#define GL_GLEXT_PROTOTYPES 1

#include <assert.h>
#include <stddef.h>
#include <stdlib.h>
#include <GL/glut.h>
#include <GL/glext.h>

typedef float vector3[3];

typedef struct
{
  vector3 position;
  vector3 colour;
} vertex;

static GLuint vertices_name;
static GLuint triangles[2];

static void
init(void)
{
  vertex *data = malloc(4 * sizeof(vertex));
  if (data == NULL) abort();

  data[0].position[0] = 0.0;
  data[0].position[1] = 0.0;
  data[0].position[2] = 0.0;
  data[0].colour[0] = 1.0;
  data[0].colour[1] = 0.0;
  data[0].colour[2] = 0.0;

  data[1].position[0] = 100.0;
  data[1].position[1] = 0.0;
  data[1].position[2] = 0.0;
  data[1].colour[0] = 0.0;
  data[1].colour[1] = 1.0;
  data[1].colour[2] = 0.0;

  data[2].position[0] = 100.0;
  data[2].position[1] = 100.0;
  data[2].position[2] = 0.0;
  data[2].colour[0] = 0.0;
  data[2].colour[1] = 0.0;
  data[2].colour[2] = 1.0;

  data[3].position[0] = 0.0;
  data[3].position[1] = 100.0;
  data[3].position[2] = 0.0;
  data[3].colour[0] = 1.0;
  data[3].colour[1] = 1.0;
  data[3].colour[2] = 0.0;

  glGenBuffers(1, &vertices_name);
  glBindBuffer(GL_ARRAY_BUFFER, vertices_name);
  glBufferData(GL_ARRAY_BUFFER, 4 * sizeof(vertex), data, GL_STATIC_DRAW);

  free(data);

  glGenBuffers(2, triangles);

  {
    unsigned char indices[] = { 0, 1, 2 };
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangles[0]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); 
  }

  {
    unsigned char indices[] = { 1, 2, 3 };
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangles[1]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); 
  }

  assert(glGetError() == GL_NO_ERROR);
}

static void
reshape(int width, int height)
{
  glViewport(0, 0, width, height);
 
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glOrtho(0, width, 0, height, -1.0, 100.0);

  assert(glGetError() == GL_NO_ERROR);
}

static void
display(void)
{
  glClearColor(0.3f, 0.3f, 0.3f, 1.0f);
  glClear(GL_COLOR_BUFFER_BIT);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();

  glPushMatrix();
  glTranslated(20.0, 20.0, 0.0);

  glEnableClientState(GL_VERTEX_ARRAY);
  glEnableClientState(GL_COLOR_ARRAY);
 
  glBindBuffer(GL_ARRAY_BUFFER, vertices_name);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangles[0]);
  glVertexPointer(3, GL_FLOAT, sizeof(vertex), (void *) 0);
  glColorPointer(3, GL_FLOAT, sizeof(vertex), (void *) offsetof(vertex, colour));
  glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, (void *) 0);
 
  glTranslated(120.0, 120.0, 0.0);
  
  glBindBuffer(GL_ARRAY_BUFFER, vertices_name);
  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangles[1]);
  glVertexPointer(3, GL_FLOAT, sizeof(vertex), (void *) 0);
  glColorPointer(3, GL_FLOAT, sizeof(vertex), (void *) offsetof(vertex, colour));
  glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, (void *) 0);

  glDisableClientState(GL_VERTEX_ARRAY);
  glDisableClientState(GL_COLOR_ARRAY);

  glPopMatrix();

  assert(glGetError() == GL_NO_ERROR);

  glutSwapBuffers();
}

int
main(int argc, char **argv)
{
  glutInit(&argc, argv);
  glutCreateWindow("Vertex array triangle");
  init();
  glutReshapeFunc(reshape);
  glutDisplayFunc(display);
  glutIdleFunc(glutPostRedisplay);

  glutMainLoop();
  return 0;
}