[aappleby]

Not bad, I like that the author stuck with plain C instead of sse/aes.

Looks like two interleaved 128-bit hashes, don't see any cross-half mixing. Code style needs a bit of cleanup, the sindex stuff obscures the algorithm a bit.

-Austin, SMHasher/Murmur author

[osrec]

Was it the inconsistent use of curly braces that made you comment on the code style?

[aappleby]

Predeclared aliased pointers, STATEM/HSTATEM constants, len/Len, that kind of stuff.

[fantastisch]

Thank you, Austin!

BTW, not sure exactly what you mean by cross-half mixing, but I'm assuming it means mixing both halves of the 128 bit state with each other. There is that, on 3 lines 91[0], 100 and 108.

The 128 bit mix, stirs two adjacent blocks of 64 bits. Those 3 lines can mix blocks 1 & 2.

Subsequent mixes on either half then propagate that cross mix. So it does use the full 256 bit state.

Is that what you meant?

[0]: https://github.com/cris691/discohash/blob/master/src/discoha...

[antirez]

Looks like a solid hash function indeed, with a very simple round.

Q: Is there any reason you don't use the same mixing function for the residue of the string (the final tail that is not a full 8 bytes block)? You could copy the bytes you have into a 8 byte array that is pre-padded with some pattern, and call the same mixing function again. Is this in order to save the distribution / avalanche properties in case the string is very short? In general would be nice to read some design note, if P and Q were obtained experimentally by checking for distribution or alike, if a different rotation length changes significantly the distribution properties and so forth.

[fantastisch]

Great question, and thank you, Salvatore!

Some reasons for that are: different hashes for s and s\0 (without explicitly using length) to make it harder to create collisions, related to as you say better distribution/avalanche for short strings, also design-wise I like the asymmetry that strings not perfectly divisible by 8 will be treated a little differently. I just think that makes it better overall.

[antirez]

Thanks for the replies!