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authorKevin B Kenny <kennykb@acm.org>2005-01-19 22:41:26 (GMT)
committerKevin B Kenny <kennykb@acm.org>2005-01-19 22:41:26 (GMT)
commitef78ca64ce6ba6a8786f083318fe536f2bd52925 (patch)
tree47f8ad0d7291237c7f9af988c5e05275ed9286ee /libtommath/etc/mersenne.c
parentb23d942a1e86ddee18c2309afd7fa7e9afa79ef8 (diff)
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Import of libtommath 0.33
Diffstat (limited to 'libtommath/etc/mersenne.c')
-rw-r--r--libtommath/etc/mersenne.c140
1 files changed, 140 insertions, 0 deletions
diff --git a/libtommath/etc/mersenne.c b/libtommath/etc/mersenne.c
new file mode 100644
index 0000000..1cd5b50
--- /dev/null
+++ b/libtommath/etc/mersenne.c
@@ -0,0 +1,140 @@
+/* Finds Mersenne primes using the Lucas-Lehmer test
+ *
+ * Tom St Denis, tomstdenis@iahu.ca
+ */
+#include <time.h>
+#include <tommath.h>
+
+int
+is_mersenne (long s, int *pp)
+{
+ mp_int n, u;
+ int res, k;
+
+ *pp = 0;
+
+ 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, s)) != MP_OKAY) {
+ goto LBL_MU;
+ }
+ if ((res = mp_sub_d (&n, 1, &n)) != MP_OKAY) {
+ goto LBL_MU;
+ }
+
+ /* set u=4 */
+ mp_set (&u, 4);
+
+ /* 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, 2, &u)) != MP_OKAY) {
+ goto LBL_MU;
+ }
+
+ /* make sure u is positive */
+ while (u.sign == MP_NEG) {
+ if ((res = mp_add (&u, &n, &u)) != MP_OKAY) {
+ goto LBL_MU;
+ }
+ }
+
+ /* reduce */
+ if ((res = mp_reduce_2k (&u, &n, 1)) != MP_OKAY) {
+ goto LBL_MU;
+ }
+ }
+
+ /* if u == 0 then its prime */
+ if (mp_iszero (&u) == 1) {
+ mp_prime_is_prime(&n, 8, pp);
+ if (*pp != 1) printf("FAILURE\n");
+ }
+
+ res = MP_OKAY;
+LBL_MU:mp_clear (&u);
+LBL_N:mp_clear (&n);
+ return res;
+}
+
+/* square root of a long < 65536 */
+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 */
+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)
+{
+ int 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 == 1) {
+ /* count time */
+ tt = clock () - tt;
+
+ /* display if prime */
+ printf ("2^%-5ld - 1 is prime, test took %ld ticks\n", k, tt);
+ }
+
+ /* goto next odd exponent */
+ k += 2;
+
+ /* but make sure its prime */
+ while (isprime (k) == 0) {
+ k += 2;
+ }
+ }
+ return 0;
+}