My implementation of MurmurHash3 in C++. All of the code here is constexpr, meaning the hashes can be computed at compile-time. There were a few obstacles that made this a little tricky to pull off, namely the fact that the original implementation used pointer type punning quite heavily for the extraction of blocks from a given key. In C++, any cast that reinterprets underlying data cannot be performed in a constexpr context, so I had to get a little bit creative in order to make things work.

chnk  = static_cast<uint32_t>(key[i * 4 + 0]) << 0;
chnk |= static_cast<uint32_t>(key[i * 4 + 1]) << 8;
chnk |= static_cast<uint32_t>(key[i * 4 + 2]) << 16;
chnk |= static_cast<uint32_t>(key[i * 4 + 3]) << 24;

As you can see, each chunk from a given key is extracted one byte at a time. This also gets rid of the punning issues that the original algorithm had in the first place, so that's a nice plus.

Each function found in murmur3.hpp accepts two arguments: a key, and a seed. The 32 bit version of the algorithm returns the hashed value directly, and the 128 bit ones return a struct containing the upper and lower 64 bits of the hash.

Lastly: note that I haven't implemented the x86 (32 bit) version of the algorithm that produces a 128 bit hash. To be honest, I didn't really need this one, and you shouldn't either if you're on a 64 bit platform, as the 64 bit version of that particular algorithm is much better regardless.