[issue34751] Hash collisions for tuples

[Tim Peters]

Tim Peters <tim at python.org> added the comment:

> >>> from itertools import product
> >>> len(set(map(hash, product([0.5, 0.25], repeat=20))))
> 32

> Good catch! Would you like me to add this to the testsuite?

It's in mine already ;-)  I've added all the "bad examples" in all the messages here.  Sooner or later they'll get folded into Python's test suite.

BTW, there were no collisions in that under whatever 64-bit Python I last compiled.  That was a SeaHash variant.  I'm not certain, but I believe it had "t ^= t << 1" at the start and with the first multiply commented out.

Having learning _something_ about why SeaHash does what it does, I'm not convinced the first multiply is of much value.  As a standalone bit-scrambler for a single 64-bit input, it does matter.  But in the tuple hash context, we're running it in a loop.  Strictly alternating "propagate left" and "propagate right" seems to me almost as good - although that's just intuition.

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<https://bugs.python.org/issue34751>
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