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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
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2 * |
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3 * LibTomMath is a library that provides multiple-precision |
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4 * integer arithmetic as well as number theoretic functionality. |
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5 * |
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6 * The library was designed directly after the MPI library by |
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7 * Michael Fromberger but has been written from scratch with |
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8 * additional optimizations in place. |
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9 * |
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10 * The library is free for all purposes without any express |
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11 * guarantee it works. |
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12 * |
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
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14 */ |
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15 #include <tommath.h> |
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16 |
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17 /* high level subtraction (handles signs) */ |
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18 int |
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19 mp_sub (mp_int * a, mp_int * b, mp_int * c) |
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20 { |
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21 int sa, sb, res; |
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22 |
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23 sa = a->sign; |
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24 sb = b->sign; |
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25 |
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26 if (sa != sb) { |
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27 /* subtract a negative from a positive, OR */ |
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28 /* subtract a positive from a negative. */ |
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29 /* In either case, ADD their magnitudes, */ |
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30 /* and use the sign of the first number. */ |
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31 c->sign = sa; |
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32 res = s_mp_add (a, b, c); |
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33 } else { |
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34 /* subtract a positive from a positive, OR */ |
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35 /* subtract a negative from a negative. */ |
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36 /* First, take the difference between their */ |
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37 /* magnitudes, then... */ |
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38 if (mp_cmp_mag (a, b) != MP_LT) { |
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39 /* Copy the sign from the first */ |
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40 c->sign = sa; |
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41 /* The first has a larger or equal magnitude */ |
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42 res = s_mp_sub (a, b, c); |
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43 } else { |
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44 /* The result has the *opposite* sign from */ |
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45 /* the first number. */ |
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46 c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS; |
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47 /* The second has a larger magnitude */ |
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48 res = s_mp_sub (b, a, c); |
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49 } |
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50 } |
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51 return res; |
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52 } |
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53 |
