#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/Xutil.h>
#include <cairo.h>
#include <cairo-xlib.h>

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

static void draw_needle(cairo_t *cr,
                        double x,
                        double y,
                        double r,
                        double min_angle,
                        double max_angle,
                        double value) {
    double angle = min_angle + ((max_angle - min_angle) * value) - 90.0;

    cairo_move_to(cr, x, y);

    cairo_set_source_rgb(cr, 0, 0.25, 1.0);

    cairo_line_to(cr,
                  x + r * cos((M_PI/180.0) * angle),
                  y + r * sin((M_PI/180.0) * angle));

    cairo_stroke(cr);
}

static void draw_face_number(cairo_t *cr,
                             double x,
                             double y,
                             double r,
                             double min_angle,
                             double max_angle,
                             double value,
                             const char *text) {
    double angle = min_angle + ((max_angle - min_angle) * value) - 95;

    cairo_set_source_rgb(cr, 1, 1, 1);

    cairo_move_to(cr,
                  x + r * cos((M_PI/180.0) * angle),
                  y + r * sin((M_PI/180.0) * angle));

    cairo_save(cr);
    cairo_rotate(cr, (M_PI/180.0) * (angle + 95));
    cairo_show_text(cr, text);
    cairo_restore(cr);
}

static void redraw(cairo_t *cr) {
    int i;

    /*
     * Set up text
     */
    cairo_select_font_face(cr, "Helvetica",
      CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);

    cairo_set_font_size(cr, 24);

    /*
     * Paint canvas black
     */
    cairo_set_source_rgb(cr, 0, 0, 0);
    cairo_paint(cr);

    /*
     * Draw two identical circles
     */
    cairo_set_source_rgb(cr, 1, 1, 1);
    cairo_arc(cr, 208, 240, 192, 0, 360);
    cairo_stroke(cr);

    cairo_arc(cr, 816, 240, 192, 0, 360);
    cairo_stroke(cr);


    /*
     * Draw face numbers
     */
    for (i=0; i<=80; i+=10) {
        char text[4];

        snprintf(text, 3, "%02d", i);
        draw_face_number(cr, 208, 240, 162, 232, 488, i / 80.0, text);
    }

    /*
     * Draw a gauge needle
     */
    draw_needle(cr, 208, 240, 148, 232, 488, 0.0);

    /*
     * Draw a tiny boi circle
     */
    cairo_set_source_rgb(cr, 1, 1, 1);
    cairo_arc(cr, 208, 240, 16, 0, 360);
    cairo_fill(cr);

    /*
     * Draw two smaller circles
     */
    cairo_arc(cr, 442, 368, 64, 0, 360 * (M_PI/180));
    cairo_stroke(cr);

    cairo_arc(cr, 582, 368, 64, 0, 360 * (M_PI/180));
    cairo_stroke(cr);

    /*
     * Draw a rectangle
     */
    cairo_move_to(cr, 432, 48);
    cairo_line_to(cr, 592, 48);
    cairo_line_to(cr, 592, 288);
    cairo_line_to(cr, 432, 288);
    cairo_line_to(cr, 432, 48);
    cairo_set_source_rgb(cr, 0.75, 0, 0);
    cairo_fill(cr);

    cairo_move_to(cr, 432, 48);
    cairo_line_to(cr, 592, 48);
    cairo_line_to(cr, 592, 288);
    cairo_line_to(cr, 432, 288);
    cairo_line_to(cr, 432, 48);
    cairo_set_source_rgb(cr, 1, 1, 1);
    cairo_stroke(cr);
}

int main(int argc, char **argv) {
    Display *display;
    Window win;
    Pixmap buf;
    GC gc;

    int screen, x = 1024, y = 480;

    XGCValues values = {
        .foreground         = 0x000000,
        .background         = 0xffffff,
        .graphics_exposures = 0
    };

    cairo_surface_t *sfc;
    cairo_t *ctx;

    XEvent e;

    if ((display = XOpenDisplay(NULL)) == NULL)
        exit(1);

    screen = DefaultScreen(display);
    win = XCreateSimpleWindow(display, DefaultRootWindow(display), 0, 0, x, y, 0, 0, 0);
    gc  = XCreateGC(display, win, GCForeground | GCBackground | GCGraphicsExposures, &values);
    buf = XCreatePixmap(display, win, x, y, 24);

    XSelectInput(display, win, ExposureMask | ButtonPressMask | KeyPressMask);
    XMapWindow(display, win);

    sfc = cairo_xlib_surface_create(display, buf, DefaultVisual(display, screen), x, y);
    cairo_xlib_surface_set_size(sfc, x, y);

    ctx = cairo_create(sfc);

    redraw(ctx);

    while (1) {
        XExposeEvent *expose;

        XNextEvent(cairo_xlib_surface_get_display(sfc), &e);

        switch (e.type) {
            case Expose:
                expose = (XExposeEvent *)&e;

                XCopyArea(display, buf, win, gc, expose->x, expose->y,
                          expose->width, expose->height,
                          expose->x, expose->y);

                break;
                    
            case ButtonPress:
            case KeyPress:
                goto done;

            default:
                fprintf(stderr, "Dropping unhandled XEvent.type = %d.\n", e.type);
        }
    }

done:
    cairo_destroy(ctx);

    cairo_surface_destroy(sfc);
    XCloseDisplay(display);

    return 0;
}