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Show HN: Compute polynomials twice as fast(thomasahle.com)
42 points by thomasahle 20 hours ago | 10 comments

A few years ago my coauthor and I was wondering if we could reduce the number of multiplications used for hashing algorithms. We had a construction and a 100 page proof, but we were not 100% sure it was correct. Now we have a full Lean proof, so we decided to publish it.

I made this website to make it easy for anyone how has polynomials to evaluate to see how it would be done using our method, as well as a number of previous approaches by Knuth and others.

voxelghost 3 hours ago | parent | next [-]

It keeps flipping back to 'monic' from e.g. 'ln(1+x)' when switching between algorithms, and then seems to lock to 'monic'? (Am I missing something?)

Also I am curious, in your version vs. horner , how do both algorithms map onto number of fmadd operations?

gowld 2 hours ago | parent [-]

"monic" is a separate switch from the example functions radio-selector. Enabling "monic" removes the leading coefficient.

vlovich123 3 hours ago | parent | prev | next [-]

Would this be applicable to fast hashes like WyHash and xxh3 or are those not using polynomials? Is this mainly for faster cryptographic hashes?

orlp a minute ago | parent [-]

It is applicable to fast universal hashes like Poly1305 and Polymer (the latter of which I'm the author). However it's not clear to me whether this work improves over the state of the art for that purpose, see some questions here: https://www.reddit.com/r/programming/comments/1wbgcke/comput....

WyHash and xxh3 do not have polynomial structures.

aetherspawn 3 hours ago | parent | prev | next [-]

I guess it’s not faster than using a table for CRC8?

gowld 2 hours ago | parent | prev | next [-]

From the abstract, a name that many on HN would recognize:

> We also give an injective polynomial construction for universal hashing that uses N multiplications to hash 2N values with a single random key. This improves the best previous construction by Daniel J. Bernstein (this http URL).

gowld 3 hours ago | parent | prev [-]

What is the tradeoff between multiplication and addition?

nraynaud 2 hours ago | parent | next [-]

Just a few years ago, mults were slower, but I think now (Intel i9) mult, add and fma are the same.

https://stackoverflow.com/a/39135689

gigatexal 2 hours ago | parent | prev [-]

I think multiplications are faster to do in computer land than adds? I too am curious.

hyperhello 2 hours ago | parent [-]

Also could use analysis of dependencies to see what can happen in parallel. Or for that matter, some real benchmarks.