code: NameWithINode getSourceFile(String parity, String prefix) throws IOException {
      if (isHarFile(parity)) {
        return null;
      }
      // remove the prefix
      String src = parity.substring(prefix.length());
      byte[][] components = INodeDirectory.getPathComponents(src);
      INode inode = namesystem.dir.getINode(components);
      return new NameWithINode(src, inode);
    }
docs: Get path for the corresponding source file for a valid parity
  file. Returns null if it does not exists
  @param parity the toUri path of the parity file
  @return the toUri path of the source file