Editing Jackson structured programming (section)

== Techniques for handling difficult design problems ==
In ''Principles of Program Design'' Jackson recognized situations that posed specific kinds of design problems, and provided techniques for handling them.

One of these situations is a case in which a program processes two input files, rather than one. In 1975, one of the standard "wicked problems" was how to design a transaction-processing program.  In such a program, a sequential file of update records is run against a sequential master file, producing an updated master file as output.  (For example, at night a bank would run a batch program that would update the balances in its customers' accounts based on records of the deposits and withdrawals that they had made that day.)  ''Principles of Program Design'' provided a standard solution for that problem, along with an explanation of the logic behind the design.

Another kind of problem involved what Jackson called "recognition difficulties" and today we would call parsing problems.  The basic JSP design technique was supplemented by POSIT and QUIT operations to allow the design of what we would now call a backtracking parser.

JSP also recognized three situations that are called "structure clashes"&mdash; a boundary clash, an ordering clash, and an interleaving clash&mdash; and provided techniques for dealing with them.  In structure clash situations the input and output data structures are so incompatible that it is not possible to produce the output file from the input file.  It is necessary, in effect, to write two programs&mdash; the first processes the input stream, breaks it down into smaller chunks, and writes those chunks to an intermediate file. The second program reads the intermediate file and produces the desired output.