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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)
commitb9cf65a08e6a59e434685e894e3189c201ac6791 (patch)
tree1f0868ef44c9f17d83d10dc94343df7b8cfe1842 /libtommath/bn_mp_n_root.c
parentc6a259aeeca4814a97cf6694814c63e74e4e18fa (diff)
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Import of libtommath 0.33
Diffstat (limited to 'libtommath/bn_mp_n_root.c')
-rw-r--r--libtommath/bn_mp_n_root.c128
1 files changed, 128 insertions, 0 deletions
diff --git a/libtommath/bn_mp_n_root.c b/libtommath/bn_mp_n_root.c
new file mode 100644
index 0000000..7b11aa2
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+++ b/libtommath/bn_mp_n_root.c
@@ -0,0 +1,128 @@
+#include <tommath.h>
+#ifdef BN_MP_N_ROOT_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
+ */
+
+/* find the n'th root of an integer
+ *
+ * Result found such that (c)**b <= a and (c+1)**b > a
+ *
+ * This algorithm uses Newton's approximation
+ * x[i+1] = x[i] - f(x[i])/f'(x[i])
+ * which will find the root in log(N) time where
+ * each step involves a fair bit. This is not meant to
+ * find huge roots [square and cube, etc].
+ */
+int mp_n_root (mp_int * a, mp_digit b, mp_int * c)
+{
+ mp_int t1, t2, t3;
+ int res, neg;
+
+ /* input must be positive if b is even */
+ if ((b & 1) == 0 && a->sign == MP_NEG) {
+ return MP_VAL;
+ }
+
+ if ((res = mp_init (&t1)) != MP_OKAY) {
+ return res;
+ }
+
+ if ((res = mp_init (&t2)) != MP_OKAY) {
+ goto LBL_T1;
+ }
+
+ if ((res = mp_init (&t3)) != MP_OKAY) {
+ goto LBL_T2;
+ }
+
+ /* if a is negative fudge the sign but keep track */
+ neg = a->sign;
+ a->sign = MP_ZPOS;
+
+ /* t2 = 2 */
+ mp_set (&t2, 2);
+
+ do {
+ /* t1 = t2 */
+ if ((res = mp_copy (&t2, &t1)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ /* t2 = t1 - ((t1**b - a) / (b * t1**(b-1))) */
+
+ /* t3 = t1**(b-1) */
+ if ((res = mp_expt_d (&t1, b - 1, &t3)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ /* numerator */
+ /* t2 = t1**b */
+ if ((res = mp_mul (&t3, &t1, &t2)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ /* t2 = t1**b - a */
+ if ((res = mp_sub (&t2, a, &t2)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ /* denominator */
+ /* t3 = t1**(b-1) * b */
+ if ((res = mp_mul_d (&t3, b, &t3)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ /* t3 = (t1**b - a)/(b * t1**(b-1)) */
+ if ((res = mp_div (&t2, &t3, &t3, NULL)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ if ((res = mp_sub (&t1, &t3, &t2)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+ } while (mp_cmp (&t1, &t2) != MP_EQ);
+
+ /* result can be off by a few so check */
+ for (;;) {
+ if ((res = mp_expt_d (&t1, b, &t2)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+
+ if (mp_cmp (&t2, a) == MP_GT) {
+ if ((res = mp_sub_d (&t1, 1, &t1)) != MP_OKAY) {
+ goto LBL_T3;
+ }
+ } else {
+ break;
+ }
+ }
+
+ /* reset the sign of a first */
+ a->sign = neg;
+
+ /* set the result */
+ mp_exch (&t1, c);
+
+ /* set the sign of the result */
+ c->sign = neg;
+
+ res = MP_OKAY;
+
+LBL_T3:mp_clear (&t3);
+LBL_T2:mp_clear (&t2);
+LBL_T1:mp_clear (&t1);
+ return res;
+}
+#endif