public int rootBlockContainingKey(final byte[] key, int offset, int length) {
      return rootBlockContainingKey(key, offset, length, null);
    }
Finds the root-level index block containing the given key. Key to find
@return Offset of block containing <code>key</code> (between 0 and the number of blocks - 1)
        or -1 if this file does not contain the request.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlockIndex.java