This paper sketches a reverse engineering discipline which combines formal and semi-formal methods. Central to the former is denotational semantics, expressed in the ISO/IEC 13817-1 standard specification language (VDM-SL). This is strengthened with algebra of pro- gramming, which is applied in “reverse order” so as to reconstruct formal specifications from legacy code. The latter include code slicing, a “shortcut” which trims down the complexity of handling the formal semantics of all program variables at the same time.
A key point of the approach is its constructive style. Reverse calculations go as far as absorbing auxiliary variables, introducing mutual recursion (if applicable) and reversing semantic denota- tions into standard generic programming schemata such as cata/paramorphisms.
The approach is illustrated for a small piece of code already studied in the code-slicing literature: Kernighan and Richtie’s word count C programming “bagatelle”.FC
