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

typedef float vector3[3];

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

static vertex data[] = {
  { { 0.0,   0.0,   0.0 }, { 1.0, 0.0, 0.0 } },
  { { 100.0, 0.0,   0.0 }, { 0.0, 1.0, 0.0 } },
  { { 100.0, 100.0, 0.0 }, { 0.0, 0.0, 1.0 } },
  { { 0.0,   100.0, 0.0 }, { 1.0, 1.0, 0.0 } },
};

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);
 
  {
    const unsigned char *data_v = (unsigned char *) &data;
    const unsigned char *data_c = ((unsigned char *) &data) + offsetof(vertex, colour);

    {
      const unsigned char indices[] = { 0, 1, 2 };
      glVertexPointer(3, GL_FLOAT, sizeof(vertex), data_v);
      glColorPointer(3, GL_FLOAT, sizeof(vertex), data_c);
      glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, indices);
    }
 
    glTranslated(120.0, 120.0, 0.0);
 
    {
      const unsigned char indices[] = { 1, 2, 3 };
      glVertexPointer(3, GL_FLOAT, sizeof(vertex), data_v);
      glColorPointer(3, GL_FLOAT, sizeof(vertex), data_c);
      glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, indices);
    }
  }

  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");
  glutReshapeFunc(reshape);
  glutDisplayFunc(display);
  glutIdleFunc(glutPostRedisplay);

  glutMainLoop();
  return 0;
}