//
// "$Id$"
//
// Arc functions for the Fast Light Tool Kit (FLTK).
//
// Copyright 1998-2016 by Bill Spitzak and others.
//
// This library is free software. Distribution and use rights are outlined in
// the file "COPYING" which should have been included with this file.  If this
// file is missing or damaged, see the license at:
//
//     http://www.fltk.org/COPYING.php
//
// Please report all bugs and problems on the following page:
//
//     http://www.fltk.org/str.php
//

/**
  \file fl_arc.cxx
  \brief Utility functions for drawing arcs and circles.
*/

// Utility for drawing arcs and circles.  They are added to
// the current fl_begin/fl_vertex/fl_end path.
// Incremental math implementation:

#include <FL/fl_draw.H>
#include <FL/math.h>

// avoid problems with some platforms that don't 
// implement hypot.
static double _fl_hypot(double x, double y) {
  return sqrt(x*x + y*y);
}

/**
 \cond DriverDev
 \addtogroup DriverDeveloper
 \{
 */

/**
 Draw an arc.

 The default implementation draws an arc using other calls into the graphics
 driver. There is no need to override the method unless the target platform
 supports drawing arcs directly.

 \see fl_arc(double x, double y, double r, double start, double end)
 */
void Fl_Graphics_Driver::arc(double x, double y, double r, double start, double end) {

  // draw start point accurately:
  
  double A = start*(M_PI/180);		// Initial angle (radians)
  double X =  r*cos(A);			// Initial displacement, (X,Y)
  double Y = -r*sin(A);			//   from center to initial point
  fl_vertex(x+X,y+Y);			// Insert initial point

  // Maximum arc length to approximate with chord with error <= 0.125
  
  double epsilon; {
    double r1 = _fl_hypot(fl_transform_dx(r,0), // Horizontal "radius"
		          fl_transform_dy(r,0));
    double r2 = _fl_hypot(fl_transform_dx(0,r), // Vertical "radius"
		          fl_transform_dy(0,r));
		      
    if (r1 > r2) r1 = r2;		// r1 = minimum "radius"
    if (r1 < 2.) r1 = 2.;		// radius for circa 9 chords/circle
    
    epsilon = 2*acos(1.0 - 0.125/r1);	// Maximum arc angle
  }
  A = end*(M_PI/180) - A;		// Displacement angle (radians)
  int i = int(ceil(fabs(A)/epsilon));	// Segments in approximation
  
  if (i) {
    epsilon = A/i;			// Arc length for equal-size steps
    double cos_e = cos(epsilon);	// Rotation coefficients
    double sin_e = sin(epsilon);
    do {
      double Xnew =  cos_e*X + sin_e*Y;
		Y = -sin_e*X + cos_e*Y;
      fl_vertex(x + (X=Xnew), y + Y);
    } while (--i);
  }
}

/**
 \}
 \endcond
 */

//
// End of "$Id$".
//
