/* Draws a circle using either cartesian or polar coordinates */

/* $Id: circle.c,v 1.2 2020/03/13 01:59:24 gl Exp gl $ */

#include <math.h>
#include <stdio.h>
#include <stdbool.h>

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>

#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif

enum { cartesian, parametric } coordSystem = { parametric };
enum { points, lines } renderMode = { lines };

bool debug = true;

int n = 32;

// Utility function for drawing axes
void drawAxes(float len)
{
#if 0
  glPushAttrib(GL_CURRENT_BIT);
#endif
  glBegin(GL_LINES);
  glColor3f(1.0, 0.0, 0.0);
  glVertex3f(0.0, 0.0, 0.0);
  glVertex3f(len, 0.0, 0.0);
  glColor3f(0.0, 1.0, 0.0);
  glVertex3f(0.0, 0.0, 0.0);
  glVertex3f(0.0, len, 0.0);
  glColor3f(0.0, 0.0, 1.0);
  glVertex3f(0.0, 0.0, 0.0);
  glVertex3f(0.0, 0.0, len);
  glEnd();
#if 0
  glPopAttrib();
#endif
}

// Cartesian coordinates
void drawCircleCartesian(float r, int n)
{
  float x, y;

  if (debug)
    printf("drawCircleCartesian: %d\n", n);

  glColor3f(1.0, 1.0, 1.0);
  if (renderMode == points) {
    glPointSize(2.0);
    glBegin(GL_POINTS);
  } else { 
    glLineWidth(2.0);
    glBegin(GL_LINE_LOOP);
  }
  
  n /= 2;

  for (int i = 0; i < n; i++) {
    x = ((i / (float)n - 0.5) * 2.0) * r;
    y = sqrt(r * r - x * x);
    if (debug)
      printf("x y: %f %f\n", x, y);
    glVertex2f(x, y);
    if (renderMode == points) {
      y = -y;
      if (debug)
	printf("x y: %f %f\n", x, y);
      glVertex2f(x, y);
    }
  }
  
  if (renderMode == lines)
    for (int i = n; i > 0; i--) {
      x = (i / (float)n - 0.5) * 2.0 * r;
      y = -sqrt(r * r - x * x);
      if (debug)
	printf("x y: %f %f\n", x, y);
      glVertex2f(x, y);
    }

  glEnd();

#if 0
  // Ignore
#endif
}

// Parametric coordinates
void drawCircleParametric(float r, int n)
{
  float x, y;
  float theta;

  if (debug)
    printf("drawCircleParametric: %d\n", n);

  glColor3f(1.0, 1.0, 1.0);
  if (renderMode == points) {
    glPointSize(2.0);
    glBegin(GL_POINTS); 
  } else {
    glLineWidth(2.0);
    glBegin(GL_LINE_LOOP);
  }
  for (int i = 0; i < n; i++) {
    theta = i / (float)n * 2.0 * M_PI;
    x = r * cosf(theta);
    y = r * sinf(theta);
    if (debug)
      printf("x y: %f %f", x, y);
    glVertex2f(x, y);
  }
  glEnd();
}

// Display callback
void display()
{
  int err;

  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  drawAxes(1.0);
  if (coordSystem == cartesian)
    drawCircleCartesian(1.0, n);
  else if (coordSystem == parametric)
    drawCircleParametric(1.0, n);

  // Always check for gl errors
  if ((err = glGetError()) != GL_NO_ERROR)
    fprintf(stderr, "Error: %s\n", gluErrorString(err));
  
  glutSwapBuffers();
}

// Keyboard callback
void keyboard(unsigned char key, int x, int y)
{
  switch (key) {
  case 'c':
    if (coordSystem == cartesian)
      coordSystem = parametric;
    else
      coordSystem = cartesian;
    break;
  case 'm':
    if (renderMode == lines)
      renderMode = points;
    else
      renderMode = lines;
    break;
  case 'd':
    debug = !debug;
    break;
  case '+':
    n *= 2;
    break;
  case '-':
    n /= 2;
    if (n < 2)
      n = 2;
    break;
  case 27: 
    // ESCAPE key 
    exit (0); 
    break;
  }
  glutPostRedisplay();
}
    

int main(int argc, char **argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
  glutInitWindowSize(500, 500);
  glutCreateWindow("Circle");
  glutReshapeWindow(500, 500);
  glutDisplayFunc(display);
  glutKeyboardFunc(keyboard);
  glutMainLoop();
}