A constexpr implementation of xxHash written in C++

If you thought the C version was fast, you ain't seen nothing yet

This is a header-only constexpr implementation of the popular hashing algorithm xxHash.
You can find the original implementation in C at Cyan4973 / xxHash

Yes. I tested against the hashes produced by the original code and I'll try to stick to that standard.

I adapted the original C code to a modern C++ standard to take advantage of C++17's constexpr functions.
The main reason for doing so was to be able to switch-case on strings, without the overhead of computing the hashes for constants every single time (or store computed hashes somewhere).

int function(const char* str)
{
    using namespace xxh;
    const size_t str_size = strlen(str); // null character not included
    const uint64_t str_hash = xxHash64(str, str_size);

    switch (str_hash)
    {
        case "a"_h:
            return 1;
        case "abcdefgh very long string"_h:
            return 2;
    }
}

• At the moment, the header only implements the 64 bit version of xxHash.
• Furthermore, the function can hash a string of up to 128 bytes.
  (I thought that would be enough for my use case)
• Lastly, no support for seeds or custom secrets.

If you want to submit a PR, I'd be happy to review it.

Feel free to do whatever you want with this code.
I don't know how licensing works, and I don't want to find out.