connection.getDiff( set, availableSets )

This determines the minimal amount of data that must be loaded from the base connection by going through each cached set (availableSets) and doing a subset check and a set difference with the requested set (set).

If set is a subset of an availableSet, {cached: set} will be returned.

If set is neither a subset of, nor intersects with any availableSets, {needed: set} is returned.

If set has an intersection with one or more availableSets, a description of the difference that has the fewest missing elements will be returned. An example diff description looks like:

{
  needed: {start: 50, end: 99}, // the difference, the set that is not cached
  cached: {start: 0, end: 49}, // the intersection, the set that is cached
  count: 49 // the size of the needed set
}