/*
 * https://www.khronos.org/opengl/wiki/Tutorial:_OpenGL_3.0_Context_Creation_(GLX)
 *
 * Modified to use an indirect context
 *
 * $ gcc -g gl3.cxx -o gl3 $(pkg-config --cflags --libs  x11) -lGL -lGLU
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <GL/gl.h>
#include <GL/glx.h>

#define GLX_CONTEXT_MAJOR_VERSION_ARB       0x2091
#define GLX_CONTEXT_MINOR_VERSION_ARB       0x2092
typedef GLXContext (*glXCreateContextAttribsARBProc)(Display*, GLXFBConfig, GLXContext, Bool, const int*);

// Helper to check for extension string presence.  Adapted from:
//   http://www.opengl.org/resources/features/OGLextensions/
static bool isExtensionSupported(const char *extList, const char *extension)
{
  const char *start;
  const char *where, *terminator;
  
  where = strchr(extension, ' ');
  if (where || *extension == '\0')
    return false;

  for (start=extList;;) {
    where = strstr(start, extension);

    if (!where)
      break;

    terminator = where + strlen(extension);

    if ( where == start || *(where - 1) == ' ' )
      if ( *terminator == ' ' || *terminator == '\0' )
        return true;

    start = terminator;
  }

  return false;
}

int main(int argc, char* argv[])
{
  Display *display = XOpenDisplay(NULL);

  if (!display)
  {
    printf("Failed to open X display\n");
    exit(1);
  }

  // Get a matching FB config
  static int visual_attribs[] =
    {
      GLX_X_RENDERABLE    , True,
      GLX_DRAWABLE_TYPE   , GLX_WINDOW_BIT,
      GLX_RENDER_TYPE     , GLX_RGBA_BIT,
      GLX_X_VISUAL_TYPE   , GLX_TRUE_COLOR,
      GLX_RED_SIZE        , 8,
      GLX_GREEN_SIZE      , 8,
      GLX_BLUE_SIZE       , 8,
      GLX_ALPHA_SIZE      , 8,
      GLX_DEPTH_SIZE      , 24,
      GLX_STENCIL_SIZE    , 8,
      GLX_DOUBLEBUFFER    , True,
      None
    };

  static int context_attribs[] =
    {
      GLX_CONTEXT_MAJOR_VERSION_ARB, 1,
      GLX_CONTEXT_MINOR_VERSION_ARB, 0,
      None
    };

  int glx_major, glx_minor;
  int fbcount;

  GLXFBConfig* fbc = glXChooseFBConfig(display, DefaultScreen(display), visual_attribs, &fbcount);
  if (!fbc)
  {
    printf( "Failed to retrieve a framebuffer config\n" );
    exit(1);
  }

  int best_fbc = -1, worst_fbc = -1, best_num_samp = -1, worst_num_samp = 999;
  int i;
  for (i=0; i<fbcount; ++i)
  {
    XVisualInfo *vi = glXGetVisualFromFBConfig( display, fbc[i] );
    if ( vi )
    {
      int samp_buf, samples;
      glXGetFBConfigAttrib( display, fbc[i], GLX_SAMPLE_BUFFERS, &samp_buf );
      glXGetFBConfigAttrib( display, fbc[i], GLX_SAMPLES       , &samples  );
      
      if ( best_fbc < 0 || samp_buf && samples > best_num_samp )
        best_fbc = i, best_num_samp = samples;
      if ( worst_fbc < 0 || !samp_buf || samples < worst_num_samp )
        worst_fbc = i, worst_num_samp = samples;
    }
    XFree( vi );
  }

  GLXFBConfig bestFbc = fbc[ best_fbc ];

  XFree( fbc );

  XVisualInfo *vi = glXGetVisualFromFBConfig( display, bestFbc );
  XSetWindowAttributes swa;
  Colormap cmap;
  swa.colormap = cmap = XCreateColormap( display,
                                         RootWindow( display, vi->screen ), 
                                         vi->visual, AllocNone );
  swa.background_pixmap = None ;
  swa.border_pixel      = 0;
  swa.event_mask        = StructureNotifyMask;

  Window win = XCreateWindow( display, RootWindow( display, vi->screen ), 
                              0, 0, 100, 100, 0, vi->depth, InputOutput, 
                              vi->visual, 
                              CWBorderPixel|CWColormap|CWEventMask, &swa );
  if ( !win )
  {
    printf( "Failed to create window.\n" );
    exit(1);
  }

  // Done with the visual info data
  XFree( vi );

  XStoreName( display, win, "https://bugs.freedesktop.org/show_bug.cgi?id=107508" );
  XMapWindow( display, win );

  const char *glxExts = glXQueryExtensionsString( display,
                                                  DefaultScreen( display ) );

  glXCreateContextAttribsARBProc glXCreateContextAttribsARB = 0;
  glXCreateContextAttribsARB = (glXCreateContextAttribsARBProc)
           glXGetProcAddressARB( (const GLubyte *) "glXCreateContextAttribsARB" );

  GLXContext ctx = 0;

  if ( !isExtensionSupported( glxExts, "GLX_ARB_create_context" ) || !glXCreateContextAttribsARB )
  {
    printf( "glXCreateContextAttribsARB() not found\n" );
    exit (1);
  }

  ctx = glXCreateContextAttribsARB( display, bestFbc, 0, False, context_attribs );

  if ( ! glXIsDirect ( display, ctx ) )
  {
    printf( "Indirect GLX rendering context\n" );
  }
  else
  {
    printf( "Direct GLX rendering context\n" );
  }

  glXMakeCurrent( display, win, ctx );

  glClearColor( 0, 0.5, 1, 1 );
  glClear( GL_COLOR_BUFFER_BIT );
  glXSwapBuffers ( display, win );

  sleep( 1 );

  glClearColor ( 1, 0.5, 0, 1 );
  glClear ( GL_COLOR_BUFFER_BIT );
  glXSwapBuffers ( display, win );

  sleep( 1 );

  glXMakeCurrent( display, 0, 0 );
  glXDestroyContext( display, ctx );

  XDestroyWindow( display, win );
  XFreeColormap( display, cmap );
  XCloseDisplay( display );

  return 0;
}