takes a batch of updates in their natural order (i.e. the order they occurred in the client code) and normalizes them to something iOS can consume when performing batch updates to a table or collection view iOS requires that all deletes be specified first with indexes relative to the source data *before* any insertions are applied it then requires insertions be specified next relative to the source data *after* any deletions are applied this code also de-duplicates as necessary. The simplest possible scenario for this is adding and immediately deleting an item. iOS should never even be told about this set of updates because they cancel each other out.

public static class IndexNormalizer