Hashing.murmur3_128 is incompatible with C++ reference implementation for negative seed values

[oertl]

The reference implementation as mentioned in the Javadoc of Hashing.murmur3_128(int) (https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp) uses an unsigned 32-bit integer as seed. Since Java only has signed integers, half of all possible seed values are represented as negative values. Within the constructor of Murmur3_128Hasher the seed value is assigned to a long:
this.h1 = seed; this.h2 = seed;
Unfortunately, for negative values, this is not equivalent to the reference implementation using unsigned integers. To fix that the two assignment statements need to be changed to
this.h1 = seed & 0xFFFFFFFFL; this.h2 = seed & 0xFFFFFFFFL;

[nick-someone]

Thanks for bringing this to our attention! At first glance, this inconsistency between the Java implementation and the linked C++ implementation appears to be correct. We'll need to decide what to do here, since people can have persisted the hash values with an explicit negative seed before.