## File: bn_s_mp_sub.c

package info (click to toggle)
libtommath 0.42.0-1
 `1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889` ``````#include #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. * * The library is free for all purposes without any express * guarantee it works. * * Tom St Denis, tomstdenis@gmail.com, http://libtom.org */ /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */ int s_mp_sub (mp_int * a, 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; { register mp_digit u, *tmpa, *tmpb, *tmpc; register 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); /* 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_digit)(CHAR_BIT * sizeof (mp_digit) - 1)); /* 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 /* \$Source\$ */ /* \$Revision: 0.41 \$ */ /* \$Date: 2007-04-18 09:58:18 +0000 \$ */ ``````