#include <assert.h>
#include <GL/gl.h>
#include <GL/glx.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>

Display *Dpy = NULL;
Window Win = 0;
GLXContext Context1, Context2;
int ExitFlag = GL_FALSE;
volatile int thread = 1;

void createCtxes()
{
    int attrib[] = { GLX_RGBA,
                     GLX_RED_SIZE, 1,
                     GLX_GREEN_SIZE, 1,
                     GLX_BLUE_SIZE, 1,
                     GLX_DEPTH_SIZE, 1,
                     GLX_DOUBLEBUFFER,
                     None };
    int scrnum;
    XSetWindowAttributes attr;
    unsigned long mask;
    Window root;
    XVisualInfo *visinfo;
    int width = 300, height = 300;

    scrnum = DefaultScreen(Dpy);
    root = RootWindow(Dpy, scrnum);

    visinfo = glXChooseVisual(Dpy, scrnum, attrib);
    if (!visinfo) {
        printf("Unable to find RGB, Z, double-buffered visual");
        exit(0);
    }

    /* window attributes */
    attr.background_pixel = 0;
    attr.border_pixel = 0;
    attr.colormap = XCreateColormap(Dpy, root, visinfo->visual, AllocNone);
    attr.event_mask = StructureNotifyMask | ExposureMask | KeyPressMask;
    mask = CWBackPixel | CWBorderPixel | CWColormap | CWEventMask;

    Win = XCreateWindow(Dpy, root, 0, 0, width, height,
                        0, visinfo->depth, InputOutput,
                        visinfo->visual, mask, &attr);
    if (!Win) {
        printf("Couldn't create window");
        exit(0);
    }

    {
        XSizeHints sizehints;
        sizehints.x = 0;
        sizehints.y = 0;
        sizehints.width  = width;
        sizehints.height = height;
        sizehints.flags = USSize | USPosition;
        XSetNormalHints(Dpy, Win, &sizehints);
        XSetStandardProperties(Dpy, Win, "glthreads", "glthreads",
                               None, (char **)NULL, 0, &sizehints);
    }

    Context1 = glXCreateContext(Dpy, visinfo, NULL, True);
    if (!Context1) {
        printf("Couldn't create GLX context");
        exit(0);
    }
    Context2 = glXCreateContext(Dpy, visinfo, Context1, True);
    if (!Context2) {
        printf("Couldn't create GLX context");
        exit(0);
    }

    XMapWindow(Dpy, Win);
    XSync(Dpy, 0);
}

static void
event_loop(Display *dpy)
{
    XEvent event;
    int i;

    while (!ExitFlag) {
       XNextEvent(dpy, &event);

        switch (event.type) {
            case KeyPress:
                /* tell all threads to exit */
                ExitFlag = GL_TRUE;
                /*printf("exit draw_loop %d\n", wt->Index);*/
                return;
            default:
            /*no-op*/ ;
        }
    }
}

static void *
thread1_function(void *p)
{
    if(glXMakeCurrent(Dpy, Win, Context1) != GL_TRUE)
    {
        printf("glXMakeCurrent error\n");
        return(0);
    }

    glEnable(GL_DEPTH_TEST);
    glColor3f(0, 0, 1);
    glDrawBuffer(GL_BACK);

    while(!ExitFlag)
    {
        printf("draw1\n");

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glBegin(GL_QUADS);
        glVertex3f(0, 0, 0);
        glVertex3f(0, 1, 0);
        glVertex3f(1, 1, 0);
        glVertex3f(1, 0, 0);
        glEnd();
        printf("done draw1\n");

        thread = 2;

        while(thread != 1 && !ExitFlag) usleep(1);
        if(ExitFlag) return;

        printf("Flip\n");
        glFinish();
        glXSwapBuffers(Dpy, Win);
    }
    return NULL;
}

static void *
thread2_function(void *p)
{
    if(glXMakeCurrent(Dpy, Win, Context2) != GL_TRUE)
    {
        printf("glXMakeCurrent error\n");
        return(0);
    }

    glColor3f(1,0, 0);
    glDrawBuffer(GL_BACK);
    glEnable(GL_DEPTH_TEST);

    while(!ExitFlag)
    {
        while(thread != 2 && !ExitFlag) usleep(1);;
        if(ExitFlag) return;
        printf("draw2\n");

        glBegin(GL_QUADS);
        glVertex3f(0.5, 0.5,  0.1);
        glVertex3f(0.5,  -1, 0.1);
        glVertex3f(-1, -1, 0.1);
        glVertex3f(-1,  0, 0.1);
        glEnd();
        printf("done draw2\n");
        glFinish();
        thread = 1;
    }
    return NULL;
}

int main(int argc, char *argv[])
{
    pthread_t Thread1, Thread2;
    XInitThreads();
    Dpy = XOpenDisplay(NULL);
    if (!Dpy) {
        fprintf(stderr, "Unable to open display %s\n", NULL);
        return -1;
    }

    createCtxes();

    pthread_create(&Thread1, NULL, thread1_function, NULL);
    pthread_create(&Thread1, NULL, thread2_function, NULL);
    event_loop(Dpy);

    return 0;
}