#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <math.h> 
#define Xrange 2
#define Yrange 2
#define r 0.1

using namespace std;

double Ex(double q1, double x1, double y1,double xt, double yt){
  //Electric field (x)
  return q1*(xt-x1)/pow((pow((x1-xt),2)+pow((y1-yt),2)),1.5);
}
double Ey(double q1, double x1, double y1,double xt, double yt){
  //Electric field (y)
  return q1*(yt-y1)/pow((pow((x1-xt),2)+pow((y1-yt),2)),1.5);
}


int main(int argc, char *argv[]){
  int i;
  double pos[2][2],q[2];//cahrges
  double tpos[2],npos[2],lpos[2];//test position
  double V,tV;
  double E[2];
  double phi,phi_best;
  double diff,diff_least;

  //charge1 -1 at (0,0)
  pos[0][0]=0;
  pos[0][1]=0;
  q[0]=-1;
 
  //initial position
  if(argc<3){
    tpos[0]=0;
    tpos[1]=0;
  }
  else{
  tpos[0]=atof(argv[1]);
  tpos[1]=atof(argv[2]);
  }
  cerr <<"initial parameter: pos=("<<tpos[0]<<", "<<tpos[1]<<")"<<endl;
  cout <<tpos[0]<<"\t"<<tpos[1]<<endl;  

  while(fabs(tpos[0])<=Xrange&&fabs(tpos[1])<=Yrange){
    //get electruic field
    E[0]=Ex(q[0], pos[0][0], pos[0][1],tpos[0], tpos[1]);
    E[1]=Ey(q[0], pos[0][0], pos[0][1],tpos[0], tpos[1]);
    //calculate the angle
    if(E[0]>0){
      phi=atan(E[1]/E[0]);
    }
    else if(E[0]<0) 
      phi=M_PI+atan(E[1]/E[0]);
    else phi=0;
    //move the test charge
    tpos[0]+=r*cos(phi);
    tpos[1]+=r*sin(phi);
    //out put
    cout <<tpos[0]<<"\t"<<tpos[1]<<endl;
    if(pow((tpos[0]-pos[0][0]),2)+pow((tpos[1]-pos[0][1]),2)<r*r){
      // test the distance to the charge
      cout << pos[0][0]<<"\t"<<pos[0][1]<<endl;
      break;
    }
  }
  return 0;
}