public DoubleWritable evaluate(DoubleWritable base, DoubleWritable a) {
    if (a == null || base == null) {
      return null;
    }
    return log(base.get(), a.get());
  }
Returns the logarithm of "a" with base "base".
ql/src/java/org/apache/hadoop/hive/ql/udf/UDFLog.java