Abstract Syntax Trees and Control Flow Graph Representation

Lecture 8 of SE602 Software Re-Engineering examines core intermediate representations (IR) used by reverse engineering tools to model program behavior and structure. Students investigate Abstract Syntax Trees (ASTs), which capture the hierarchical syntactic structure of source code while discarding superficial details like whitespace and punctuation. The lecture transitions to Control Flow Graphs (CFGs), where directed graphs represent all computational paths traversed by a program during execution. Basic blocks—maximal sequences of consecutive program statements entered only at the first statement and exited only at the last—are formally defined. Students learn how to compute Thomas McCabe Cyclomatic Complexity V(G) = E - N + 2P from control flow graphs to identify overly complex, error-prone legacy subroutines that require immediate modular restructuring. Students investigate the trade-offs between system replacement, wrapping legacy components, and iterative code refactoring transformations. The discussion highlights how Lehman laws of evolution, system complexity growth, and declining quality necessitate continuous code restructuring. Key concepts including static program analysis, dynamic execution tracing, and characterization test safety nets are thoroughly demystified. The lecture details classical object-oriented re-engineering patterns demonstrated across standard enterprise software architectures. Structured refactoring methodologies are demonstrated using Martin Fowler catalogs, method extraction, and polymorphism replacement. Software quality attributes, maintainability index equations, and technical debt principal valuations are evaluated alongside empirical benchmarks. Architectural recovery processes are explored through module dependency analysis, component clustering, and high-level design reconstruction. Database schema normalization, legacy record migration, and automated ETL extraction pipelines are systematically analyzed. Monolithic application decomposition strategies including service-oriented architecture (SOA) and the Strangler Fig pattern are examined in depth. Scholars examine program slicing algorithms, forward and backward dependency slices, and impact assessment for proposed modifications. Automated tool support, bytecode instrumentation, and static code analyzers are evaluated to detect architectural violations across repositories. Risk management frameworks, business case evaluations, and cost estimation models such as COCOMO II are analyzed for migration projects. Legal and ethical constraints surrounding reverse engineering, clean-room design implementations, and software intellectual property rights are reviewed.