1 /*	$NetBSD: bn_mp_sub_d.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $	*/
2 
3 #include <tommath.h>
4 #ifdef BN_MP_SUB_D_C
5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
6  *
7  * LibTomMath is a library that provides multiple-precision
8  * integer arithmetic as well as number theoretic functionality.
9  *
10  * The library was designed directly after the MPI library by
11  * Michael Fromberger but has been written from scratch with
12  * additional optimizations in place.
13  *
14  * The library is free for all purposes without any express
15  * guarantee it works.
16  *
17  * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18  */
19 
20 /* single digit subtraction */
21 int
mp_sub_d(mp_int * a,mp_digit b,mp_int * c)22 mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
23 {
24   mp_digit *tmpa, *tmpc, mu;
25   int       res, ix, oldused;
26 
27   /* grow c as required */
28   if (c->alloc < a->used + 1) {
29      if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
30         return res;
31      }
32   }
33 
34   /* if a is negative just do an unsigned
35    * addition [with fudged signs]
36    */
37   if (a->sign == MP_NEG) {
38      a->sign = MP_ZPOS;
39      res     = mp_add_d(a, b, c);
40      a->sign = c->sign = MP_NEG;
41 
42      /* clamp */
43      mp_clamp(c);
44 
45      return res;
46   }
47 
48   /* setup regs */
49   oldused = c->used;
50   tmpa    = a->dp;
51   tmpc    = c->dp;
52 
53   /* if a <= b simply fix the single digit */
54   if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
55      if (a->used == 1) {
56         *tmpc++ = b - *tmpa;
57      } else {
58         *tmpc++ = b;
59      }
60      ix      = 1;
61 
62      /* negative/1digit */
63      c->sign = MP_NEG;
64      c->used = 1;
65   } else {
66      /* positive/size */
67      c->sign = MP_ZPOS;
68      c->used = a->used;
69 
70      /* subtract first digit */
71      *tmpc    = *tmpa++ - b;
72      mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
73      *tmpc++ &= MP_MASK;
74 
75      /* handle rest of the digits */
76      for (ix = 1; ix < a->used; ix++) {
77         *tmpc    = *tmpa++ - mu;
78         mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
79         *tmpc++ &= MP_MASK;
80      }
81   }
82 
83   /* zero excess digits */
84   while (ix++ < oldused) {
85      *tmpc++ = 0;
86   }
87   mp_clamp(c);
88   return MP_OKAY;
89 }
90 
91 #endif
92 
93 /* Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v  */
94 /* Revision: 1.6  */
95 /* Date: 2006/12/28 01:25:13  */
96