97 lines
1.4 KiB
C
97 lines
1.4 KiB
C
#include "os.h"
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#include <mp.h>
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#include "dat.h"
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// res = b**e
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//
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// knuth, vol 2, pp 398-400
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enum {
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Freeb= 0x1,
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Freee= 0x2,
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Freem= 0x4,
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};
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//int expdebug;
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void
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mpexp(mpint *b, mpint *e, mpint *m, mpint *res)
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{
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mpint *t[2];
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int tofree;
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mpdigit d, bit;
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int i, j;
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assert(m == nil || m->flags & MPnorm);
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assert((e->flags & MPtimesafe) == 0);
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res->flags |= b->flags & MPtimesafe;
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i = mpcmp(e,mpzero);
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if(i==0){
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mpassign(mpone, res);
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return;
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}
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if(i<0)
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jehanne_sysfatal("mpexp: negative exponent");
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t[0] = mpcopy(b);
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t[1] = res;
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tofree = 0;
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if(res == b){
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b = mpcopy(b);
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tofree |= Freeb;
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}
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if(res == e){
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e = mpcopy(e);
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tofree |= Freee;
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}
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if(res == m){
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m = mpcopy(m);
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tofree |= Freem;
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}
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// skip first bit
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i = e->top-1;
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d = e->p[i];
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for(bit = mpdighi; (bit & d) == 0; bit >>= 1)
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;
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bit >>= 1;
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j = 0;
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for(;;){
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for(; bit != 0; bit >>= 1){
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if(m != nil)
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mpmodmul(t[j], t[j], m, t[j^1]);
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else
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mpmul(t[j], t[j], t[j^1]);
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if(bit & d) {
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if(m != nil)
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mpmodmul(t[j^1], b, m, t[j]);
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else
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mpmul(t[j^1], b, t[j]);
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} else
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j ^= 1;
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}
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if(--i < 0)
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break;
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bit = mpdighi;
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d = e->p[i];
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}
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if(t[j] == res){
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mpfree(t[j^1]);
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} else {
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mpassign(t[j], res);
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mpfree(t[j]);
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}
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if(tofree){
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if(tofree & Freeb)
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mpfree(b);
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if(tofree & Freee)
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mpfree(e);
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if(tofree & Freem)
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mpfree(m);
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}
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}
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