/* Put a zoneplate texture on the framebuffer.
 *
 * This does work on (my) Linux console, but in X11 it has no visible
 * effect.
 */

#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <stdio.h>

int main()
{
  int fb = open("/dev/fb0", O_RDWR);
  if (fb < 0) {
    perror("open /dev/fb0");
    return -1;
  }

  struct fb_fix_screeninfo fix;
  if (ioctl(fb, FBIOGET_FSCREENINFO, &fix) < 0) {
    perror("FBIOGET_FSCREENINFO");
    return -1;
  }

  int pagesize = sysconf(_SC_PAGESIZE);
  if (fix.smem_start & (pagesize-1)) {
    fprintf(stderr, "smem_start %x not %d-aligned\n", (int)fix.smem_start,
            pagesize);
    return -1;
  }

  if (fix.type != FB_TYPE_PACKED_PIXELS) {
    fprintf(stderr, "framebuffer type %u isn’t FB_TYPE_PACKED_PIXELS\n",
            (unsigned)fix.type);
    return -1;
  }

  if (fix.visual != FB_VISUAL_TRUECOLOR) {
    fprintf(stderr, "visual %u isn’t FB_VISUAL_TRUECOLOR\n",
            (unsigned)fix.visual);
    return -1;
  }

  struct fb_var_screeninfo var;
  if (ioctl(fb, FBIOGET_VSCREENINFO, &var) < 0) {
    perror("FBIOGET_VSCREENINFO");
    return -1;
  }

  /* e.g. https://www.raspberrypi.org/forums/viewtopic.php?t=29844
     suggests that we can enable page-flipping double-buffering with

         var.yres_virtual = var.yres * 2;
         ioctl(fb, FBIOPUT_VSCREENINFO, &var);

     and then to flip:

         var.activate = FB_ACTIVATE_VBL;
         var.yoffset = offset;
         ioctl(fb, FBIOPUT_VSCREENINFO, &var);

     or at least the author cleverca22 claimed that worked on Android
     and the RasPi fork of Android in 2013.  Also in
     https://lb.raspberrypi.org/forums/viewtopic.php?t=30720&start=25
     hglm claimed that it worked on the RasPi in 2015, but required
     kernel patches, although now https://github.com/hglm/dgl is using
     `FBIOPAN_DISPLAY` instead.
     https://bugs.launchpad.net/qemu/+bug/1777672 describes a
     different Pi-3-specific way to do page-flipping on Linux in 2018,
     and explains how QEMU support for emulating it was added.
     https://unix.stackexchange.com/questions/33596/no-framebuffer-device-how-to-enable-it
     mentions the DirectFB library.
     https://github.com/toradex/cairo-fb-examples/blob/master/rectangles/rectangles.c#L51
     also uses `FBIOPAN_DISPLAY`, and also `FBIO_WAITFORVSYNC`.
  */

  if (var.bits_per_pixel != 32) {
    fprintf(stderr, "bits_per_pixel is %d, not 32\n", (int)var.bits_per_pixel);
    return -1;
  }

  if (var.xres_virtual >= 32768 || var.yres_virtual >= 32768) {
    fprintf(stderr, "%u×%u resolution is unlikely\n",
            (unsigned)var.xres_virtual,
            (unsigned)var.yres_virtual);
    return -1;
  }
  char *fb_buf =
    mmap(NULL, fix.smem_len, PROT_READ | PROT_WRITE, MAP_SHARED, fb, 0);

  if (!fb_buf || (intptr_t)fb_buf == -1) {
    perror("mmap");
    return -1;
  }

  uint32_t *p = (uint32_t*)fb_buf;
  uint32_t line_len = var.xres_virtual;

  for (uint32_t y = 0; y != var.yres_virtual; y++) {
    for (uint32_t x = 0; x != line_len; x++) {
      p[x + line_len * y] = x*x + y*y;
    }
  }

  return 0;
}