#include "nr3.h"
#include "ran.h"

int main() {
// random walk
// test the probability that 2 persons find eachother, in a 10 by 10 grid.
// they find eachother if they arrive at the same square.
// they move 1 step in random direction every even second, (person 1) and odd
// second (person 2)
// test the average time it takes for an encounter 

	const int Ntrial = 1000000;
	double steps=0; // average and mean number of time steps needed
	double stepssqr=0;
	Ran ran1(1234567);
	for (int i=0; i<Ntrial; i++) {
		int trialstep=0;
		int x1= (int) (ran1.doub()*10);
		int y1= (int) (ran1.doub()*10);
		int x2= (int) (ran1.doub()*10);
		int y2= (int) (ran1.doub()*10);
		while ((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2) > 0) {
			trialstep+=1;
			if (trialstep%2) {
			   double z=ran1.doub();
			   if (z<0.25) x1=((x1+1)%10); // step to right
			   else if (z<0.5) x1=(x1+9)%10; // step to left
			   else if (z<0.75) y1=(y1+9)%10; // step down
			   else y1=(y1+1)%10; // step up
			} else {
			   double z=ran1.doub();
			   if (z<0.25) x2=((x2+1)%10); // step to right
			   else if (z<0.5) x2=(x2+9)%10; // step to left
			   else if (z<0.75) y2=(y2+9)%10; // step down
			   else y2=(y2+1)%10; // step up
			}
		}
		steps+=trialstep;
		stepssqr+=trialstep*trialstep;
	}
	cout << "Ntrials: " << Ntrial << endl;
	cout << "average number of steps : " << steps/Ntrial;
	cout << " RMS : " << sqrt(stepssqr/Ntrial-steps*steps/Ntrial/Ntrial) << endl;
	return 0;
}