FA2S Interpreter Library (C++17)

This refactors the single-file interpreter into a reusable library.

Highlights

• Library target: fa2s_interpreter
• Public API (namespace fa2s):
  • Exceptions: ParseError, RuntimeFault
  • Data types: Program, Transition, InGuard, TopGuard
  • Class: Interpreter
    • loadProgramFromString / loadProgram(istream&)
    • setInput(string), setOutput(ostream*), setTrace(bool), setMaxSteps(size_t)
    • run() -> int (0=accept, 1=reject; throws on error)
    • stepsExecuted() -> size_t
• CLI example target: fa2s (optional)

Build

• CMake
  • mkdir build && cd build
  • cmake .. -DCMAKE_BUILD_TYPE=Release
  • cmake --build .
• Run CLI
  • ./fa2s examples/echo.fa2s --input "hello"
  • ./fa2s examples/reverse.fa2s --input "abcd"
  • ./fa2s examples/balparen.fa2s --input "(())()"

Notes

• Semantics match the Python and the original C++ single-file version:
  • read at EOF, empty stack pop, or no matching transition -> RuntimeFault
  • Step limit default 1,000,000 (configurable)
  • Trace prints to stderr; program output to setOutput(ostream*) (default cout)