bnldev.c

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#include <math.h>
#define PI 3.141592654

float bnldev(float pp, int n, long *idum)
{
	float gammln(float xx);
	float ran1(long *idum);
	int j;
	static int nold=(-1);
	float am,em,g,angle,p,bnl,sq,t,y;
	static float pold=(-1.0),pc,plog,pclog,en,oldg;

	p=(pp <= 0.5 ? pp : 1.0-pp);
	am=n*p;
	if (n < 25) {
		bnl=0.0;
		for (j=1;j<=n;j++)
			if (ran1(idum) < p) ++bnl;
	} else if (am < 1.0) {
		g=exp(-am);
		t=1.0;
		for (j=0;j<=n;j++) {
			t *= ran1(idum);
			if (t < g) break;
		}
		bnl=(j <= n ? j : n);
	} else {
		if (n != nold) {
			en=n;
			oldg=gammln(en+1.0);
			nold=n;
		} if (p != pold) {
			pc=1.0-p;
			plog=log(p);
			pclog=log(pc);
			pold=p;
		}
		sq=sqrt(2.0*am*pc);
		do {
			do {
				angle=PI*ran1(idum);
				y=tan(angle);
				em=sq*y+am;
			} while (em < 0.0 || em >= (en+1.0));
			em=floor(em);
			t=1.2*sq*(1.0+y*y)*exp(oldg-gammln(em+1.0)
				-gammln(en-em+1.0)+em*plog+(en-em)*pclog);
		} while (ran1(idum) > t);
		bnl=em;
	}
	if (p != pp) bnl=n-bnl;
	return bnl;
}
#undef PI