remove the redundant hasher in Bloom #6404
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| if k_i < NUMBER_OF_HASHERS as u32 { | ||
| let mut sip = self.sips[k_i as usize].clone(); | ||
| if k_i < 2 { | ||
| let mut sip = self.sip.clone(); | ||
| item.hash(&mut sip); | ||
| let hash = sip.finish(); | ||
| hashes[k_i as usize] = hash; |
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The thing is that the hashes array will contain exactly the same number twice, so the whole thing is redundant as well.
I think we should initialize two different SipHashers and keep the logic here, but that will break backward compatibility.
If we want to use only a single hasher then this code should be simplified as well, and I think we should review if that hashing method doesn't generate too many collisions.
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Hi, could you explain in which way using different hashers will break backward compatibility, thank you.
Without the bc problem, using two different independent hashers is indeed the best choice according to Kirsch and Mitzenmacher's paper.
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A bloom filter of non-empty accounts is stored as part of the state and loaded via Bloom::from_parts. If we change the bloom_hash method it will affect check and therefore the restored bitmap will have no correlation to accounts we are checking it against.
The change is possible but will require a database migration similar to: https://github.com/paritytech/parity/blob/2b81b982197d27efeea80c8a176c385114d95fdb/ethcore/src/migrations/v10.rs#L17
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After speaking with some team members we decided that it would be best to leave the hashing logic as is (i.e. avoid migration) but simplify it as much as we can (perhaps gaining some performance).
So you can probably avoid calling item.hash(&mut sip) twice (just copy the value for the next iteration) and avoid storing the SipHasher alltogether (as it now suggests that the hasher state is actually maintained, which is not true because it's being cloned before use).
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@tomusdrw Thanks for explaining it.
I just updated the PR according to your suggestion.
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I can't think of any such test besides constant comparison like this: let bloom = Bloom::new(128, 16);
bloom.set(<anything in 0..128 range>, <anything T: Hash>);
assert_eq!(bloom.drain_journal().drain(), <constant from previous version>);and It would be nice if it will be added before merge |
* remove the redundant hasher in Bloom * add the test to check the hash backward compatibility
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Great! Awesome set of samples also 👍 |
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Follow up: I guess we should add backward compatibility test for the Sorry for not considering the comprehensive aspects and add it in this PR. If you guys consider it necessary, I can create another PR for it. @tomusdrw @NikVolf |
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@Hawstein it would be good to have such test indeed. Feel free to submit another PR if you are game. |
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@tomusdrw PR created. |
Fix the TODO stuff in Bloom.