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#include "os.h"
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#include <mp.h>
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#include "dat.h"
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mpint*
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mpfactorial(ulong n)
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{
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	int i;
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	ulong k;
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	unsigned cnt;
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	int max, mmax;
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	mpdigit p, pp[2];
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	mpint *r, *s, *stk[31];
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	cnt = 0;
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	max = mmax = -1;
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	p = 1;
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	r = mpnew(0);
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	for(k=2; k<=n; k++){
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		pp[0] = 0;
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		pp[1] = 0;
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		mpvecdigmuladd(&p, 1, (mpdigit)k, pp);
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		if(pp[1] == 0)	/* !overflow */
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			p = pp[0];
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		else{
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			cnt++;
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			if((cnt & 1) == 0){
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				s = stk[max];
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				mpbits(r, Dbits*(s->top+1+1));
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				memset(r->p, 0, Dbytes*(s->top+1+1));
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				mpvecmul(s->p, s->top, &p, 1, r->p);
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				r->sign = 1;
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				r->top = s->top+1+1;		/* XXX: norm */
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				mpassign(r, s);
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				for(i=4; (cnt & (i-1)) == 0; i=i<<1){
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					mpmul(stk[max], stk[max-1], r);
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					mpassign(r, stk[max-1]);
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					max--;
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				}
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			}else{
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				max++;
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				if(max > mmax){
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					mmax++;
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					if(max > nelem(stk))
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						abort();
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					stk[max] = mpnew(Dbits);
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				}
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				stk[max]->top = 1;
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				stk[max]->p[0] = p;
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			}
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			p = (mpdigit)k;
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		}
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	}
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	if(max < 0){
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		mpbits(r, Dbits);
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		r->top = 1;
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		r->sign = 1;
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		r->p[0] = p;
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	}else{
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		s = stk[max--];
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		mpbits(r, Dbits*(s->top+1+1));
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		memset(r->p, 0, Dbytes*(s->top+1+1));
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		mpvecmul(s->p, s->top, &p, 1, r->p);
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		r->sign = 1;
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		r->top = s->top+1+1;		/* XXX: norm */
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	}
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	while(max >= 0)
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		mpmul(r, stk[max--], r);
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	for(max=mmax; max>=0; max--)
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		mpfree(stk[max]);
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	mpnorm(r);
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	return r;
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}