GB1336817A - Optical projection system - Google Patents

Abstract

1336817 Information retrieval INTERNATIONAL BUSINESS MACHINES CORP 22 Sept 1971 [21 Dec 1970] 44104/71 Heading G4A The Specification describes algorithms for maintaining a multi-level compressed key index by making any necessary modifications to the compressed keys resulting from the insertion, deletion or updating of data records. Each maintenance operation includes a multilevel search through the index using a search argument representing a data record, each compressed key level being exited at a compressed key (CK) first comparing high with the search argument and the exiting pointers (printing to the next lower level) being stored, retrieval of a data record with the last pointer stored, derivation of an uncompressed key (UK) from the retrieval data record, and comparison of the UK with the search argument to generate a signal indicating whether the argument is high, low or equal with respect to the UK. If on insertion of a new record, the subsequent addition of a new CK to an index block results in that block exceeding a maximum length, the index block is split, the pointer associated with that block being assigned to one of the new blocks and a new pointer taken from a pool is assigned to the other new block, with appropriate modification of the CK's. The lowest index level associates a single CK with each pointer which addresses a data level block. The higher index levels each associate a pair of CK's with a pointer addressing the next lower level, the first CK indicating the index change within the block referenced by the pointer and the second CK indicating the index change between the end of the block referenced by the pointer and the beginning of the following block in the index sequence at the same level. Specification 1,280,487 is referred to.