prevent unnecessary hash collisions for Pair
#57285
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nsajko
added
hashing
collections
nsajko
commented
Feb 6, 2025
nsajko
commented
Feb 6, 2025
nsajko
commented
Feb 6, 2025
base/pair.jl
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| const _pairhash_seed = if UInt === UInt64 | ||
| 0x94cb2bb20a28ce96 | ||
| else | ||
| 0x1f60a087 | ||
| end::UInt | ||
| hash(p::Pair, h::UInt) = hash(p.second, hash(p.first, hash(_pairhash_seed, h))) |
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FWIW, this also passes the new tests while being simpler/cheaper:
Suggested change
| const _pairhash_seed = if UInt === UInt64 | |
| 0x94cb2bb20a28ce96 | |
| else | |
| 0x1f60a087 | |
| end::UInt | |
| hash(p::Pair, h::UInt) = hash(p.second, hash(p.first, hash(_pairhash_seed, h))) | |
| hash(p::Pair, h::UInt) = hash(p.second, hash(p.first, ~h)) |
I think it relies on hash(p.first, ~h) != ~hash(p.first, h). Not sure whether it has any other disadvantages.
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The problem with this suggestion is that another collection, unrelated to Pair, could define a hash method with the same body, resulting in unnecessary collisions between Pair values and values of this other collection. That's why I used a random seed.
nsajko
commented
Feb 8, 2025
nsajko
commented
Feb 8, 2025
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Issue pointed out by foobar_lv2 on Discourse.
Updates #57284