#include <math.h>

double gasdev(long *idum)
{
	double ran1(long *idum);
	static int iset=0;
	static double gset;
	double fac,rsq,v1,v2;

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