private List<MockId> generateIds(NavigableMap<MockId, NavigableSet<IndexValue>> referenceIndex, int count) {
    List<MockId> generatedIds = new ArrayList<>();
    for (int i = 0; i < count; i++) {
      MockId id;
      do {
        id = new MockId(TestUtils.getRandomString(CUSTOM_ID_SIZE));
      } while (referenceIndex.containsKey(id) || generatedIds.contains(id));
      generatedIds.add(id);
    }
    return generatedIds;
  }
Generates {@code count} unique ids that arent in {@code referenceIndex}.
@param referenceIndex the current reference index
@param count the number of unique ids to generate
@return a set with {@code count} unique ids
ambry-store/src/test/java/com/github/ambry/store/IndexSegmentTest.java