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

typedef float vector3[3];

/*
 * Interleaved vertex and colour data.
 */

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

static vertex data0[] = {
  { { 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 } }
};

/*
 * Vertex data followed by colour data.
 */

static vector3 data1[] = {
  { 0.0,   0.0,   0.0 },
  { 100.0, 0.0,   0.0 },
  { 100.0, 100.0, 0.0 },

  { 1.0, 0.0, 0.0 },
  { 0.0, 1.0, 0.0 },
  { 0.0, 0.0, 1.0 }
};

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);

  {
    const unsigned char *data0_v = (unsigned char *) &data0;
    const unsigned char *data0_c = ((unsigned char *) &data0) + offsetof(vertex, colour);

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);
    glVertexPointer(3, GL_FLOAT, sizeof(vertex), data0_v);
    glColorPointer(3, GL_FLOAT, sizeof(vertex), data0_c);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);
  }

  glTranslated(120.0, 120.0, 0.0);

  {
    const unsigned char *data1_v = (unsigned char *) &data1;
    const unsigned char *data1_c = ((unsigned char *) &data1) + (3 * sizeof(vector3));

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);
    glVertexPointer(3, GL_FLOAT, sizeof(vector3), data1_v);
    glColorPointer(3, GL_FLOAT, sizeof(vector3), data1_c);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glDisableClientState(GL_VERTEX_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;
}