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/* Finds Mersenne primes using the Lucas-Lehmer test
 *
 * Tom St Denis, tomstdenis@gmail.com
 */
#include <time.h>
#include <tommath.h>

static mp_err is_mersenne(long s, bool *pp)
{
   mp_int  n, u;
   mp_err  res;
   int     k;

   *pp = false;

   if ((res = mp_init(&n)) != MP_OKAY) {
      return res;
   }

   if ((res = mp_init(&u)) != MP_OKAY) {
      goto LBL_N;
   }

   /* n = 2^s - 1 */
   if ((res = mp_2expt(&n, (int)s)) != MP_OKAY) {
      goto LBL_MU;
   }
   if ((res = mp_sub_d(&n, 1uL, &n)) != MP_OKAY) {
      goto LBL_MU;
   }

   /* set u=4 */
   mp_set(&u, 4uL);

   /* for k=1 to s-2 do */
   for (k = 1; k <= (s - 2); k++) {
      /* u = u^2 - 2 mod n */
      if ((res = mp_sqr(&u, &u)) != MP_OKAY) {
         goto LBL_MU;
      }
      if ((res = mp_sub_d(&u, 2uL, &u)) != MP_OKAY) {
         goto LBL_MU;
      }

      /* make sure u is positive */
      while (mp_isneg(&u)) {
         if ((res = mp_add(&u, &n, &u)) != MP_OKAY) {
            goto LBL_MU;
         }
      }

      /* reduce */
      if ((res = mp_reduce_2k(&u, &n, 1uL)) != MP_OKAY) {
         goto LBL_MU;
      }
   }

   /* if u == 0 then its prime */
   if (mp_iszero(&u)) {
      mp_prime_is_prime(&n, 8, pp);
      if (!*pp) printf("FAILURE\n");
   }

   res = MP_OKAY;
LBL_MU:
   mp_clear(&u);
LBL_N:
   mp_clear(&n);
   return res;
}

/* square root of a long < 65536 */
static long i_sqrt(long x)
{
   long    x1, x2;

   x2 = 16;
   do {
      x1 = x2;
      x2 = x1 - ((x1 * x1) - x) / (2 * x1);
   } while (x1 != x2);

   if ((x1 * x1) > x) {
      --x1;
   }

   return x1;
}

/* is the long prime by brute force */
static int isprime(long k)
{
   long    y, z;

   y = i_sqrt(k);
   for (z = 2; z <= y; z++) {
      if ((k % z) == 0)
         return 0;
   }
   return 1;
}


int main(void)
{
   bool pp;
   long    k;
   clock_t tt;

   k = 3;

   for (;;) {
      /* start time */
      tt = clock();

      /* test if 2^k - 1 is prime */
      if (is_mersenne(k, &pp) != MP_OKAY) {
         printf("Whoa error\n");
         return -1;
      }

      if (pp) {
         /* count time */
         tt = clock() - tt;

         /* display if prime */
         printf("2^%-5ld - 1 is prime, test took %ld ticks\n", k, (long)tt);
      }

      /* goto next odd exponent */
      k += 2;

      /* but make sure its prime */
      while (isprime(k) == 0) {
         k += 2;
      }
   }
}
c3-lang/libtommath/etc/mersenne.c

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etc/mersenne.c
