ref: 909323e92a4e694bff5908e500d8655ac1bbd932
dir: /sys/src/libmp/port/mpexp.c/
#include "os.h" #include <mp.h> #include "dat.h" // res = b**e // // knuth, vol 2, pp 398-400 enum { Freeb= 0x1, Freee= 0x2, Freem= 0x4, }; //int expdebug; void mpexp(mpint *b, mpint *e, mpint *m, mpint *res) { mpint *t[2]; int tofree; mpdigit d, bit; int i, j; assert(m == nil || m->flags & MPnorm); assert((e->flags & MPtimesafe) == 0); res->flags |= b->flags & MPtimesafe; i = mpcmp(e,mpzero); if(i==0){ mpassign(mpone, res); return; } if(i<0) sysfatal("mpexp: negative exponent"); t[0] = mpcopy(b); t[1] = res; tofree = 0; if(res == b){ b = mpcopy(b); tofree |= Freeb; } if(res == e){ e = mpcopy(e); tofree |= Freee; } if(res == m){ m = mpcopy(m); tofree |= Freem; } // skip first bit i = e->top-1; d = e->p[i]; for(bit = mpdighi; (bit & d) == 0; bit >>= 1) ; bit >>= 1; j = 0; for(;;){ for(; bit != 0; bit >>= 1){ if(m != nil) mpmodmul(t[j], t[j], m, t[j^1]); else mpmul(t[j], t[j], t[j^1]); if(bit & d) { if(m != nil) mpmodmul(t[j^1], b, m, t[j]); else mpmul(t[j^1], b, t[j]); } else j ^= 1; } if(--i < 0) break; bit = mpdighi; d = e->p[i]; } if(t[j] == res){ mpfree(t[j^1]); } else { mpassign(t[j], res); mpfree(t[j]); } if(tofree){ if(tofree & Freeb) mpfree(b); if(tofree & Freee) mpfree(e); if(tofree & Freem) mpfree(m); } }