/* Hello World for OpenGL.
 * Following Cedric Fung's tutorial
 * <https://vec.io/posts/head-to-opengl-hello-triangle>
 * Sadly he seems to have stopped after the first part…
 * also following
 * <http://stackoverflow.com/questions/10984928/hello-world-opengl-program-glenablevertexattribarray-was-not-declared-it-this>
 * which gives me segfaults, and so I'm trying
 * <http://www.lighthouse3d.com/tutorials/glut-tutorial/initialization/>
 * which doesn't use glDrawArrays but also then my code doesn’t segfault.
 */
#include <math.h>
#include <stdio.h>
#include <sys/time.h>
// This #define allows us to get glVertexAttribPointer and the like
// without resorting to GLEW.
#define GL_GLEXT_PROTOTYPES
#include <GL/glut.h>
#include <GL/glu.h>
#include <GL/gl.h>

void draw(int _)
{
  glClearColor(0, 1, 0, 1); // RGBA
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // ???

  struct timeval now;
  gettimeofday(&now, NULL);
  double theta = now.tv_sec % 6283 + .000001 * now.tv_usec;

  glLoadIdentity();
  // <http://www.lighthouse3d.com/tutorials/glut-tutorial/keyboard-example-moving-around-the-world/>
  gluLookAt(0, 0, 3,
            0, 0, 0,
            0, 1, 0);
  glRotatef(theta*100, 0, 1, 0);
  glRotatef(theta*61, 0, 0, 1);

  glBegin(GL_TRIANGLES);
  glVertex3f(-.5, -.5, 0);
  glVertex3f(+.5,   0, 0);
  glVertex3f(  0, +.5, 0);
  glEnd();

  /* GLfloat geometry[] = */
  /*   { */
  /*     -.5, +.5, 0, 1, */
  /*     +.5, -5 , 0, 1, */
  /*     0  , +.5, 0, 1, */
  /*   }; */

  /* GLuint buf; */
  /* glGenBuffers(1, &buf); */
  /* glBindBuffer(GL_ARRAY_BUFFER, buf); */
  /* glBufferData(GL_ARRAY_BUFFER, sizeof(geometry), geometry, GL_STATIC_DRAW); */
  /* glBindBuffer(GL_ARRAY_BUFFER, 0); */

  /* glBindBuffer(GL_ARRAY_BUFFER, buf); */
  /* glEnableVertexAttribArray(0); // ??? */
  /* // https://www.opengl.org/sdk/docs/man/html/glVertexAttribPointer.xhtml */
  /* int index = 0, componentsPerAttribute = 4, strideBytes = 0; */
  /* glVertexAttribPointer(index, */
  /*                       componentsPerAttribute, */
  /*                       GL_FLOAT, */
  /*                       GL_FALSE, /\* not normalized ints *\/ */
  /*                       strideBytes, */
  /*                       0);     /\* offset into buffer *\/ */

  /* GLenum err = glGetError(); */
  /* if (err != GL_NO_ERROR) { */
  /*   fprintf(stderr, "%d: %s\n", err, gluErrorString(err)); */
  /*   abort(); */
  /* } */

  /* glDrawArrays(GL_TRIANGLES, 0, 3); // ??? */
  /* glBindBuffer(GL_ARRAY_BUFFER, 0); */

  glutSwapBuffers();
  
  glutTimerFunc(22, draw, 0);
}

int main(int argc, char **argv)
{
  glutInit(&argc, argv);
  // <https://www.opengl.org/resources/libraries/glut/spec3/node12.html>
  glutInitDisplayMode(GLUT_DOUBLE | // Double-buffering.
                      GLUT_ALPHA |
                      GLUT_DEPTH | // Depth-buffering.
                      GLUT_STENCIL // “Stencil buffering”, whatever that is.
                      );
  glutInitWindowSize(512, 512);
  glutInitWindowPosition(64, 128);
  glutCreateWindow(argv[0]);

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  // Without this gluPerspective call, the gluLookAt call results in
  // seeing nothing.
  gluPerspective(45, 1, 0.1, 100);
  glMatrixMode(GL_MODELVIEW);

  draw(0);
  glutMainLoop();
  return 0;
}