public String getPrefix(LogMessage msg) {
        calendar.setTimeInMillis(msg.getTimestamp().toEpochMilli());
        StringBuilder result = new StringBuilder(absoluteRootDir.length()
                                                       + 1 // slash
                                                       + 4 // year
                                                       + 1 // slash
                                                       + 2 // month
                                                       + 1 // slash
                                                       + 2 // day
                                                       + 1 // slash
                                                       + 2 // hour
        );
        result.append(absoluteRootDir).append("/")
              .append(dateformat.format(calendar.getTime()));
        return result.toString();
    }
Generate prefix for log filenames based on log message.
<p>
<EM>This message is <code>public</code> only because we need
access to it in unit tests.  For all practical purposes this
method does not exist to you, the application programmer, OK? :-)</EM>
<p>
XXX optimize!
logserver/src/main/java/com/yahoo/logserver/handlers/archive/ArchiverHandler.java