#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <GL/glut.h>
#include <GL/glu.h>

/* declare functions */
void init(int width, int height);
void idle(void);
void reshape(int width, int height);
void define_earth_list(void);
void define_moon_list(void);
void display(void);

/* declare globals */
GLint Earth;
GLint Moon;
GLfloat EARTH_ORBIT_ANGLE = 0.0;
GLfloat EARTH_ROTATION_ANGLE = 0.0;
GLfloat MOON_ORBIT_ANGLE = 0.0;
GLfloat MOON_ROTATION_ANGLE = 0.0;

/*
* main function - set up window
* set handlers
* and run main loop
*/
int main(int argc, char **argv)
{
  /* Initialize glut */
  glutInit (&argc, argv);		
  glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
  glutInitWindowSize (600, 600);	
  glutCreateWindow ("Earth and Moon");	
  glutIdleFunc (idle);			
  glutReshapeFunc(reshape);		
  glutDisplayFunc (display);

  init(600,600);
  glutMainLoop();
  return 1;
}


/* INIT
* Use this function to set global variables and states.
* Open GL function that are only done once can also be done here;
* Setting the initial projection matrix,
* Object creation and definition
* Display list creation
* Light creation and setup
* Material definition and creation
*/
void init(int width, int height) 
{
  glClearColor (0.0, 0.0, 0.0, 0.0);
  glMatrixMode (GL_PROJECTION);
  glShadeModel (GL_FLAT);
  glLoadIdentity ();

  gluPerspective (45.0f, (GLfloat) width/(GLfloat) height, 0.1f, 300.0f);
  gluLookAt (150.0,50.0,50.0,  0.0,0.0,0.0,   0.0,1.0,0.0);
  glEnable (GL_DEPTH_TEST);
  
  /* initialize lists */
  Earth = glGenLists(1);
  define_earth_list();
  Moon = glGenLists(1);
  define_moon_list();
  glMatrixMode (GL_MODELVIEW);
  glLoadIdentity();
}


/* IDLE
* these events are done every frame but aren't drawn
* put pre-frame computations here
* useful for changing animation variables between frames
* also a good place to change over-all machine states  
*/
void idle(void) 
{
  EARTH_ROTATION_ANGLE += 1.0;
  EARTH_ORBIT_ANGLE = (1.0/365.0)*EARTH_ROTATION_ANGLE;
  MOON_ORBIT_ANGLE = (1.0/30.0)*EARTH_ROTATION_ANGLE;
  MOON_ROTATION_ANGLE = MOON_ORBIT_ANGLE;
}

/* RESHAPE
* width and height are generated by glut
* These commands are done everytime the window is moved or resized.
*/
void reshape(int width, int height) 
{
  glViewport(0, 0, width, height);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluPerspective (45.0f, (GLfloat) width/(GLfloat) height, 0.1f, 300.0f);
  gluLookAt (150.0,50.0,50.0,  0.0,0.0,0.0,   0.0,1.0,0.0);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();				
}

/*  DISPLAY
* call your command and function calls that handle drawing here
*/
void display(void) 
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glPushMatrix();\
    glLoadIdentity();
    glRotatef(EARTH_ORBIT_ANGLE,0.0,1.0,0.0);
    glTranslatef(30.0,0.0,0.0);
    glColor3f(0.0,0.0,1.0);
    glRotatef(EARTH_ROTATION_ANGLE,0.0,1.0,0.0);
    glCallList(Earth);
    glPushMatrix();
      glRotatef(MOON_ORBIT_ANGLE,0.0,1.0,0.0);
      glTranslatef(20.0,0.0,0.0);
      glColor3f(1.0,0.0,1.0);
      glRotatef(MOON_ROTATION_ANGLE,0.0,1.0,0.0);
      glCallList(Moon);
    glPopMatrix();
  glPopMatrix();
  
  glutSwapBuffers();
  glutPostRedisplay();
}

/*
*
* glList definitions
*
*/

/* 
* Earth List
*/
void define_earth_list(void) 
{    
  glNewList(Earth,GL_COMPILE);    
    glutWireSphere(10.0,16,16);
  glEndList();
}

/*
* Moon List
*/
void define_moon_list(void) 
{    
  glNewList(Moon,GL_COMPILE);    
    glutWireSphere(2.0,16,16);
  glEndList();
}