annotate bn_mp_mod_2d.c @ 2:86e0b50a9b58 libtommath-orig ltm-0.30-orig

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author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:25:22 +0000
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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 /* calc a value mod 2**b */
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18 int
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19 mp_mod_2d (mp_int * a, int b, mp_int * c)
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20 {
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21 int x, res;
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22
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23 /* if b is <= 0 then zero the int */
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24 if (b <= 0) {
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25 mp_zero (c);
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26 return MP_OKAY;
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27 }
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28
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29 /* if the modulus is larger than the value than return */
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30 if (b > (int) (a->used * DIGIT_BIT)) {
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31 res = mp_copy (a, c);
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32 return res;
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33 }
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34
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35 /* copy */
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36 if ((res = mp_copy (a, c)) != MP_OKAY) {
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37 return res;
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38 }
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39
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40 /* zero digits above the last digit of the modulus */
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41 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
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42 c->dp[x] = 0;
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43 }
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44 /* clear the digit that is not completely outside/inside the modulus */
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45 c->dp[b / DIGIT_BIT] &=
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46 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
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47 mp_clamp (c);
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48 return MP_OKAY;
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49 }