Differential testing

[mave9159]

Overview

This PR implements a differential oracle for comparing two Mealy machine models, and a report generator for the results.

New classes

• DifferentialOracle: uses an BFS algorithm over the product state space to find divergences between two Mealy machine models. Returns a list of DivergenceRecord instances.
• DivergenceRecord: contains a witness sequence and the two differing outputs, for a single divergence.
• DifferentialReport: generates a test file and a report file from the found divergences.

Tests

• DifferentialOracleTest: contains unit tests for the core BFS algorithm.
• DifferentialReportTest: contains unit tests for the report writer.

Test resources

16 .dot files used for the unit tests. Some are basic hand-crafted models and some are taken from the dtls-fuzzer repo (https://github.com/assist-project/dtls-fuzzer/tree/master/experiments/results/clients).

[actyp]

Thank you for making the changes so quickly!
They are looking good overall, but we need to focus on the alphabet handling.

I think we should follow the way that TestRunner is implemented. If we want to be totally symmetric we can do the following:

1. [like TestRunner] Create a DiffTester to be our general diff testing interface

2. [like TestRunnerEnabler] Create a DiffTesterEnabler to provide the necessary configs (LearnerConfig, DiffTesterConfig) to the DiffTesterStandard

3. [like TestRunnerStandard] Rename the current DiffTester to DiffTesterStandard that implements the interface.

The DiffTesterStandard constructor should take the DiffTesterEnabler and some AlphabetBuilder as inputs.

Since we need an Alphabet<String> for our use case, I think this presents a nice opportunity to use the AlphabetBuilderTransformer (PSF class) as the argument constructor possibly like this:

constructor(..., AlphabetBuilderTransformer<I, String> ...)...

This way the user should provide the implementation of the transformation from their symbols to String.

4. [like TestRunnerBuilder] We also need to create a DiffTesterBuilder and add it as an argument to the CommandLineParser constructor.

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Let me know if you have any questions.