C++ std::hash for GMP's big integer types mpz_class and mpz_t

#ifndef HASH_MPZ_H_
#define HASH_MPZ_H_

#include <cstddef>
#include <gmpxx.h>

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

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

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

#endif /* HASH_MPZ_H_ */

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

static 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(
            "this sizeof(size_t) is not supported. only 32 or 64 bits are supported.");
    }
}

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

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

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

    return result;
}

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

size_t std::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 std::cout;
using std::hash;
using std::unordered_map;

int main() {
    mpz_class a;

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

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

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

me@hostname:~/Downloads$ unzip hash_gmp_mpz_1_0_0.zip
Archive:  hash_gmp_mpz_1_0_0.zip
   creating: hash_gmp_mpz_1_0_0/
  inflating: hash_gmp_mpz_1_0_0/main.cpp  
  inflating: hash_gmp_mpz_1_0_0/hash_mpz.cpp  
  inflating: hash_gmp_mpz_1_0_0/hash_mpz.h  
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ cd hash_gmp_mpz_1_0_0/
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ g++ -std=c++17 *.cpp -o main -lgmp -lgmpxx
hash_mpz.h:5:10: fatal error: gmpxx.h: No such file or directory
 #include <gmpxx.h>
          ^~~~~~~~~
[...]
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ sudo apt-get install libgmp-dev
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  gmp-doc libmpfr-dev
The following NEW packages will be installed:
  libgmp-dev
[...]
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ g++ -std=c++17 *.cpp -o main -lgmp -lgmpxx
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ ./main 
a       : 402669288768856477614113920779288576
hash( a): 776996915127542148
hash(-a): 8581308568227109393
map[a]  : 2
me@hostname:~/Downloads/hash_gmp_mpz_1_0_0$ █