Lifting infinite normal form definitions from term rewriting to term graph rewriting

Infinite normal forms are a way of giving semantics to non-terminating rewrite systems. The notion is a generalization of the Boehm tree in the lambda calculus. It was first introduced in [AB97] to provide semantics for a lambda calculus on terms with letrec. In that paper infinite normal forms were defined directly on the graph rewrit e system. In [Blo01] the framework was improved by defining the infinite normal form of a term graph using the infinite normal form on terms. This approach of lifting the definition makes the non-confluence problems introduced into term graph rewriting by substitution rules much easier to deal with. In this paper, we give a simplified presentation of the latter approach

Simulated time for testing railway interlockings with TTCN-3

In this report, we first give an overview of software systems based on Vital Processor Interlocking (VPI). Interlockings guarantee safety of railway control systems, so testing these software systems is a key issue. We show why testing such systems with real time and scaled time is inefficient. We also provide a time semantics for simulated time that is more suitable for testing VPI's software. We provide a solution that allows simulated time for TTCN-3 test systems. TTCN-3 is a standard language for specifying and executing test suites. In the context of the TT-MEDAL project, TTCN-3 is applied to various domains, in particular to testing railway and automotive systems. TTCN-3 supports real-time and scaled-time testing but not simulated-time testing. The solution is based on a distributed termination detection algorithm that we extend to provide the main ingredients of simulated time: idleness detection and correct time progress. We implemented our solution as a TTCN-3 module and several Java classes that can be reused for testing other systems that have characteristics similar to those of VPI

Parallel Recursive State Compression for Free

This paper focuses on reducing memory usage in enumerative model checking, while maintaining the multi-core scalability obtained in earlier work. We present a tree-based multi-core compression method, which works by leveraging sharing among sub-vectors of state vectors. An algorithmic analysis of both worst-case and optimal compression ratios shows the potential to compress even large states to a small constant on average (8 bytes). Our experiments demonstrate that this holds up in practice: the median compression ratio of 279 measured experiments is within 17% of the optimum for tree compression, and five times better than the median compression ratio of SPIN's COLLAPSE compression. Our algorithms are implemented in the LTSmin tool, and our experiments show that for model checking, multi-core tree compression pays its own way: it comes virtually without overhead compared to the fastest hash table-based methods.Comment: 19 page

Timed verification with muCRL

muCRL is a process algebraic language for specification and verification of distributed systems. muCRL allows to describe temporal properties of distributed systems but it has no explicit reference to time. In this work we propose a manner of introducing discrete time without extending the language. The semantics of discrete time we use makes it possible to reduce the time progress problem to the diagnostics of 'no action is enabled' situations. The synchronous nature of the language facilitates the task. We show some experimental verification results obtained on a timed communication protocol