#define _GNU_SOURCE
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define GL_GLEXT_PROTOTYPES
#include <GL/glx.h>

static double get_timestamp()
{
  struct timespec tp;
  clock_gettime(CLOCK_MONOTONIC, &tp);
  return tp.tv_sec + 1e-9 * tp.tv_nsec;
}

int main(int argc, char *argv[])
{
  int width = 0, height = 0;

  if (argc != 3 || !(width = atoi(argv[1])) || !(height = atoi(argv[2])))
    {
      fprintf(stderr, "usage: %s WIDTH HEIGHT\n", argv[0]);
      return 1;
    }
  Display *dpy = XOpenDisplay(NULL);

  int ncfg, attrs[] = {GLX_DOUBLEBUFFER, GL_TRUE, None};
  GLXFBConfig *dconfigs = glXChooseFBConfig(dpy, 0, attrs, &ncfg);
  fprintf(stderr, "FBConfig %p\n", dconfigs[0]);

  int pbattrs[] = {GLX_PBUFFER_WIDTH, width, GLX_PBUFFER_HEIGHT, height, GLX_PRESERVED_CONTENTS, True, None};
  GLXPbuffer pbuffer = glXCreatePbuffer(dpy, dconfigs[0], pbattrs);

  GLXContext ctx = glXCreateNewContext(dpy, dconfigs[0], GLX_RGBA_TYPE, NULL, True);
  glXMakeCurrent(dpy, pbuffer, ctx);

  GLuint pbo;
  glGenBuffers(1, &pbo);
  glBindBuffer(GL_PIXEL_UNPACK_BUFFER_EXT, pbo);

  int size = width*height*4;
  glBufferData(GL_PIXEL_UNPACK_BUFFER_EXT, size, NULL, GL_STREAM_DRAW);
  void *pixeldata = malloc(size);
  memset(pixeldata, 0x80, size);

  double end = 0.2 + get_timestamp();
  int iters = 0;
  do {
    glBufferSubData(GL_PIXEL_UNPACK_BUFFER_EXT, 0, size, pixeldata);
    glDrawPixels(width, height, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
    glXSwapBuffers(dpy, pbuffer);
    iters++;
  } while (end > get_timestamp());

  free(pixeldata);
  glDeleteBuffers(1, &pbo);
  glXDestroyPbuffer(dpy, pbuffer);

  fprintf(stderr, "%d iters\n", iters);
  return 0;
}