1#include <tommath.h>
2#ifdef BN_S_MP_SUB_C
3/* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
16 */
17
18/* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
19int
20s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
21{
22  int     olduse, res, min, max;
23
24  /* find sizes */
25  min = b->used;
26  max = a->used;
27
28  /* init result */
29  if (c->alloc < max) {
30    if ((res = mp_grow (c, max)) != MP_OKAY) {
31      return res;
32    }
33  }
34  olduse = c->used;
35  c->used = max;
36
37  {
38    register mp_digit u, *tmpa, *tmpb, *tmpc;
39    register int i;
40
41    /* alias for digit pointers */
42    tmpa = a->dp;
43    tmpb = b->dp;
44    tmpc = c->dp;
45
46    /* set carry to zero */
47    u = 0;
48    for (i = 0; i < min; i++) {
49      /* T[i] = A[i] - B[i] - U */
50      *tmpc = *tmpa++ - *tmpb++ - u;
51
52      /* U = carry bit of T[i]
53       * Note this saves performing an AND operation since
54       * if a carry does occur it will propagate all the way to the
55       * MSB.  As a result a single shift is enough to get the carry
56       */
57      u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
58
59      /* Clear carry from T[i] */
60      *tmpc++ &= MP_MASK;
61    }
62
63    /* now copy higher words if any, e.g. if A has more digits than B  */
64    for (; i < max; i++) {
65      /* T[i] = A[i] - U */
66      *tmpc = *tmpa++ - u;
67
68      /* U = carry bit of T[i] */
69      u = *tmpc >> ((mp_digit)(CHAR_BIT * sizeof (mp_digit) - 1));
70
71      /* Clear carry from T[i] */
72      *tmpc++ &= MP_MASK;
73    }
74
75    /* clear digits above used (since we may not have grown result above) */
76    for (i = c->used; i < olduse; i++) {
77      *tmpc++ = 0;
78    }
79  }
80
81  mp_clamp (c);
82  return MP_OKAY;
83}
84
85#endif
86
87/* $Source: /cvs/libtom/libtommath/bn_s_mp_sub.c,v $ */
88/* $Revision: 1.3 $ */
89/* $Date: 2006/03/31 14:18:44 $ */
90