMPTY) {
return DKIX_EMPTY;
}
}
to this:
size_t perturb;
....
perturb = hash;
i = mask & ((i << 1) + perturb + 1); /* <---- key line */
ix = dk_get_index(k, i);
if (ix == index) {
return i;
}
if (ix == DKIX_EMPTY) {
return DKIX_EMPTY;
}
for (;;) {
/* nothing changes in this loop */
perturb >>= PERTURB_SHIFT;
i = mask & ((i << 2) + i + perturb + 1);
ix = dk_get_index(k, i);
if (ix == index) {
return i;
}
if (ix == DKIX_EMPTY) {
return DKIX_EMPTY;
}
}
And, of course, it would mean adding the same precheck in front of all loops
which go through this secondary index calculation.
This prevents preventable collisions for the hashes, h, such that
h mod 2**k is equal to (h >> 5) mod 2**k,
where 2**k is the dict size. This frequency of such occurrence for each dict
size is what's printed by
print_collision_counts(py3_6_1_lookdict_perturb)
in either of the attached scripts. Given that, for instance, it's 91 for
dict's of size 256, it seems rather ripe for improvement.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30671>
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