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, tomstdenis@gmail.com, 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: /cvsroot/tcl/libtommath/bn_mp_sub.c,v $ */ |
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58 | /* $Revision: 1.1.1.3 $ */ |
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59 | /* $Date: 2006/12/01 00:08:11 $ */ |
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