Fast libgmpxx implementation of 300 decimal digit factoring puzzle

puzzle.cc

/*
https://www.mersenneforum.org/showthread.php?t=29584
 
Factors 300 decimal digits semiprime n, knowing its prime factors consist of
digits 3 and 7 only, 10,000 times in less than 1s in total on 7950X CPU.

f=puzzle 
g++ -O3 -Wall -Wextra -pedantic $f.cc -o $f -lgmp -lgmpxx
cpplint --filter=-legal/copyright,-runtime/references $f.cc
cppcheck --enable=all --suppress=missingIncludeSystem $f.cc --check-config
*/
#include <gmpxx.h>
#include <iostream>

mpz_class n = mpz_class("275749941173280562233206455736390095733872087404999918648104846244787100445192883149543759714060622705730237359190372814923054256546971783443405641914960603655306172124864651388859278743238322029857347654069385380300498785472834485212409650369402081148638740386329616025152776319904754759259065304901"); // NOLINT

void g(mpz_class& R,
       mpz_class n, mpz_class a = 0, mpz_class b = 0, mpz_class m = 1) {
  for (unsigned ad = 3; ad <= 7; ad += 4) {
    for (unsigned bd = 3; bd <= 7; bd += 4) {
      mpz_class d = n-(ad*b+bd*a+ad*bd*m);
      if (d % 10 != 0) continue;
      d /= 10;
      if (d == 0) { R = a + ad * m; return; }
      mpz_class A = a + m * ad;
      mpz_class B = b + m * bd;
      mpz_class M = m  * 10;
      g(R, d, A, B, M);
      if (R != 0) return;
    }
  }
}

int main(void) {
  clock_t start = clock();

  for (int i = 0; i < 10000; ++i) {
    mpz_class r = 0;
    g(r, n);
  }

  std::cout << static_cast<float>(clock() - start) / CLOCKS_PER_SEC << "s\n";

  mpz_class r = 0;
  g(r, n);
  std::cout << r << std::endl;

  return 0;
}

hermann@7950x:~$ f=puzzle 
hermann@7950x:~$ g++ -O3 -Wall -Wextra -pedantic $f.cc -o $f -lgmp -lgmpxx
hermann@7950x:~$ cpplint --filter=-legal/copyright,-runtime/references $f.cc
Done processing puzzle.cc
hermann@7950x:~$ cppcheck --enable=all --suppress=missingIncludeSystem $f.cc --check-config
Checking puzzle.cc ...
hermann@7950x:~$ ./puzzle 
0.980455s
737737733773377773337733777733777377737333773733377737737377373337377333733777373337373737777337777777333337337733773777737773777337777777773773373373
hermann@7950x:~$