!7

p-substance {
  if (l(args) != 0) new Viewer().showFile(new File(join(" ", args)));
  else inputFilePath("Image to view", voidfunc(File file) { new Viewer().showFile(file) });
}

sclass Viewer {
  ImageSurface is;
  File file;

  void showFile(final File file) {
    assertTrue("File not found: " + absolutePath(file), file.exists());
    this.file = file;
    bool first = is == null;
    is = showImage_centered(is, file.getName(), loadBufferedImage(file));
    awt { is.zoomToWindow(); }
    
    if (first) {  
      registerLeftKey(getFrame(is), r { nextFile(-1) });
      registerRightKey(getFrame(is), r { nextFile(1) });
      registerMinusKeys(is, r { is.zoomOut(1.5) });
      registerPlusKeys(is, r { is.zoomIn(1.5) });
      registerKey(is, KeyEvent.VK_A, "A", r { is.setZoom(1.0) });
      registerKey(is, KeyEvent.VK_W, "W", r { is.zoomToWindow() });
      hideConsole();
    }
  }
    
  void nextFile(int offset) {
    L<File> files = asList(listFilesOnly(file.getParentFile()));
    int i = indexOfPred(files, func(File f) { eq(f.getName(), file.getName()) });
    showFile(get(files, mod(i+offset, l(files))));
  }
}

Began life as a copy of #1006685