Add gccHash function

[sundy-li]

I hereby agree to the terms of the CLA available at: https://yandex.ru/legal/cla/?lang=en

Category (leave one):

• New Feature

Short description (up to few sentences):

Add gccHash function, use the same hash seed as gcc

[alexey-milovidov]

I'm interested, what is intended use case for it?

Because the hash function that is used in libstdc++ for std::unordered_map is an implementation specific detail that is not exposed in documentation. It may change in future versions, and in fact, it was already changed at least one time (in earlier versions of gcc, FNV1a is used for strings, now it is MurmurHash64A). Even more, if you replace libstdc++ to libc++ (that is valid option for both gcc and clang) or just compile with clang with default options on different platforms (Linux and FreeBSD), different standatd C++ library implementations will be used, that will lead to different hash functions used inside std::unordered_map.

We can add this hash function, but the name should be more specific (because there is no such thing as "gcc hash function").

[sundy-li]

The intention is that we used std::hash of libstdc++ to split the users for A/B testing experiment.
Well I agree with what you said, the purpose of this function may be very limited, and I can't come up with a proper name.

Since this hash function use MurmurHash64A , the difference with murmurHash2_64 is just the seed number. So will it be better to specify the seed number for some hash functions?

Eg.

murmurHash2_64('a')  ->  murmurHash2_64(0)('a'),  MurmurHash64A using the seed number 0
murmurHash2_64(0xc70f6907)('a')  ->  MurmurHash64A using the seed number 0xc70f6907

[alexey-milovidov]

Let's name it gccMurmurHash :)