Abstract: A method comprises: sending at a server to one or more user devices first data and group of first hashes, the group comprising a subset of first hashes stored in a hash store. Each first hash is stored in association with a respective first data portion. The server subsequently receives from each user device one or more second hashes and second data. The first data has been modified at the user device and the second data comprises the modified first data excluding one or more second data portions from which each second hash can be hashed. For each second hash, an indication that the second hash has been received with the matching, is then associated with the stored first hash. Based on the indications, the group is updated to comprises first hashes that are more likely to be received than the first hashes not in the group.

Abstract: A computer-implemented method of providing unique identification information for an element in a tree data structure where the element does not have an independent, unique identifier associated therewith by which it can be identified without reference to any further element. The method comprises determining (130) identification information uniquely identifying an element in a structure model, by reference to another element in the structure having a unique identifier associated therewith by which the other element can be uniquely identified without reference to any further element in the structure. The unique identification information comprises a unique identifier of the other element by which said other element can be uniquely identified in the structure without reference to any further element in the structure, and path information uniquely identifying the element in the structure relative to the other element.