private void colAdvanceRow(int selCol, SelectedColumn col) throws IOException {
      if (col.runLength > 0) {
        --col.runLength;
      } else {
        int length = (int) WritableUtils.readVLong(colValLenBufferReadIn[selCol]);
        if (length < 0) {
          // we reach a runlength here, use the previous length and reset
          // runlength
          col.runLength = (~length) - 1;
        } else {
          col.prvLength = length;
          col.runLength = 0;
        }
      }
    }
Advance column state to the next now: update offsets, run lengths etc
@param selCol - index among selectedColumns
@param col - column object to update the state of.  prvLength will be
       set to the new read position
@throws IOException
ql/src/java/org/apache/hadoop/hive/ql/io/RCFile.java