1. A method comprising, by a computing device: at time t 2 , determining a delta set of first objects representing a difference between a first set of first objects at time t 1 and a second set of first objects at time t 2 ; comparing the delta set of first objects with a set of second objects represented as a balanced k-dimensional tree; and identifying each second object whose content substantially matches content of at least one first object from the delta set of first objects.