compile-time string hashing

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.

i.e., 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.

can you use struct with static string instead of string directly? perhaps a macro to generate them too? — Anycorn
– Anycorn, Commented Feb 12, 2011 at 4:19
@aaa: macros don't split tokens, they can stringize or concatenate them, but not split them. — Matthieu M.
– Matthieu M., Commented Feb 12, 2011 at 10:26
@Matt macros to generate struct with name and static string. eg #define str(n) struct n { ... } — Anycorn
– Anycorn, Commented Feb 12, 2011 at 18:31
Any thoughts of using gperf and wrapping the results with a type? — CoreyStup
– CoreyStup, Commented Feb 16, 2011 at 17:41

niXman · Accepted Answer · 2013-02-22 06:37:20Z

9

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!

edited Feb 22, 2013 at 6:37

answered Feb 16, 2011 at 17:07

niXman

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4 Comments

Damon Over a year ago

Are you actually getting this to compile with gcc 4.6 without a recursion error?

Damon Over a year ago

Funny, my gcc-4.6.1 (MingW-TDM) rejects that code with said error (is the reason why I asked).

Damon Over a year ago

In instantiation of 'static constexpr size_t hash_calc<N, N>::apply(const char (&)[N]) [with unsigned int N = 10u, size_t = unsigned int]': recursively instantiated from 'static constexpr size_t hash_calc<N, I>::apply(const char (&)[N]) [with unsigned int N = 10u, unsigned int I = 1u, size_t = unsigned int]' instantiated from 'static constexpr size_t hash_calc<N, I>::apply(const char (&)[N]) [with unsigned int N = 10u, unsigned int I = 0u, size_t = unsigned int]' instantiated from 'constexpr size_t hash(const char (&)[N]) [with unsigned int N = 10u, size_t = unsigned int]'

niXman Over a year ago

I'm not sure about the TDM builds. I use these builds: code.google.com/p/mingw-builds

Mark B · Accepted Answer · 2011-02-12 06:05:41Z

5

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.

answered Feb 12, 2011 at 6:05

Mark B

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Comments

hkaiser · Accepted Answer · 2011-02-15 17:44:37Z

1

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).

edited Feb 15, 2011 at 17:44

answered Feb 12, 2011 at 17:03

hkaiser

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Comments

jtlim · Accepted Answer · 2014-10-29 14:58:42Z

1

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.

answered Oct 29, 2014 at 14:58

jtlim

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