问Iter 2:适用于GMP大整数型的可重用、健壮的c++ std::hash<mpz_class>

EN

#ifndef HASH_MPZ_H_
#define HASH_MPZ_H_

#include <gmpxx.h>

namespace std {

template<> struct hash<mpz_srcptr> {
    size_t operator()(const mpz_srcptr x) const;
};

template<> struct hash<mpz_t> {
    size_t operator()(const mpz_t x) const;
};

template<> struct hash<mpz_class> {
    size_t operator()(const mpz_class &x) const;
};

}

#endif /* HASH_MPZ_H_ */

#include "hash_mpz.h"
#include <cstddef>
#include <string_view>

constexpr size_t pi_size_t() {
    if (sizeof(size_t) == 4) {
        return 0xc90fdaa2; // floor(pi/4 * 2^32)
    } else if (sizeof(size_t) == 8) {
        return 0xc90fdaa22168c234; // floor(pi/4 * 2^64)
    } else {
        throw std::logic_error(
                "sizeof(size_t) not supported. only 32 or 64 bits are supported. you can easily add the required code for other sizes.");
    }
}

inline size_t scramble(size_t v) {
    return v ^ (pi_size_t() + (v << 6) + (v >> 2));
}

namespace std {

size_t std::hash<mpz_srcptr>::operator()(const mpz_srcptr x) const {
    string_view view { reinterpret_cast<char*>(x->_mp_d), abs(x->_mp_size)
            * sizeof(mp_limb_t) };
    size_t result = hash<string_view> { }(view);

    // produce different hashes for negative x
    if (x->_mp_size < 0) {
        result = scramble(result);
    }

    return result;
}

size_t hash<mpz_t>::operator()(const mpz_t x) const {
    return hash<mpz_srcptr> { }(static_cast<mpz_srcptr>(x));
}

size_t hash<mpz_class>::operator()(const mpz_class &x) const {
    return hash<mpz_srcptr> { }(x.get_mpz_t());
}

}

#include <iostream>
#include <gmpxx.h>
#include <unordered_map>

#include "hash_mpz.h"

using namespace std;

int main() {
    mpz_class a;

    mpz_ui_pow_ui(a.get_mpz_t(), 168, 16);

    cout << "a      : " << a << endl;
    cout << "hash( a): " << (hash<mpz_class> { }(a)) << endl;
    cout << "hash(-a): " << (hash<mpz_class> { }(-a)) << endl;

    unordered_map<mpz_class, int> map;
    map[a] = 2;
    cout << "map[a] : " << map[a] << endl;

    return 0;
}