private int getNumTopics(int segIdx) {
        if (segIdx < 0) {
            return 0;
        }
        ByteBuffer buf =  getIndirectRecordEntry(segIdx, 4);
        return buf.getInt(0);
    }
Returns the number of topics the FSA contains for the input
segment.
@return Number of topics for the segment.
fsa/src/main/java/com/yahoo/fsa/topicpredictor/TopicPredictor.java