92 lines
2.1 KiB
C
92 lines
2.1 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtommath.iahu.ca
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*/
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#include <tommath.h>
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/* low level addition, based on HAC pp.594, Algorithm 14.7 */
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int
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s_mp_add (mp_int * a, mp_int * b, mp_int * c)
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{
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mp_int *x;
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int olduse, res, min, max, i;
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mp_digit u;
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/* find sizes, we let |a| <= |b| which means we have to sort
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* them. "x" will point to the input with the most digits
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*/
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if (a->used > b->used) {
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min = b->used;
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max = a->used;
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x = a;
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} else if (a->used < b->used) {
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min = a->used;
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max = b->used;
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x = b;
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} else {
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min = max = a->used;
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x = NULL;
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}
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/* init result */
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if (c->alloc < max + 1) {
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if ((res = mp_grow (c, max + 1)) != MP_OKAY) {
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return res;
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}
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}
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olduse = c->used;
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c->used = max + 1;
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/* add digits from lower part */
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/* set the carry to zero */
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u = 0;
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for (i = 0; i < min; i++) {
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/* Compute the sum at one digit, T[i] = A[i] + B[i] + U */
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c->dp[i] = a->dp[i] + b->dp[i] + u;
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/* U = carry bit of T[i] */
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u = (c->dp[i] >> DIGIT_BIT) & 1;
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/* take away carry bit from T[i] */
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c->dp[i] &= MP_MASK;
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}
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/* now copy higher words if any, that is in A+B if A or B has more digits add those in */
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if (min != max) {
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for (; i < max; i++) {
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/* T[i] = X[i] + U */
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c->dp[i] = x->dp[i] + u;
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/* U = carry bit of T[i] */
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u = (c->dp[i] >> DIGIT_BIT) & 1;
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/* take away carry bit from T[i] */
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c->dp[i] &= MP_MASK;
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}
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}
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/* add carry */
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c->dp[i] = u;
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/* clear digits above used (since we may not have grown result above) */
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for (i = c->used; i < olduse; i++) {
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c->dp[i] = 0;
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}
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mp_clamp (c);
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return MP_OKAY;
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}
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