## File: bn_s_mp_sub.c

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libtommath 1.1.0-3
 `12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182838485` ``````#include "tommath_private.h" #ifdef BN_S_MP_SUB_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. * * SPDX-License-Identifier: Unlicense */ /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ int s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c) { int olduse, res, min, max; /* find sizes */ min = b->used; max = a->used; /* init result */ if (c->alloc < max) { if ((res = mp_grow(c, max)) != MP_OKAY) { return res; } } 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 >> (((size_t)CHAR_BIT * sizeof(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 >> (((size_t)CHAR_BIT * sizeof(mp_digit)) - 1u); /* Clear carry from T[i] */ *tmpc++ &= MP_MASK; } /* clear digits above used (since we may not have grown result above) */ for (i = c->used; i < olduse; i++) { *tmpc++ = 0; } } mp_clamp(c); return MP_OKAY; } #endif /* ref: HEAD -> master, tag: v1.1.0 */ /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */ /* commit time: 2019-01-28 20:32:32 +0100 */ ``````