PHBBox.h

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// Tell emacs that this is a C++ source
//  -*- C++ -*-.
#ifndef G4MAIN_PHBBOX_H
#define G4MAIN_PHBBOX_H

#include <iostream>
#include <iomanip>
#include <bitset>

// Clip a line using the Cohen-Southerland algorithm

class PHBBox 
{
public:

  PHBBox(const double x0, const double y0, const double x1, const double y1) :
    _x0(x0), _y0(y0), _x1(x1), _y1(y1) 
  {}

  bool ClipLine(double& x0, double& y0, double& x1, double& y1) const
  {
    int clipCode0 = ClipCode(x0,y0); // clipping code for end point 0
    int clipCode1 = ClipCode(x1,y1); // clipping code for end point 1

    while ( clipCode0 || clipCode1 )
      {
        //std::cout << "clipCode0 = " << std::bitset<4>(clipCode0).to_string() << std::endl;
        //std::cout << "clipCode1 = " << std::bitset<4>(clipCode1).to_string() << std::endl;

        if ( clipCode0 & clipCode1 ) return false;

        int code = 0;
        if ( clipCode0 > 0 ) code = clipCode0; // clip the first point
        else                 code = clipCode1; // clip the last point

        double x = 0, y = 0;

        if ( (code & BOTTOM) == BOTTOM )
          {
            // Clip the line to the bottom of the box
            //std::cout << "Clip the line to the bottom of the box" << std::endl;
            y = _y0;
            x = x0 + (x1-x0)*(y-y0)/(y1-y0);
          }
        else if ( (code & TOP) == TOP ) 
          {
            // Clip the line to the top of the box
            //std::cout << "Clip the line to the top of the box" << std::endl;
            y = _y1;
            x = x0 + (x1-x0)*(y-y0)/(y1-y0);
          }
        else if ( (code & LEFT) == LEFT )
          {
            //std::cout << "Clip the line to the left of the box" << std::endl;
            x = _x0;
            y = y0 + (y1-y0)*(x-x0)/(x1-x0);
          }
        else if ( (code & RIGHT) == RIGHT )
          {
            //std::cout << "Clip the line to the right of the box" << std::endl;
            x = _x1;
            y = y0 + (y1-y0)*(x-x0)/(x1-x0);
          }

        //std::cout << "x = " << x << ", y = " << y << std::endl;

        if ( code == clipCode0 )
          {
            // modify the first coord
            //std::cout << "modify the first coord" << std::endl;
            x0 = x;
            y0 = y;
            clipCode0 = ClipCode(x0,y0);
          }
        else 
          {
            // modify the second coord
            //std::cout << "modify the second coord" << std::endl;
            x1 = x;
            y1 = y;
            clipCode1 = ClipCode(x1,y1);
          }
      }

    return true;
  }

  void Print(std::ostream& os = std::cout)
  {
    os << _x0 << " " << _y0 << ", " << _x1 << " " << _y1 << std::endl;
  }

private:
  enum { RIGHT=1, BOTTOM=2, LEFT=4, TOP=8 };

  int ClipCode(const double x, const double y) const
  {
    int code = 0;
    if      ( x > _x1 ) code |= RIGHT;
    else if ( x < _x0 ) code |= LEFT;
    if      ( y > _y1 ) code |= TOP;
    else if ( y < _y0 ) code |= BOTTOM;
    return code;
  }

  double _x0;
  double _y0;
  double _x1;
  double _y1; 
};

#endif // __PHBBOX_H__