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, 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, arr.length, 42);
auto l = arr[0..1], r = arr[1..$];
auto b2a = bytesHash(&l, 1, 42);
auto b2b= bytesHash(&r, 1, b2a);
assert(b1 == b2b);
}
However with std.digest you can "save" the current state of a
hash function and continue to give new items to the hasher:
void main()
{
import std.digest.murmurhash; // required Phobos nightly
uint a = 4, b = 23;
auto arr = [a, b];
auto l = arr[0..1], r = arr[1..$];
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);
}
