1 #include "os.h"
2 #include <mp.h>
3 #include <libsec.h>
4 #include "dat.h"
5 
6 mpint*
7 mpfactorial(ulong n)
8 {
9 	int i;
10 	ulong k;
11 	unsigned cnt;
12 	int max, mmax;
13 	mpdigit p, pp[2];
14 	mpint *r, *s, *stk[31];
15 
16 	cnt = 0;
17 	max = mmax = -1;
18 	p = 1;
19 	r = mpnew(0);
20 	for(k=2; k<=n; k++){
21 		pp[0] = 0;
22 		pp[1] = 0;
23 		mpvecdigmuladd(&p, 1, (mpdigit)k, pp);
24 		if(pp[1] == 0)	/* !overflow */
25 			p = pp[0];
26 		else{
27 			cnt++;
28 			if((cnt & 1) == 0){
29 				s = stk[max];
30 				mpbits(r, Dbits*(s->top+1+1));
31 				memset(r->p, 0, Dbytes*(s->top+1+1));
32 				mpvecmul(s->p, s->top, &p, 1, r->p);
33 				r->sign = 1;
34 				r->top = s->top+1+1;		/* XXX: norm */
35 				mpassign(r, s);
36 				for(i=4; (cnt & (i-1)) == 0; i=i<<1){
37 					mpmul(stk[max], stk[max-1], r);
38 					mpassign(r, stk[max-1]);
39 					max--;
40 				}
41 			}else{
42 				max++;
43 				if(max > mmax){
44 					mmax++;
45 					if(max > nelem(stk))
46 						abort();
47 					stk[max] = mpnew(Dbits);
48 				}
49 				stk[max]->top = 1;
50 				stk[max]->p[0] = p;
51 			}
52 			p = (mpdigit)k;
53 		}
54 	}
55 	if(max < 0){
56 		mpbits(r, Dbits);
57 		r->top = 1;
58 		r->sign = 1;
59 		r->p[0] = p;
60 	}else{
61 		s = stk[max--];
62 		mpbits(r, Dbits*(s->top+1+1));
63 		memset(r->p, 0, Dbytes*(s->top+1+1));
64 		mpvecmul(s->p, s->top, &p, 1, r->p);
65 		r->sign = 1;
66 		r->top = s->top+1+1;		/* XXX: norm */
67 	}
68 
69 	while(max >= 0)
70 		mpmul(r, stk[max--], r);
71 	for(max=mmax; max>=0; max--)
72 		mpfree(stk[max]);
73 	mpnorm(r);
74 	return r;
75 }