53 lines
1.2 KiB
C
53 lines
1.2 KiB
C
#include "amplify_tommath_private.h"
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#ifdef AMPLIFY_BN_MP_RSHD_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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/* Modifications Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. */
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/* shift right a certain amount of digits */
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void amplify_mp_rshd(amplify_mp_int *a, int b)
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{
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int x;
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amplify_mp_digit *bottom, *top;
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/* if b <= 0 then ignore it */
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if (b <= 0) {
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return;
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}
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/* if b > used then simply zero it and return */
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if (a->used <= b) {
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amplify_mp_zero(a);
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return;
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}
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/* shift the digits down */
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/* bottom */
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bottom = a->dp;
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/* top [offset into digits] */
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top = a->dp + b;
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/* this is implemented as a sliding window where
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* the window is b-digits long and digits from
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* the top of the window are copied to the bottom
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*
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* e.g.
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b-2 | b-1 | b0 | b1 | b2 | ... | bb | ---->
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/\ | ---->
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\-------------------/ ---->
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*/
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for (x = 0; x < (a->used - b); x++) {
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*bottom++ = *top++;
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}
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/* zero the top digits */
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AMPLIFY_MP_ZERO_DIGITS(bottom, a->used - x);
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/* remove excess digits */
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a->used -= b;
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}
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#endif
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