Mercurial > dropbear
view libtommath/bn_s_mp_sub.c @ 1899:ab9c5467970d
Merge pull request #152 from HansH111/tracelevel2
author | Matt Johnston <matt@ucc.asn.au> |
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date | Wed, 23 Mar 2022 21:55:33 +0800 |
parents | 1051e4eea25a |
children |
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#include "tommath_private.h" #ifdef BN_S_MP_SUB_C /* LibTomMath, multiple-precision integer library -- Tom St Denis */ /* SPDX-License-Identifier: Unlicense */ /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c) { int olduse, min, max; mp_err err; /* find sizes */ min = b->used; max = a->used; /* init result */ if (c->alloc < max) { if ((err = mp_grow(c, max)) != MP_OKAY) { return err; } } olduse = c->used; c->used = max; { mp_digit u, *tmpa, *tmpb, *tmpc; int i; /* alias for digit pointers */ tmpa = a->dp; tmpb = b->dp; tmpc = c->dp; /* set carry to zero */ u = 0; for (i = 0; i < min; i++) { /* T[i] = A[i] - B[i] - U */ *tmpc = (*tmpa++ - *tmpb++) - u; /* U = carry bit of T[i] * Note this saves performing an AND operation since * if a carry does occur it will propagate all the way to the * MSB. As a result a single shift is enough to get the carry */ u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u); /* Clear carry from T[i] */ *tmpc++ &= MP_MASK; } /* now copy higher words if any, e.g. if A has more digits than B */ for (; i < max; i++) { /* T[i] = A[i] - U */ *tmpc = *tmpa++ - u; /* U = carry bit of T[i] */ u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u); /* Clear carry from T[i] */ *tmpc++ &= MP_MASK; } /* clear digits above used (since we may not have grown result above) */ MP_ZERO_DIGITS(tmpc, olduse - c->used); } mp_clamp(c); return MP_OKAY; } #endif