A system receives an association of first item with first system user, generates first hash value by applying first hash function associated with first system user to first item identifier associated with first item, and sets a bit corresponding to first hash value in array. The system receives an association of second item with second system user, generates second hash value by applying second hash function associated with second user to second item identifier associated with second item, and sets a bit corresponding to second hash value in array. The system receives a request to determine whether third item is associated with first system user, generates third hash value by applying first hash function to third item identifier associated with third item, and outputs message that third item is not associated with first user if a bit corresponding to third hash value is not set in array.