docs: public BitSet getAvailablePieces() {
      if (!this.isInitialized()) {
        throw new IllegalStateException("Torrent not yet initialized!");
      }
  
      BitSet availablePieces = new BitSet(this.pieces.length);
  
      synchronized (this.pieces) {
        for (Piece piece : this.pieces) {
          if (piece.available()) {
            availablePieces.set(piece.getIndex());
          }
        }
      }
  
      return availablePieces;
    }