private long getSegmentCount() {
        if (segmentCount == null) {
            if (segmentInputStream == null) {
                segmentInputStream = new RASegmentInputStream(data, indexInputStream);
            }
            segmentCount = segmentInputStream.count();
        }
        return segmentCount;
    }
Wrapper for a nested input stream.
<p>
You must call getNextEntry and closeEntry like the ZIP API.
stroom-data/stroom-data-store-impl-fs/src/main/java/stroom/data/store/impl/fs/RANestedInputStream.java