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annotate libtommath/bn_mp_sub.c @ 670:349fb2dda2dd

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author Matt Johnston <matt@ucc.asn.au>
date Thu, 12 Apr 2012 21:57:30 +0800
parents 5ff8218bcee9
children 60fc6476e044
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1 #include <tommath.h>
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2 #ifdef BN_MP_SUB_C
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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.com
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16 */
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17
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18 /* high level subtraction (handles signs) */
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19 int
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20 mp_sub (mp_int * a, mp_int * b, mp_int * c)
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21 {
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22 int sa, sb, res;
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23
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24 sa = a->sign;
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25 sb = b->sign;
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26
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27 if (sa != sb) {
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28 /* subtract a negative from a positive, OR */
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29 /* subtract a positive from a negative. */
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30 /* In either case, ADD their magnitudes, */
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31 /* and use the sign of the first number. */
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32 c->sign = sa;
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33 res = s_mp_add (a, b, c);
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34 } else {
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35 /* subtract a positive from a positive, OR */
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36 /* subtract a negative from a negative. */
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37 /* First, take the difference between their */
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38 /* magnitudes, then... */
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39 if (mp_cmp_mag (a, b) != MP_LT) {
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40 /* Copy the sign from the first */
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41 c->sign = sa;
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42 /* The first has a larger or equal magnitude */
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43 res = s_mp_sub (a, b, c);
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44 } else {
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45 /* The result has the *opposite* sign from */
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46 /* the first number. */
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47 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
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48 /* The second has a larger magnitude */
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49 res = s_mp_sub (b, a, c);
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50 }
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51 }
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52 return res;
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53 }
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54
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55 #endif
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56
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57 /* $Source: /cvs/libtom/libtommath/bn_mp_sub.c,v $ */
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58 /* $Revision: 1.3 $ */
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59 /* $Date: 2006/03/31 14:18:44 $ */