public void purgeUnreferencedEntries() {
        for (Segment segment : this.segments) {
            segment.restructureIfNecessary(false);
        }
    }
Remove any entries that have been garbage collected and are no longer referenced.
Under normal circumstances garbage collected entries are automatically purged as
items are added or removed from the Map. This method can be used to force a purge,
and is useful when the Map is read frequently but updated less often.
analyzer/src/main/java/nl/basjes/parse/useragent/utils/springframework/util/ConcurrentReferenceHashMap.java