annotate bn_mp_div_2d.c @ 145:a96ff234ff19 libtommath

propagate of fc94c38452d9fd684a8e1eb9e3a73120aac0d38f and 3e4de4cbef3d9035a7b2f0c25e9f86e297f9f6d1 from branch 'au.asn.ucc.matt.ltm-orig' to 'au.asn.ucc.matt.ltm-db'
author Matt Johnston <matt@ucc.asn.au>
date Sun, 19 Dec 2004 15:57:19 +0000
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d29b64170cf0 import of libtommath 0.32
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1 #include <tommath.h>
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2 #ifdef BN_MP_DIV_2D_C
2
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * The library is free for all purposes without any express
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13 * guarantee it works.
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14 *
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15 * Tom St Denis, [email protected], http://math.libtomcrypt.org
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16 */
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17
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18 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
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19 int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d)
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20 {
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21 mp_digit D, r, rr;
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22 int x, res;
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23 mp_int t;
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24
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25
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26 /* if the shift count is <= 0 then we do no work */
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27 if (b <= 0) {
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28 res = mp_copy (a, c);
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29 if (d != NULL) {
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30 mp_zero (d);
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31 }
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32 return res;
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33 }
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34
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35 if ((res = mp_init (&t)) != MP_OKAY) {
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36 return res;
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37 }
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38
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39 /* get the remainder */
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40 if (d != NULL) {
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41 if ((res = mp_mod_2d (a, b, &t)) != MP_OKAY) {
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42 mp_clear (&t);
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43 return res;
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44 }
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45 }
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46
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47 /* copy */
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48 if ((res = mp_copy (a, c)) != MP_OKAY) {
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49 mp_clear (&t);
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50 return res;
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51 }
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52
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53 /* shift by as many digits in the bit count */
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54 if (b >= (int)DIGIT_BIT) {
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55 mp_rshd (c, b / DIGIT_BIT);
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56 }
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57
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58 /* shift any bit count < DIGIT_BIT */
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59 D = (mp_digit) (b % DIGIT_BIT);
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60 if (D != 0) {
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61 register mp_digit *tmpc, mask, shift;
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62
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63 /* mask */
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64 mask = (((mp_digit)1) << D) - 1;
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65
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66 /* shift for lsb */
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67 shift = DIGIT_BIT - D;
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68
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69 /* alias */
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70 tmpc = c->dp + (c->used - 1);
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71
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72 /* carry */
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73 r = 0;
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74 for (x = c->used - 1; x >= 0; x--) {
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75 /* get the lower bits of this word in a temp */
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76 rr = *tmpc & mask;
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77
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78 /* shift the current word and mix in the carry bits from the previous word */
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79 *tmpc = (*tmpc >> D) | (r << shift);
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80 --tmpc;
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81
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82 /* set the carry to the carry bits of the current word found above */
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83 r = rr;
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84 }
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85 }
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86 mp_clamp (c);
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87 if (d != NULL) {
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88 mp_exch (&t, d);
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89 }
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90 mp_clear (&t);
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91 return MP_OKAY;
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92 }
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93 #endif