Infer qubit number from STIM program#162
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Pull request overview
Adds a qubit-count inference path for running validated Stim programs from the Python API, so callers can omit n_qubits in common cases while keeping the Rust backend’s explicit sizing requirements.
Changes:
- Added
StimProgram.num_qubits(Python-visible) backed by a newExtendedProgram::num_qubits()AST helper instim-parser. - Made
n_qubitsoptional inppvm.sample_stimandGeneralizedTableau.sample, inferring it fromprog.num_qubitswhen omitted. - Added Python and Rust tests covering qubit-count inference and edge cases (annotations ignored, repeats handled).
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
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| File | Description |
|---|---|
| ppvm-python/test/generalized_tableau/test_stim.py | Adds tests for StimProgram.num_qubits and for omitting n_qubits in sampling APIs. |
| ppvm-python/src/ppvm/generalized_tableau.py | Makes n_qubits optional for sampling and infers from prog.num_qubits. |
| ppvm-python/src/ppvm/_core.pyi | Exposes the new StimProgram.num_qubits property in type stubs. |
| crates/stim-parser/src/ast/extended.rs | Implements ExtendedProgram::num_qubits() via a recursive max-qubit walk, with unit tests. |
| crates/ppvm-python-native/src/stim_program.rs | Exposes num_qubits via a PyO3 #[getter] on StimProgram. |
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Roger-luo
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Jul 13, 2026
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| /// Number of qubits the program operates on: one past the highest qubit | ||
| /// index any instruction references (`0` if it touches no qubits). Lets | ||
| /// callers size a tableau when no explicit qubit count is given. |
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| When ``n_qubits`` is ``None`` (the default) the qubit count is inferred | ||
| from the program via ``prog.num_qubits`` (one past the highest qubit | ||
| index it references), falling back to 1 for a program that touches no | ||
| qubits. Pass an explicit ``n_qubits`` to size the tableau larger. |
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| // Empty program, and an annotation-only program, both touch no qubits. | ||
| let empty = ExtendedProgram { | ||
| instructions: vec![], | ||
| line_map: Arc::new(LineMap::new("")), | ||
| }; | ||
| assert_eq!(empty.num_qubits(), 0); | ||
| } |
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One more nice convenience thing when running STIM programs.
n_qubitsin the python wrapper (sample_stimandGeneralizedTableau.sample) is now optional and inferred from the program if omitted.GeneralizedTableau.dostill require it (even though the inference is exposed, so you could construct the stim program first and infer from that), because of the compile-time values that depend on the qubit number.FYI, @rafaelha: is this okay or should we somehow have
GeneralizedTableau.doalso omit the qubit number? It's a little trickier, but I guess we could just add a hack.