code: public PeerRequest mapFile(int file, long offset, int size) {
          return new PeerRequest(ti.map_file(file, offset, size));
      }
docs: This function will map a range in a specific file into a range in the torrent.
  The {@code offset} parameter is the offset in the file, given in bytes, where
  0 is the start of the file.
  <p>
  The input range is assumed to be valid within the torrent. {@code offset + size}
  is not allowed to be greater than the file size. {@code index}
  must refer to a valid file, i.e. it cannot be {@code >= numFiles()}.
  
  @param file
  @param offset
  @param size
  @return
  @see com.frostwire.jlibtorrent.PeerRequest