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