92 lines
2.3 KiB
C
92 lines
2.3 KiB
C
#include "tommath_private.h"
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#ifdef S_MP_SQR_COMBA_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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/* the jist of squaring...
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* you do like mult except the offset of the tmpx [one that
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* starts closer to zero] can't equal the offset of tmpy.
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* So basically you set up iy like before then you min it with
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* (ty-tx) so that it never happens. You double all those
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* you add in the inner loop
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After that loop you do the squares and add them in.
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*/
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mp_err s_mp_sqr_comba(const mp_int *a, mp_int *b)
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{
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int oldused, pa, ix;
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mp_digit MP_ALLOC_WARRAY(W);
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mp_word W1;
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mp_err err;
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MP_CHECK_WARRAY(W);
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/* grow the destination as required */
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pa = a->used + a->used;
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if ((err = mp_grow(b, pa)) != MP_OKAY) {
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MP_FREE_WARRAY(W);
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return err;
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}
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/* number of output digits to produce */
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W1 = 0;
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for (ix = 0; ix < pa; ix++) {
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int tx, ty, iy, iz;
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mp_word _W;
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/* clear counter */
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_W = 0;
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/* get offsets into the two bignums */
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ty = MP_MIN(a->used-1, ix);
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tx = ix - ty;
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/* this is the number of times the loop will iterate, essentially
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while (tx++ < a->used && ty-- >= 0) { ... }
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*/
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iy = MP_MIN(a->used-tx, ty+1);
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/* now for squaring tx can never equal ty
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* we halve the distance since they approach at a rate of 2x
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* and we have to round because odd cases need to be executed
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*/
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iy = MP_MIN(iy, ((ty-tx)+1)>>1);
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/* execute loop */
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for (iz = 0; iz < iy; iz++) {
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_W += (mp_word)a->dp[tx + iz] * (mp_word)a->dp[ty - iz];
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}
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/* double the inner product and add carry */
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_W = _W + _W + W1;
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/* even columns have the square term in them */
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if (((unsigned)ix & 1u) == 0u) {
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_W += (mp_word)a->dp[ix>>1] * (mp_word)a->dp[ix>>1];
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}
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/* store it */
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W[ix] = (mp_digit)_W & MP_MASK;
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/* make next carry */
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W1 = _W >> (mp_word)MP_DIGIT_BIT;
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}
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/* setup dest */
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oldused = b->used;
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b->used = a->used+a->used;
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for (ix = 0; ix < pa; ix++) {
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b->dp[ix] = W[ix] & MP_MASK;
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}
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/* clear unused digits [that existed in the old copy of c] */
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s_mp_zero_digs(b->dp + b->used, oldused - b->used);
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mp_clamp(b);
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MP_FREE_WARRAY(W);
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return MP_OKAY;
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}
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#endif
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