how to download the file range pieces? about bt HOT 2 OPEN

	private final FilePrioritySkipSelector filePrioritySkipSelector =
			torrentFile ->
					torrent.getFiles().indexOf(torrentFile)==selectedFileIndex
							? FilePriority.HIGH_PRIORITY
							: FilePriority.SKIP;

	private final PieceSelector pieceSelector = new PieceSelector() {
		@Override
		public IntStream getNextPieces(BitSet bitSet, PieceStatistics pieceStatistics) {
			return IntStream.of(piecesToDownload);
		}
	};

	private boolean piecesDownloaded() {
		for (int i:piecesToDownload) if (!pieceMap[i]) return false;
		return true;
	}

	private BtClient createClient(URI uri) {
		try {
			return Bt.client(runtime)
					.storage(storage)
					.torrent(uri.toURL())
					.afterTorrentFetched(tf -> {
...
						pieceMap = new boolean[piecesInTorrent];
					        piecesToDownload = IntStream.concat(IntStream.range(firstFilePiece, firstFilePiece+piecesToPrepare), IntStream.rangeClosed(lastFilePiece-criticalEndPieces+1, lastFilePiece)).toArray();
						runtime.getEventSource().onPieceVerified(torrent.getTorrentId(), event -> {
							pieceMap[event.getPieceIndex()]=true;
							if (piecesDownloaded()) {
                                                             // update piecesToDownload array to supply next portion of pieces
							}
						});
					})
					.fileSelector(filePrioritySkipSelector)
					.selector(pieceSelector)
					.stopWhenDownloaded()
					.build();
		} catch (Exception e) {
			log("Error creating BT client", e);
			return null;
		}
	}