// cancel single key
 @Override
 void implCancelKey(WatchKey obj) {
   WindowsWatchKey key = (WindowsWatchKey) obj;
   if (key.isValid()) {
     fk2key.remove(key.fileKey());
     ck2key.remove(key.completionKey());
     key.invalidate();
   }
 }
    /**
     * Register a directory for changes as follows:
     *
     * <p>1. Open directory 2. Read its attributes (and check it really is a directory) 3. Assign
     * completion key and associated handle with completion port 4. Call ReadDirectoryChangesW to
     * start (async) read of changes 5. Create or return existing key representing registration
     */
    @Override
    Object implRegister(
        Path obj, Set<? extends WatchEvent.Kind<?>> events, WatchEvent.Modifier... modifiers) {
      WindowsPath dir = (WindowsPath) obj;
      boolean watchSubtree = false;

      // FILE_TREE modifier allowed
      for (WatchEvent.Modifier modifier : modifiers) {
        if (modifier == ExtendedWatchEventModifier.FILE_TREE) {
          watchSubtree = true;
        } else {
          if (modifier == null) return new NullPointerException();
          if (modifier instanceof com.sun.nio.file.SensitivityWatchEventModifier)
            continue; // ignore
          return new UnsupportedOperationException("Modifier not supported");
        }
      }

      // open directory
      long handle;
      try {
        handle =
            CreateFile(
                dir.getPathForWin32Calls(),
                FILE_LIST_DIRECTORY,
                (FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE),
                OPEN_EXISTING,
                FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED);
      } catch (WindowsException x) {
        return x.asIOException(dir);
      }

      boolean registered = false;
      try {
        // read attributes and check file is a directory
        WindowsFileAttributes attrs;
        try {
          attrs = WindowsFileAttributes.readAttributes(handle);
        } catch (WindowsException x) {
          return x.asIOException(dir);
        }
        if (!attrs.isDirectory()) {
          return new NotDirectoryException(dir.getPathForExceptionMessage());
        }

        // check if this directory is already registered
        FileKey fk =
            new FileKey(attrs.volSerialNumber(), attrs.fileIndexHigh(), attrs.fileIndexLow());
        WindowsWatchKey existing = fk2key.get(fk);

        // if already registered and we're not changing the subtree
        // modifier then simply update the event and return the key.
        if (existing != null && watchSubtree == existing.watchSubtree()) {
          existing.setEvents(events);
          return existing;
        }

        // Can overflow the int type capacity.
        // Skip WAKEUP_COMPLETION_KEY value.
        int completionKey = ++lastCompletionKey;
        if (completionKey == WAKEUP_COMPLETION_KEY) completionKey = ++lastCompletionKey;

        // associate handle with completion port
        try {
          CreateIoCompletionPort(handle, port, completionKey);
        } catch (WindowsException x) {
          return new IOException(x.getMessage());
        }

        // allocate memory for events, including space for other structures
        // needed to do overlapped I/O
        int size = CHANGES_BUFFER_SIZE + SIZEOF_DWORD + SIZEOF_OVERLAPPED;
        NativeBuffer buffer = NativeBuffers.getNativeBuffer(size);

        long bufferAddress = buffer.address();
        long overlappedAddress = bufferAddress + size - SIZEOF_OVERLAPPED;
        long countAddress = overlappedAddress - SIZEOF_DWORD;

        // zero the overlapped structure
        UNSAFE.setMemory(overlappedAddress, SIZEOF_OVERLAPPED, (byte) 0);

        // start async read of changes to directory
        try {
          createAndAttachEvent(overlappedAddress);

          ReadDirectoryChangesW(
              handle,
              bufferAddress,
              CHANGES_BUFFER_SIZE,
              watchSubtree,
              ALL_FILE_NOTIFY_EVENTS,
              countAddress,
              overlappedAddress);
        } catch (WindowsException x) {
          closeAttachedEvent(overlappedAddress);
          buffer.release();
          return new IOException(x.getMessage());
        }

        WindowsWatchKey watchKey;
        if (existing == null) {
          // not registered so create new watch key
          watchKey =
              new WindowsWatchKey(dir, watcher, fk)
                  .init(
                      handle,
                      events,
                      watchSubtree,
                      buffer,
                      countAddress,
                      overlappedAddress,
                      completionKey);
          // map file key to watch key
          fk2key.put(fk, watchKey);
        } else {
          // directory already registered so need to:
          // 1. remove mapping from old completion key to existing watch key
          // 2. release existing key's resources (handle/buffer)
          // 3. re-initialize key with new handle/buffer
          ck2key.remove(existing.completionKey());
          releaseResources(existing);
          watchKey =
              existing.init(
                  handle,
                  events,
                  watchSubtree,
                  buffer,
                  countAddress,
                  overlappedAddress,
                  completionKey);
        }
        // map completion map to watch key
        ck2key.put(completionKey, watchKey);

        registered = true;
        return watchKey;

      } finally {
        if (!registered) CloseHandle(handle);
      }
    }