//=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
//   Program turtle
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#include <stdio.h>
#include <math.h>
#include "nxgraph.h"

//---- physical setting
#define N          (9)            // number of drawing lines
#define dL         (1.0/128)      // turtle's one step
#define dV         (0.5)          // step of volatage

//---- graphic setting
#define WIN_WIDTH  (256)
#define WIN_HEIGHT (256)


double Direction( double x, double y, int& status )
{
  double r1, r2, fx, fy, theta;

  r1 = hypot(x+1.0,y);
  r2 = hypot(x-1.0,y);
  fx = (x+1.0)/(r1*r1*r1) - (x-1.0)/(r2*r2*r2);
  fy = (y)/(r1*r1*r1)     - (y)/(r2*r2*r2);

  theta = atan2( fy, fx );

  if( r2<0.01 || r2>16.0 ){
    status = 1;
  }else{
    status = 0;
  }

  return( theta );
}


//---- main function
int main(void)
{
  XEvent ev;
  NXOpenWindow("Electric line and Volate line", WIN_WIDTH, WIN_HEIGHT );

  int n, flag, status;
  double x, y, theta, V;

  NXSetColor( NXGetColor("green") );
  for( n=0 ; n<N ; n++ ){
    theta = 2*M_PI/N*n;
    x = -1.0;
    y =  0.0;
    NXDrawMoveto( WIN_WIDTH/2+(int)(64*x), WIN_HEIGHT/2+(int)(64*y) );

    while(1){
      x += dL*cos(theta);
      y += dL*sin(theta);
      NXDrawLineto( WIN_WIDTH/2+(int)(64*x), WIN_HEIGHT/2+(int)(64*y) );
      theta = Direction( x, y, status );
      if( status ) break;
    }
  }

  NXSetColor( NXGetColor("red") );
  for( n=0 ; n<N ; n++ ){
    V = dV*(n-N/2);
    x = -V/(1+sqrt(1+V*V));
    y = 0.0;
    NXDrawMoveto( WIN_WIDTH/2+(int)(64*x), WIN_HEIGHT/2+(int)(64*y) );

    flag=0;
    while(1){
      theta = Direction( x, y, status );
      if( status ) break;
      theta += M_PI/2;
      x += dL*cos(theta);
      y += dL*sin(theta);
      NXDrawLineto( WIN_WIDTH/2+(int)(64*x), WIN_HEIGHT/2+(int)(64*y) );
      if( flag==0 && y<0.0 ) flag=1;
      if( flag==1 && y>0.0 ) break;
    }
  }

  NXCheckEvent( NX_WAIT, ev );
  NXCloseWindow();

  return(0);
}