private Set<MockId> getIdsToDelete(NavigableMap<MockId, NavigableSet<IndexValue>> referenceIndex,
      int outOfSegmentIdCount) {
    Set<MockId> idsToDelete = new HashSet<>();
    // return every alternate id in the map
    boolean include = true;
    for (MockId id : referenceIndex.keySet()) {
      if (include) {
        idsToDelete.add(id);
      }
      include = !include;
    }
    // generate some ids for delete
    idsToDelete.addAll(generateIds(referenceIndex, outOfSegmentIdCount));
    return idsToDelete;
  }
Gets some IDs for deleting. Picks alternate IDs from {@code referenceIndex} and randomly generates
{@code outOfSegmentIdCount} ids.
@param referenceIndex the index entries to pick for delete from.
@param outOfSegmentIdCount the number of ids to be generated that are not in {@code referenceIndex}.
@return a {@link Set} of IDs to create delete entries for.
ambry-store/src/test/java/com/github/ambry/store/IndexSegmentTest.java