#if __APPLE__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif

#include <stdlib.h>
#include <time.h>

// -----------------------------------------------------------------

int window_width, window_height ;

double s = 0.1 ;
double delta = 0.01 ;

int timer_active = 0 ;
int timer_interval = 1000/50 ; // 50 Hz

// -----------------------------------------------------------------

void
reshape(int width, int height) {
  window_width = width ;
  window_height = height ;
}

void
display(void) {
  glClearColor(1.0,1.0,1.0,1.0) ;
  glClear(GL_COLOR_BUFFER_BIT) ;

  glMatrixMode(GL_PROJECTION) ;
  glLoadIdentity() ;

  glMatrixMode(GL_MODELVIEW) ;
  glLoadIdentity() ;

  glColor3f(0.0,0.0,s) ;
  glRectf(-s,-s,s,s) ;

  glutSwapBuffers() ;
}

void
animate(int theTimer) {
  if (!timer_active) return ;
  s+=delta ;
  if (s>=0.5) { s=0.5 ; delta*=-1 ; }
  if (s<=0.0) { s=0 ; delta*=-1 ; }
  glutPostRedisplay() ;
  glutTimerFunc(timer_interval, animate, theTimer) ;
}

void
keyboard(unsigned char key, int x, int y) {
  switch (key) {
  case ' ':
  case 'a':
    if (!timer_active) glutTimerFunc(timer_interval, animate, 0) ;
    timer_active = ! timer_active ;
    break ;
  case 27: // Escape
    exit(0) ;
  }
}

// -----------------------------------------------------------------

int
main(int argc, char *argv[]) {
  glutInit(&argc, argv) ;

  glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE) ;

  glutInitWindowPosition(0, 0) ;
  glutInitWindowSize(400, 300) ;
  glutCreateWindow("transform") ;

  glutReshapeFunc(reshape) ;
  glutDisplayFunc(display) ;
  glutKeyboardFunc(keyboard) ;

  srandom((unsigned int)(time(0)%737)) ;

  glutMainLoop() ;
  return 0 ;
}