Improvements relating to hash tables

Methods for inserting objects into a hash table, searching for objects in a hash table, and deleting objects from a hash table. The hash table comprising a multiplicity of buckets. The hash table has a corresponding hash function, and a probe sequence that defines for a given hash value a sequence of buckets in the hash table. Each bucket in the hash table has an extend flag to indicate if there are subsequent objects in the hash table with the same hash value. The invention is particularly applicable to mapping genome sequences onto reference genome sequences.