/*************************    Program 2.D    ****************************/
/*                                                                      */
/************************************************************************/
/* Please Note:                                                         */
/*                                                                      */
/* (1) This computer program is written by Tao Pang in conjunction with */
/*     his book, "An Introduction to Computational Physics," published  */
/*     by Cambridge University Press in 1997.                           */
/*                                                                      */
/* (2) No warranties, express or implied, are made for this program.    */
/*                                                                      */
/************************************************************************/

#include <stdio.h>
#include <time.h>
#include <math.h>
#define NMAX 1000
int iseed;
double pi,x,y;

main()
/* Another example program to use the Gaussian random number generator.
   Copyright (c) Tao Pang 1999. */
{
int i,n;
int t1,t2,t3,t4,t5,t6;
time_t ts,tp;
struct tm t;
void grnf();
extern int iseed;
extern double pi,x,y;

pi =  4.0*atan(1.0);
n = NMAX;

/* Initial seed from the system time and and forced to be odd */

ts = time(&tp);
t  = *gmtime(&tp);
t1 = t.tm_sec;
t2 = t.tm_min;
t3 = t.tm_hour;
t4 = t.tm_mday;
t5 = t.tm_mon;
t6 = t.tm_year;
iseed = t6+70*(t5+12*(t4+31*(t3+23*(t2+59*t1))));
if ((iseed%2) == 0)
  {
  iseed = iseed-1;
  }

/* Generate the Gaussian random number set */

for (i = 0; i < n; ++i)
  {
  grnf();
  printf("%16.8lf %16.8lf\n", x,y);
  }
}

void grnf()
/* Two Gaussian random numbers generated from two uniform
   random numbers.  Copyright (c) Tao Pang 1999. */
{
extern int iseed;
extern double pi,x,y;
double r1,r2;
double ranf();

r1 = -log(1.0-ranf());
r2 = 2.0*pi*ranf();
r1 = sqrt(2.0*r1);
x  = r1*cos(r2);
y  = r1*sin(r2);
}

double ranf()
/* Uniform random number generator x(n+1)= a*x(n) mod c
   with a = pow(7,5) and c = pow(2,31)-1.
   Copyright (c) Tao Pang 1997. */
{
const int ia=16807,ic=2147483647,iq=127773,ir=2836;
int il,ih,it;
double rc;
extern int iseed;
ih = iseed/iq;
il = iseed%iq;
it = ia*il-ir*ih;
if (it > 0)
  {
  iseed = it;
  }
else
  {
iseed = ic+it;
  }
rc = ic;
return iseed/rc;
}