The task of software production is to build software systems which are to fulfil certain requirements. For years the approach has been to build up by trial and error a program which, having satisfied carefully prepared test data, offers a plausible solution to the problem. But is it correct? Even for toy examples this is not obvious. In particular, it is often not even clear whether the original problem has been properly understood. The reason for this dilemma is that the transition from the informal problem statement to the final program is too big to be intellectually manageable. To master these problems, we advocate a software development method where the whole process is split into smaller steps by introducing formal specifications for (parts of) the problem and then stepwisely deriving efficient programs by correctness-preserving transformations.

Abstract Highly optimised algorithms are, in general, hard to understand. This is a consequence of the designer's sacrifice of clarity and modularity in favour of efficiency. In this paper we present a formal derivation of a rather ingenious algorithm, viz., the fast pattern matching algorithm of Boyer and Moore. The development illustrates that transformational programming combined with assertional reasoning provides an appropriate approach for developing and understanding highly optimised algorithms.

Formal program construction by transformations is a method of software development in which a program is derived from a formal problem specification by manageable, controlled transformation steps which guarantee that the final product meets the initial specification. This methodology has been investigated in the Munich project CIP (computer-aided intuition-guided programming). The research includes the design of a wide-spectrum language specifically tailored to the needs of transformational programming, the construction of a transformation system to support the methodology, and the study of transformation rules and other methodological issues. Particular emphasis has been laid on developing a sound theoretical basis for the overall approach.>