#ifndef CA_UWATERLOO_ALUMNI_DWHARDER_RATIO
#define CA_UWATERLOO_ALUMNI_DWHARDER_RATIO

// Author:  Douglas Wilhelm Harder
// Copyright (c) 2012 by Douglas Wilhelm Harder.  All rights reserved.
// Reference:  Donald E. Knuth, "The Art of Computer Programming, Vol. 2, Seminumerical Algorithms", 3rd Ed., Addison Wesley, Toronto, 1998, p.130-2.

// On successive calls to randn_ratio(), this function returns
// a random standard normal value (a mean of zero and standard deviation of one).

#include <stdlib.h>
#include <math.h>

double randn_ratio() {
	while ( 1 ) {
		double u1 = drand48();
		double u2 = drand48();
		double s = sqrt( 8.0/exp(1.0) ) * (u2 - 0.5)/u1;

		double ss = s*s;

		if ( ( ss <= 5.0 - 4.0*exp(0.25)*u1 ) || (
			( ss < 4.0*exp(-1.35)/u1 + 1.4 ) && ( ss <= -4.0/log( u1 ) )
		) ) {
			return s;
		}
	}
}

#endif