#include <math.h>

float gasdev(idum)
int *idum;
{
	static int iset=0;
	static float gset;
	float fac,r,v1,v2;
	float ran1();

	if  (iset == 0) {
		do {
			v1=2.0*ran1(idum)-1.0;
			v2=2.0*ran1(idum)-1.0;
			r=v1*v1+v2*v2;
		} while (r >= 1.0 || r == 0.0);
		fac=sqrt(-2.0*log(r)/r);
		gset=v1*fac;
		iset=1;
		return v2*fac;
	} else {
		iset=0;
		return gset;
	}
}