In general, in one aspect, the invention features a data structure representing a schedule for a plurality of N resources, and apparatus, methods, and computer programs for generating and using the data structure, the data structure comprising: M records each representing a different one of M slots in the schedule, wherein M>1, wherein each of the M slots is allocated to one of the N resources, and wherein each of the M records comprises identifiers of the N ones of the M slots in the schedule allocated to a next occurrence of each of the N resources following the one of the M slots represented by the record, and an identifier of an order in which the N resources occur in the schedule following the one of the M slots represented by the record.