EQ-Robin: Constraint-Resilient Generation of Multiple Minimal Unique-Cause MC/DC Test Suites

Unique-Cause Modified Condition/Decision Coverage (MC/DC) is widely required in safety-critical verification. A recent deterministic algorithm, Robin's Rule, constructs the theoretical minimum of N+1 test cases for Singular Boolean Expressions (SBEs), providing strong guarantees when all generated test vectors are executable. However, industrial systems impose feasibility constraints: some input combinations are illegal, unachievable, or unsafe to execute. If a single illegal vector appears in a minimal suite, it can destroy a required independence pair and invalidate 100% Unique-Cause MC/DC, even though the underlying decision logic remains unchanged. This paper presents EQ-Robin, a lightweight, constraint-resilient pipeline that generates a family of minimal (N+1) Unique-Cause MC/DC test suites and selects a feasible suite that satisfies domain constraints. EQ-Robin systematically enumerates semantically equivalent but structurally distinct SBEs by applying algebraic rearrangements on the expression's Abstract Syntax Tree (AST). Because Robin's Rule is sensitive to structural order, each variant yields a distinct minimal suite. EQ-Robin then filters and ranks candidate suites using practical constraint checks and cost heuristics (e.g., setup cost, oracle complexity). To ensure usability at scale, we introduce a budgeted exploration mode (early exit, guided rearrangement) and a fallback repair strategy when no fully feasible N+1 suite exists under given constraints. We demonstrate the core failure mode and recovery mechanism on a TCAS-II-derived expression and outline an empirical evaluation of EQ-Robin on TCAS-II SBEs under constraint scenarios representative of industrial feasibility limitations.