编译时字符串哈希

Question

I need to use a string as the ID to obtain some object. At implement this in a run-time, and works well. But this makes the static type checking impossible, for obvious reasons.

I've Googled for the algorithm for calculating the hash-sum of string in the compile-time: C++ compile-time string hashing with Boost.MPL .

It seems to be the perfect solution to my problem, except that the sring which is necessary to the algorithm should be split into pieces by 4 characters, or character-by-character, as well, for obvious reasons.

ie, instead of the usual current record of the ID's, I'll have to write this way:

hash_cstring<boost::mpl::string<'obje', 'ct.m', 'etho', 'd'>>::value

This is absolutely unusable.

The question is, how to pass correctly the string such as "object.method" to this algorithm?

Thank you all.

Answer 1

Solution with gcc-4.6:

#include <iostream>
template<size_t N, size_t I=0>
struct hash_calc {
    static constexpr size_t apply (const char (&s)[N]) {
       return  (hash_calc<N, I+1>::apply(s) ^ s[I]) * 16777619u;
    };
};

template<size_t N>
struct hash_calc<N,N> {
    static constexpr size_t apply (const char (&s)[N]) {
       return  2166136261u;
    };
};

template<size_t N>
constexpr size_t hash ( const char (&s)[N] ) {
    return hash_calc<N>::apply(s);
}

int main() {
   char a[] = "12345678";
   std::cout << std::hex << hash(a) << std::endl;
   std::cout << std::hex << hash("12345678") << std::endl;
}

http://liveworkspace.org/code/DPObf

I`m happy!

Answer 2

I don't know of a way to do this with the preprocessor or with templates. I suspect your best bet is to create a separate pre-compile step (say with perl or such) to generate the hash_cstring statements from a set of source statements. Then at least you don't have to split the strings manually when you add new ones, and the generation is fully automated and repeatable.

Answer 3

In case anyone is interested, I walk through how to create a compile time hash of Murmur3_32 using C++11 constexpr functions and variadic templates here:

http://roartindon.blogspot.sg/2014/10/compile-time-murmur-hash-in-c.html

Most of the examples I've seen deal with hashes that are based on consuming one character of the string at a time. The Murmur3_32 hash is a bit more interesting in that it consumes 4 characters at a time and needs some special case code to handle the remaining 0, 1, 2 or 3 bytes.

Answer 4

Templates can be instantiated with any external symbol, therefore this should work as expected:

external char const* object_method = "object.method";
... = hash_cstring<object_method>::value;

(given the template hash_cstring<> is able to deal with pointer values).