Kumite

#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <assert.h>
#include <criterion/criterion.h>

long long mySol(long long n, long long d) ;

static void generate_n_d(long long *n, long long *d, long long min_d, long long max_d, long long max_delta) {
  *d = min_d + rand() % (max_d + 1 - min_d);
  unsigned long long max_n = *d + 1;
  long double prod = *d + 1;
  unsigned long long prodi = *d + 1;

  while (prod < LLONG_MAX && max_delta > 0) {
    --max_delta;
    ++max_n;
    prod *= max_n;
    prodi *= max_n;
    printf("min_d = %lld, max_d = %lld, max_n = %lld, prod = %.0Lf, prodi = %llu\n",
        max_d, min_d, max_n, prod, prodi
    );
    fflush(stdout);

    assert((long double)prodi == prod);
  }
  *n = *d + (rand() % (max_n - 1 - *d)) + 1;
}

Test(tests, generate_n_d_overflow) {
  long long tests_param[3][4] = {
  /*{test count, min d  , max d          , max delta } */
    {25ll      , 4ll    , 512ll          , 20ll      },
    {25ll      , 256ll  , 262144ll       , 200ll     },
    {50ll      , 65536ll, 0x1p31         , 1000ll    },
                        //0x1p31 = 2^32
  };    

  for (size_t i=0; i<3; ++i) {
    
    for (long long j=0; j<tests_param[i][0]; ++j) {
      
      long long n;
      long long d;

      generate_n_d(&n, &d, tests_param[i][1], tests_param[i][2], tests_param[i][3]);

      printf("%lld %lld %lld\n", n, d, mySol(n, d));
      
    }
  }
}