/* A hello-world program with XPutImage() and event handling.
 *
 * If you want to write a program that puts graphics on the screen,
 * regardless of how you do it, you need a certain amount of
 * system-dependent boilerplate just to open a window and slam a
 * framebuffer into it and get notified of mouse and keyboard events.
 * These two pages of total BULLSHIT are what you
 * need to do it with Xlib.
 *
 * This doesn’t cover the need to poll for events so you can do
 * background work when no events need processing; for that you can
 * use XPending(display) to see if events have been received and
 * select() on the ConnectionNumber(display) to see if any might be
 * waiting to be received.
 *
 * See xshmu.c and friends for a saner approach using XShmPutImage.
 */

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

#include <unistd.h>

#include <X11/Xlib.h>
#include <X11/Xutil.h>

void err(char *fmt, ...);
void draw(uint32_t *buf, int width, int height, int cx, int cy);
void redraw(Display *d, Window w, int screen, XImage *im, int ww, int hh);

int main(int argc, char **argv)
{
  int width = 640, height = 480;
  uint32_t *buf = malloc(width*height*sizeof(*buf));
  if (!buf) {
    err("malloc");
  }

  Display *d = XOpenDisplay(NULL);
  if (!d) {
    err("XOpenDisplay");
  }
  int screen = DefaultScreen(d);
  Window w = XCreateSimpleWindow(d, DefaultRootWindow(d),
                                 0, 0, /* x, y */
                                 width, height,
                                 0, 0, /* borderwidth, border */
                                 WhitePixel(d, screen));

  XSelectInput(d, w, ExposureMask
               | KeyPressMask
               | KeyReleaseMask
               | ButtonPressMask
               | ButtonReleaseMask
               | EnterWindowMask
               | LeaveWindowMask
               | PointerMotionMask
               | StructureNotifyMask);

  XMapWindow(d, w);

  draw(buf, width, height, width/2, height/2);

  XImage *im = XCreateImage(d,
                            DefaultVisual(d, screen),
                            24,      /* depth */
                            ZPixmap, /* format */
                            0,       /* offset (in X pixels) */
                            (char*)buf, /* data */
                            width,
                            height,
                            32, /* bitmap_pad */
                            0); /* let Xlib calculate bytes_per_line */

  XEvent ev;
  for (;;) {
    XNextEvent(d, &ev);
    if (ev.type == Expose) {
      redraw(d, w, screen, im, width, height);
    } else if (ev.type == KeyPress) {
      break;
    } else if (ev.type == MotionNotify) {
      draw(buf, width, height, ev.xmotion.x, ev.xmotion.y);
      redraw(d, w, screen, im, width, height);
    }
  }

  XDestroyImage(im);
  XCloseDisplay(d);
  return 0;
}

void draw(uint32_t *buf, int width, int height, int cx, int cy) {
  for (int yy = 0; yy < height; yy++) {
    for (int xx = 0; xx < width; xx++) {
      buf[yy*width + xx] = (xx-cx)*(yy-cy);
    }
  }
}

void redraw(Display *d, Window w, int screen, XImage *im, int ww, int hh) {
      XPutImage(d, w, DefaultGC(d, screen), im,
                0, 0,           /* src_x, src_y */
                0, 0,           /* dest_x, dest_y */
                ww, hh);
}

void err(char *fmt, ...)
{
  va_list args;
  va_start(args, fmt);
  perror(fmt);
  vfprintf(stderr, fmt, args);
  exit(-1);
  va_end(args);
}