#include <math.h>

/* used for uniform random variable (0.1) */
#define A       16807.0         /* multiplier */
#define M       2147483647.0    /* modulus */
static double r_seed=1.0;       /* seed number */

/************************************************************
 * setSeed -- set the seed                                  *
 ************************************************************/
void setSeed( double s ) {
  r_seed = fmod( s, M );
}

/************************************************************
 * rnd -- generates a uniform random variable (0, 1)        *
 ************************************************************/
double rnd( )
{
  r_seed = fmod( A*r_seed, M);
  return (r_seed*4.656612875e-10);
}

/************************************************************
 * gaussian -- generates a Gaussian random variable         *
 *             with mean and standard deviation d           *
 ************************************************************/
double gaussian( a, d ) 
double a, d;
{
  static double t = 0.0;
  double x, v1, v2, r;

  if( t==0.0 ) {
    do {
      v1 = 2.0*rnd()-1.0;
      v2 = 2.0*rnd()-1.0;
      r = v1*v1 + v2*v2;
    } while( r>=1.0 );
    r = sqrt((-2.0*log(r))/r);
    t = v2*r;
    return (a+v1*r*d);
  } else {
    x = t;
    t = 0.0;
    return (a + x*d);
  }
}