#define _GNU_SOURCE

#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/keysym.h>
#include <X11/extensions/Xrandr.h>

#include <stdio.h>
#include <stdlib.h>

static int get_active_window(Display *dpy, Window root, Window *active)
{
    Atom prop = XInternAtom(dpy, "_NET_ACTIVE_WINDOW", True);

    if (!prop)
        return 0;

    Atom actual;
    int format;
    unsigned long nitems;
    unsigned long bytes_after;
    unsigned char *data = NULL;

    int status = XGetWindowProperty(
        dpy,
        root,
        prop,
        0,
        1,
        False,
        XA_WINDOW,
        &actual,
        &format,
        &nitems,
        &bytes_after,
        &data
    );

    if (status != Success || !data || nitems == 0) {
        if (data)
            XFree(data);
        return 0;
    }

    *active = *(Window *)data;

    XFree(data);
    return 1;
}

static int get_window_center(
    Display *dpy,
    Window window,
    int *cx,
    int *cy
)
{
    Window root;
    Window child;

    int x;
    int y;

    unsigned int width;
    unsigned int height;
    unsigned int border;
    unsigned int depth;

    if (!XGetGeometry(
            dpy,
            window,
            &root,
            &x,
            &y,
            &width,
            &height,
            &border,
            &depth))
        return 0;

    int root_x;
    int root_y;

    if (!XTranslateCoordinates(
            dpy,
            window,
            root,
            0,
            0,
            &root_x,
            &root_y,
            &child))
        return 0;

    *cx = root_x + (int)width / 2;
    *cy = root_y + (int)height / 2;

    return 1;
}

int main(void)
{
    Display *dpy = XOpenDisplay(NULL);

    if (!dpy) {
        fprintf(stderr, "popup: cannot open X display\n");
        return EXIT_FAILURE;
    }

    int screen = DefaultScreen(dpy);
    Window root = RootWindow(dpy, screen);

    /*
     * ------------------------------------------------------------
     * Find the currently focused window.
     * ------------------------------------------------------------
     */

    Window active = None;

    int cursor_x = DisplayWidth(dpy, screen) / 2;
    int cursor_y = DisplayHeight(dpy, screen) / 2;

    if (get_active_window(dpy, root, &active)) {
        get_window_center(
            dpy,
            active,
            &cursor_x,
            &cursor_y
        );
    }

    /*
     * ------------------------------------------------------------
     * Find the monitor containing the active window.
     * ------------------------------------------------------------
     */

    int monitor_x = 0;
    int monitor_y = 0;

    int monitor_width = DisplayWidth(dpy, screen);
    int monitor_height = DisplayHeight(dpy, screen);

    int monitor_count = 0;

    XRRMonitorInfo *monitors =
        XRRGetMonitors(
            dpy,
            root,
            True,
            &monitor_count
        );

    if (monitors) {

        for (int i = 0; i < monitor_count; i++) {

            XRRMonitorInfo *m = &monitors[i];

            if (cursor_x >= m->x &&
                cursor_x < m->x + (int)m->width &&
                cursor_y >= m->y &&
                cursor_y < m->y + (int)m->height) {

                monitor_x = m->x;
                monitor_y = m->y;

                monitor_width = m->width;
                monitor_height = m->height;

                break;
            }
        }

        XRRFreeMonitors(monitors);
    }

    /*
     * ------------------------------------------------------------
     * Desired geometry.
     *
     * Half the monitor width and half the monitor height,
     * centered on that monitor.
     * ------------------------------------------------------------
     */

    int width = monitor_width / 2;
    int height = monitor_height / 2;

    int x = monitor_x + (monitor_width - width) / 2;
    int y = monitor_y + (monitor_height - height) / 2;

    /*
     * ------------------------------------------------------------
     * Create the window.
     *
     * override_redirect means the window manager does not manage
     * this window. This prevents it from moving/resizing it.
     * ------------------------------------------------------------
     */

    XSetWindowAttributes attributes;

    attributes.override_redirect = True;
    attributes.background_pixel =
        BlackPixel(dpy, screen);

    attributes.event_mask =
        KeyPressMask |
        FocusChangeMask |
        StructureNotifyMask;

    Window window = XCreateWindow(
        dpy,
        root,

        x,
        y,

        width,
        height,

        0,

        CopyFromParent,
        InputOutput,
        CopyFromParent,

        CWOverrideRedirect |
        CWBackPixel |
        CWEventMask,

        &attributes
    );

    if (!window) {
        fprintf(stderr, "popup: XCreateWindow failed\n");
        XCloseDisplay(dpy);
        return EXIT_FAILURE;
    }

    /*
     * ------------------------------------------------------------
     * Map the window.
     * ------------------------------------------------------------
     */

    XMapRaised(dpy, window);
    XFlush(dpy);

    /*
     * ------------------------------------------------------------
     * Wait until the X server has actually mapped the window.
     *
     * Calling XSetInputFocus() before this can result in:
     *
     *     BadMatch (invalid parameter attributes)
     *
     * ------------------------------------------------------------
     */

    for (;;) {

        XEvent event;

        XNextEvent(dpy, &event);

        if (event.type == MapNotify &&
            event.xmap.window == window) {
            break;
        }
    }

    /*
     * ------------------------------------------------------------
     * Now that the window is viewable, take keyboard focus.
     * ------------------------------------------------------------
     */

    XSetInputFocus(
        dpy,
        window,
        RevertToParent,
        CurrentTime
    );

    XRaiseWindow(dpy, window);

    XFlush(dpy);

    /*
     * ------------------------------------------------------------
     * Event loop.
     *
     * Escape:
     *     exit
     *
     * FocusOut:
     *     exit
     *
     * The window is override-redirect, so the WM won't resize or
     * reposition it.
     * ------------------------------------------------------------
     */

    for (;;) {

        XEvent event;

        XNextEvent(dpy, &event);

        switch (event.type) {

        case KeyPress:
        {
            KeySym key =
                XLookupKeysym(
                    &event.xkey,
                    0
                );

            if (key == XK_Escape) {

                XDestroyWindow(
                    dpy,
                    window
                );

                XCloseDisplay(dpy);

                return EXIT_SUCCESS;
            }

            break;
        }

        case FocusOut:
        {
            /*
             * Any loss of keyboard focus causes the popup
             * to immediately terminate.
             */

            XDestroyWindow(
                dpy,
                window
            );

            XCloseDisplay(dpy);

            return EXIT_SUCCESS;
        }

        default:
            break;
        }
    }

    return EXIT_SUCCESS;
}