I’m trying to implement a wrapper for running a fermat-factorization test on a public key. I’ve generated the shared library successfully using SWIG:

fermat.h:

#ifndef FERMAT_H
#define FERMAT_H

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

class Fermat
{
private:
    mpz_class key;
    mpz_class ceil_sqrt(const mpz_class&);
public:
    Fermat(mpz_class);
    std::string test();
};

#endif

fermat.cpp:

#include "fermat.h"
#include <iostream>
#include <sstream>
#include <gmpxx.h>

Fermat::Fermat(mpz_class n)
{
  key = n;
}

std::string Fermat::test()
{
// Test logic. Not important
}

mpz_class Fermat::ceil_sqrt(const mpz_class& n)
{
// A helper function
}

I compiled this with the below Makefile and .i file required by SWIG.

Makefile:

all:
    swig -c++ -python -v -I/usr/include/ fermat.i
    python setup.py build_ext --inplace
    g++ -O3 -fPIC -c -lgmp -lgmpxx fermat.cpp
    g++ -O3 -fPIC -c -lgmp -lgmpxx -I/usr/include/python3.10 fermat_wrap.cxx
    g++ -shared fermat.o fermat_wrap.o -lgmp -lgmpxx -o _fermat.so

fermat.i:

%module fermat
%{
#define SWIG_FERMAT_WITH_INIT
#include "fermat.h"
%}
%include "std_string.i"
%include "fermat.h"

Now, I can include this SWIG module in my main.py code that calls this test method. I pass the public key’s e value as a decimal number to the constructor.

import fermat as f

key_decimal = 98372489374731875343284493738234 # arbitrarily long decimal number
test_fermat = f.Fermat(key_decimal)
print(test_fermat.test())

In summary: I’m trying to pass an integer in Python to a function that accepts mpz_class type arguments. As far as I know, there’s no native way to convert an int to mpz_class, so this renders my SWIG module useless, unless I find a way to have the constructor accept that large integer in Python.

Running the above Python code would result in TypeError: in method 'new_Fermat', argument 1 of type 'mpz_class'

I’ve read about typemaps in SWIG, which might come in handy, but I’m not sure if they would solve my problem. I thought I’d ask for reassurance here first.