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68 lines
1.4 KiB
68 lines
1.4 KiB
#include "tommath_private.h"
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#ifdef BN_MP_SQRT_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* this function is less generic than mp_n_root, simpler and faster */
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mp_err mp_sqrt(const mp_int *arg, mp_int *ret)
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{
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mp_err err;
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mp_int t1, t2;
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/* must be positive */
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if (arg->sign == MP_NEG) {
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return MP_VAL;
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}
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/* easy out */
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if (MP_IS_ZERO(arg)) {
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mp_zero(ret);
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return MP_OKAY;
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}
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if ((err = mp_init_copy(&t1, arg)) != MP_OKAY) {
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return err;
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}
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if ((err = mp_init(&t2)) != MP_OKAY) {
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goto E2;
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}
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/* First approx. (not very bad for large arg) */
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mp_rshd(&t1, t1.used/2);
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/* t1 > 0 */
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if ((err = mp_div(arg, &t1, &t2, NULL)) != MP_OKAY) {
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goto E1;
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}
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if ((err = mp_add(&t1, &t2, &t1)) != MP_OKAY) {
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goto E1;
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}
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if ((err = mp_div_2(&t1, &t1)) != MP_OKAY) {
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goto E1;
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}
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/* And now t1 > sqrt(arg) */
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do {
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if ((err = mp_div(arg, &t1, &t2, NULL)) != MP_OKAY) {
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goto E1;
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}
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if ((err = mp_add(&t1, &t2, &t1)) != MP_OKAY) {
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goto E1;
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}
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if ((err = mp_div_2(&t1, &t1)) != MP_OKAY) {
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goto E1;
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}
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/* t1 >= sqrt(arg) >= t2 at this point */
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} while (mp_cmp_mag(&t1, &t2) == MP_GT);
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mp_exch(&t1, ret);
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E1:
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mp_clear(&t2);
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E2:
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mp_clear(&t1);
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return err;
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}
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#endif
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