// rnd_walk2.cxx
// Random walk in two dimension
// ak 4/30/2002
#include <iostream>
#include <fstream>
#include <math.h>
#include <iomanip>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

#include "TROOT.h"
#include "TTree.h"	// we will use a tree to store our values
#include "TApplication.h"
#include "TFile.h"


int rand(void);
void srand(unsigned int);

void main(int argc, char **argv)
{

	//structure for our random walk variables
	struct walk_t { double x;  // the position after a step
			double y; //
			int nstep; // the step number
			int jloop; // number of outer loops
		       };
	 walk_t walk;
		
	 int i_loop;	//inner loop
	 int j_loop;	//outer loop
	 int jloop_max=5000; // the max number of different trials
	
	 
	  unsigned int seed = 557 ; // here is the starting value or seed

	   int loop_max=100; // the maximum number of steps	

	  double rnd1;
	  double rnd2;
	  double y_temp; // temporary variable for y


 	TROOT root("hello","computational physics");
	
	// initialize root
	   TApplication theApp("App", &argc, argv);


	//open output file
	TFile *out_file = new TFile("rnd_walk557_2.root","RECREATE","example of
	random random walk"); // create root file
	
	// Declare tree
	TTree *ran_walk = new TTree("ran_walk","tree with random walk variables");
	 
	 ran_walk->Branch("walk",&walk.x, "x/D:y/D:nstep/I:jloop/I");

	// set seed value
	srand(seed);
	
	// the outer loop, trying the walk jloop times
	for(j_loop=0;j_loop < jloop_max ;j_loop= j_loop+1)
        {
		walk.x=0.;
		walk.y=0.;
		walk.nstep=0;
		walk.jloop=j_loop+1;

	
	 for(i_loop=0;i_loop < loop_max ;i_loop= i_loop+1)
	 {
	  // here we get the step
	 	rnd1=double(rand())/double(RAND_MAX);
		 rnd1=2*rnd1-1.;
	          walk.x=walk.x+rnd1;
		
		   if(rnd1*rnd1>1.) rnd1=1.; //safety for square root.
		   y_temp=sqrt(1.-rnd1*rnd1);
		
	 	rnd2=double(rand())/double(RAND_MAX);
 	  	 if((rnd2-.5)<=0.)
		  {
		    walk.y=walk.y-y_temp;
		  }
		  else 
		  {
		    walk.y=walk.y+y_temp;
		   }
	     
	     walk.nstep=walk.nstep+1;
	     // fill the tree
	       ran_walk->Fill();
	    
	  }
	 }
	out_file->Write();

}