#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/extensions/Xcomposite.h>
#include <GL/glx.h>
#include <GL/glu.h>
#include <GL/glxext.h>
#include <sys/time.h>

static double timeOfDay()
{
    struct timeval t;
    gettimeofday( &t, NULL );
    return (double)(t.tv_sec) + (double)( t.tv_usec )/1000000.0;
}

int main(void)
{
    Display *display = XOpenDisplay( NULL );
    int screen = DefaultScreen( display );

    int width = DisplayWidth( display, screen );
    int height = DisplayHeight( display, screen );

    if( !glXQueryExtension( display, NULL, NULL ) )
    {
        fprintf( stderr, "No GLX extension\n" );
        return 1;
    }

    int attrib_list[] = { GLX_RGBA,
                          GLX_DOUBLEBUFFER,
                          GLX_DEPTH_SIZE, 0,
                          GLX_STENCIL_SIZE, 0,
                          GLX_ALPHA_SIZE, 0,
                          None };

    XVisualInfo *visual_info = glXChooseVisual( display, screen, attrib_list );

    int dummy;
    XCompositeQueryVersion( display, &dummy, &dummy );

    Window window = XCompositeGetOverlayWindow( display, RootWindow( display, screen ) );

    GLXContext glContext = glXCreateContext( display, visual_info, NULL, True );
    if( !glXIsDirect( display, glContext ) )
    {
        fprintf( stderr, "Not direct\n" );
        return 1;
    }
    if( !glXMakeCurrent( display, window, glContext ) )
    {
        fprintf( stderr, "Could not make context current\n" );
        return 2;
    }

    glClearColor( 0.0, 0.0, 0.0, 0.0 );
    glShadeModel( GL_FLAT );
    glViewport( 0, 0, width, height );
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    gluOrtho2D( 0, width, height, 0 );
    glFinish();

    double start_time = timeOfDay();
    double lastTime = timeOfDay();
    int last_dropped_frames = 0;
    int i=0;
    for( ;; ++i )
    {
        glClear( GL_COLOR_BUFFER_BIT );

        /* Animated rectangle */
        {
            const int rect_width = 256;
            int x = (i * 6) % (width - rect_width);
            glColor4f( 1.0, 1.0, 1.0, 1.0 );
            glRecti( x, 0, x + rect_width, height );
        }

        /* Signal here - back buffer is drawn, request it be flipped
         * to front buffer. Note that the glFinish above will stall us
         * until this glXSwapBuffers and all drawing has happened. */
        glXSwapBuffers( display, window );

        double now = timeOfDay();
        double run_time = now - start_time;
        int expected_frame = (int)(run_time*60.);
        int dropped_frames = expected_frame - i;

        double interval = (now - lastTime) * 1000;
        if( dropped_frames != last_dropped_frames )
        {
            printf( "Delta %7.3lfms error %7.3lf frame %6d expected_frame %6d dropped_frames %4d (1 dropped every %.3fs)\n",
                interval, interval - (1000./60.),
                i, expected_frame,
                dropped_frames, ((double)expected_frame)/((double)dropped_frames)/60. );

            last_dropped_frames = dropped_frames;
        }
        lastTime = now;
    }
    return 0;
}