1. A method comprising: storing in a compression array a first possible representation of an input data item; comparing a first value of a compression efficiency parameter relating to one or more subsequences of said first representation, with a second value of said compression efficiency parameter relating to one or more subsequences of a second possible representation of said input data item; and replacing in said compression with said one or more subsequences of said first possible representation with said one or more subsequences of the second possible representation if said comparison indicates that said second possible representation is more efficient for compression of said input data item. wherein the first and second possible representations correspond to first and second partitions, respectively, of said input data item into subsequences looked up in a compression dictionary.