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1 #include <tommath.h> |
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2 #ifdef BN_MP_SUB_C |
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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 /* 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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142
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55 #endif |
