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

double gauss_rand(void)
{
    static double V1, V2, S;
    static int phase = 0;
    double ret;

    if ( phase == 0 ) {
        do {
            double U1 = (double)rand() / RAND_MAX;
            double U2 = (double)rand() / RAND_MAX;

            V1 = 2.0 * U1 - 1.0;
            V2 = 2.0 * U2 - 1.0;
            S = V1 * V1 + V2 * V2;
        } while ( S >=1 || S == 0 );
            ret = V1 * sqrt( -2.0 * log(S) / S);
        } else {
            ret = V2 * sqrt( -2.0 * log(S) / S);
        }
        phase = 1 - phase;

        return ret;
}