Re: MurmurHash3 behaviour
On Saturday, 20 August 2016 at 09:15:00 UTC, Ilya Yaroshenko wrote: On Friday, 19 August 2016 at 20:28:13 UTC, Cauterite wrote: Regarding the MurmurHash3 implementation in core.internal.hash, it is my understanding that: // assuming a and b are uints bytesHash([a, b], 0) == bytesHash([b], bytesHash([a], 0)) Is this correct? I'm just not quite certain of this property when I try to read the code myself, and I don't know much about hash algorithms. DRuntime has Murmurhash2, not 3. Oh, maybe I am wrong. Anyway 128bit Murmurhash3 hash from std.digest would much faster on 64 bit CPUs.
Re: MurmurHash3 behaviour
On Friday, 19 August 2016 at 20:28:13 UTC, Cauterite wrote: Regarding the MurmurHash3 implementation in core.internal.hash, it is my understanding that: // assuming a and b are uints bytesHash([a, b], 0) == bytesHash([b], bytesHash([a], 0)) Is this correct? I'm just not quite certain of this property when I try to read the code myself, and I don't know much about hash algorithms. DRuntime has Murmurhash2, not 3.
Re: MurmurHash3 behaviour
On Friday, 19 August 2016 at 21:29:43 UTC, Cauterite wrote: On Friday, 19 August 2016 at 21:03:22 UTC, Seb wrote: http://dlang.org/phobos-prerelease/std_digest_murmurhash.html Ah great, I just finished writing my own murmurhash digest module ( https://github.com/Cauterite/phobos/blob/murmur/std/digest/murmur.d ), and now I discover someone's already done it. Glad I wasted the last 3 hours of my life ;_; Well, we all learn for life? For example you could start to keep your fork up to date with upstream. It was merged two months ago ;-)
Re: MurmurHash3 behaviour
On Friday, 19 August 2016 at 21:03:22 UTC, Seb wrote: http://dlang.org/phobos-prerelease/std_digest_murmurhash.html Ah great, I just finished writing my own murmurhash digest module ( https://github.com/Cauterite/phobos/blob/murmur/std/digest/murmur.d ), and now I discover someone's already done it. Glad I wasted the last 3 hours of my life ;_;
Re: MurmurHash3 behaviour
On Friday, 19 August 2016 at 20:28:13 UTC, Cauterite wrote:
Regarding the MurmurHash3 implementation in core.internal.hash,
it is my understanding that:
// assuming a and b are uints
bytesHash([a, b], 0) == bytesHash([b], bytesHash([a], 0))
Is this correct?
I'm just not quite certain of this property when I try to read
the code myself, and I don't know much about hash algorithms.
Phobos nightly contains MurmurHash3 in std.digest (I don't know
why there are two implementations though), but I think this one
will be easier to use ;-)
http://dlang.org/phobos-prerelease/std_digest_murmurhash.html
The API for bytesHash is
size_t bytesHash(const(void)* buf, size_t len, size_t seed)
so shouldn't be?
auto arr = [a,b];
bytesHash(, arr.length, 42);
I'm just not quite certain of this property when I try to read
AFAIK the output of a hash isn't generally equal to the its
upcoming seed as pre & postpreprocessing may apply. In the case
of Murmurhash3 its post-processing (h1 ^= len):
void main() {
import std.stdio;
import core.internal.hash;
uint a = 4, b = 23;
auto arr = [a, b];
auto b1 = bytesHash(, arr.length, 42);
auto l = arr[0..1], r = arr[1..$];
auto b2a = bytesHash(, 1, 42);
auto b2b= bytesHash(, 1, b2a);
assert(b1 == b2b);
}
However with std.digest you can "save" the current state of a
hash function and continue to give new items to the hasher:
void main()
{
import std.digest.murmurhash; // required Phobos nightly
uint a = 4, b = 23;
auto arr = [a, b];
auto l = arr[0..1], r = arr[1..$];
MurmurHash3!32 hasher;
hasher.put(cast(ubyte[]) l);
hasher.put(cast(ubyte[]) r);
auto v1 = hasher.finish();
hasher = MurmurHash3!32();
hasher.put(cast(ubyte[]) arr);
auto v2 = hasher.finish();
assert(v1 == v2);
}
MurmurHash3 behaviour
Regarding the MurmurHash3 implementation in core.internal.hash, it is my understanding that: // assuming a and b are uints bytesHash([a, b], 0) == bytesHash([b], bytesHash([a], 0)) Is this correct? I'm just not quite certain of this property when I try to read the code myself, and I don't know much about hash algorithms.
