Reduction and Slicing of
Hierarchical State Machines
Mats Heimdahl et al.
University of Minnesota
Presented by Tom McMullen
For CISC836
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Outline
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Primer
Problem Space
Paper Overview
Application (Case Study)
Limitations
Critique
Discussion
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Primer
• Presented at 1997 Proceedings 6th European
Software Engineering Conference
• Cited by 65 (source: Google Scholar)
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Problem Space
• Formal Specification Languages
– Difficult to understand and use
– Not well understood by application experts
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Problem Space
• State Machine Representation
– Better, but…
Inevitable Complexity for large
systems
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Problem Space
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What this paper proposes…
• Address complexity of HSMs
• In Order To:
– Present information in digestible chunks
• Method:
– Step 1: Simplify based on scenario
– Step 2: Slice for desired values
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A Quick Refresher…
• Hierarchical State Machines (HSM)
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A Quick Refresher…
• Slicing
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A quick refresher…
• Program Slicing
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Application
• Applied to HSMs
– RSML (Requirements State Machine Language)
• Specification of safety-critical systems
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RSML
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Testbed Specification
• TCAS II
– Traffic alert and Collision Avoidance System
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Step 1: Reduce
• Produce simplified RSML model
• Interpretation based on scenario
– Domain restriction of next-state relation
– How do we classify an intruder who has stopped
reporting altitude?
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Step 1: Reduce
• Eliminate infeasible columns for scenario
• Reduction Algorithm limited to enumerated vars
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Step 2: Slice
• Slicing Algorithm
– Based on marking of Abstract Syntax Tree
– RSML parser part of earlier research
• Data Flow Slices (if a transition can be taken)
– Data Dependency of Guarding Transition
• Control Flow Slices (when a transition is taken)
– Generation of a trigger event
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Step 3: Profit?...
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Iterative slices are then combined
Attempt to answer our questions
Reduced complexity
Increased Understandability
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Case Study
• Applied to most complex part of TCAS II RSML
Model
• Subjective notion of complexity /
understandability
• Metrics:
– # of transitions
– Perceived Table Size
– Effective Table Size
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Case Study
• Results
– Promising
• Significant reduction in table size
– But..
• Reduction in # of transitions not as expected
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Critique
• Case Study details lacking
– Which TCAS model? What Questions/Scenario?
– 1998 paper has a more detailed case study/empirical
data
• Too much emphasis on RSML
– Small application
– More detail on slicing HSM in general
• Reduction and Slicing algorithms not presented
– Data flow / control flow slices not explained in depth
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Discussion
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