🐛 Support BQSKit conversion of IQM's native r gate

[flowerthrower]

Description

Fixes #547, which reports

ValueError: The 'r' gate of device 'iqm_crystal_20' is not supported in BQSKIT.

Assisted-by: GPT-5.5 via Codex

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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📝 Walkthrough

Walkthrough

This PR adds support for IQM's native r gate in BQSKit by mapping it to U1qGate, implementing a new conversion function that translates BQSKit circuits to Qiskit through OpenQASM 2 rewriting, and updating imports and tests across the codebase.

Changes

IQM r-gate support in BQSKit

Layer / File(s)	Summary
Type annotations and native gate mapping src/mqt/predictor/rl/parsing.py	Improved type system introduces BqskitCircuit, TketCircuit, and QuantumCircuit annotations. Parameter type for PreProcessTKETRoutingAfterQiskitLayout.apply narrowed to TketCircuit. Native gate mapping in get_bqskit_native_gates gains entry for "r" gate mapped to gates.U1qGate().
BQSKit to Qiskit conversion function src/mqt/predictor/rl/parsing.py	New bqskit_to_qiskit function converts BqskitCircuit to QuantumCircuit by encoding to OpenQASM 2, rewriting U1q( gate name to r(, and registering custom Qiskit r instruction. Parameter annotation for final_layout_pytket_to_qiskit updated to TketCircuit.
Test coverage for r-gate support tests/compilation/test_helper_rl.py	Import Circuit and gates from bqskit.ir. Add test validating IQM crystal device includes U1qGate in BQSKit native gates. Add test verifying bqskit_to_qiskit converts U1qGate to Qiskit r operation with matching parameters.
Import refactoring and integration test updates src/mqt/predictor/rl/predictorenv.py, tests/compilation/test_integration_further_SDKs.py	Remove bqskit_to_qiskit from bqskit.ext imports and import instead from mqt.predictor.rl.parsing. Update BQSKit synthesis test to use GatesInBasis(basis_gates=device.operation_names) instead of target=device, and remove "iqm" from unsupported device condition.
Changelog documentation CHANGELOG.md	Document BQSKit conversion support for IQM's native r gate under Unreleased/Changed section. Add PR reference link.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

bug

Poem

🐰 A rabbit hops through quantum gates,
Finding r where U1q waits,
From BQSKIT's home to Qiskit's door,
IQM's crystal shines once more! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding BQSKit conversion support for IQM's native 'r' gate, which directly addresses the issue.
Linked Issues check	✅ Passed	The PR successfully implements the core requirement from issue #547: mapping IQM's 'r' gate to BQSKit's U1qGate and handling the QASM naming issue through conversion logic.
Out of Scope Changes check	✅ Passed	All changes are directly related to the objective of supporting IQM's 'r' gate conversion: updates to gate mapping, QASM conversion logic, imports, tests, and documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The PR description addresses the core issue (#547), includes AI tool disclosure, and checks all required items, but lacks detailed motivation, context, and dependency information.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix-bqskit-r-gate

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/mqt/predictor/rl/parsing.py`:
- Around line 189-206: Add a Google-style docstring to bqskit_to_qiskit
explaining purpose, Args (circuit: BqskitCircuit), Returns (QuantumCircuit) and
any Raises, and then replace the fragile qasm.replace("U1q(", "r(") with a
robust transformation: obtain the QASM via OPENQASM2Language().encode(circuit),
then perform a targeted substitution that only replaces the U1q gate tokens (not
occurrences inside comments or identifiers) before calling qasm2.loads; update
references to the r gate constructor (r_gate/r_gate_constructor) and the
qasm2.loads custom_instructions usage accordingly so the new replacement
preserves token boundaries and avoids touching comments or gate names.

In `@tests/compilation/test_helper_rl.py`:
- Around line 64-73: The test test_bqskit_to_qiskit_converts_u1q_to_r_gate uses
exact equality to compare floating params; replace the direct equality checks on
qc.data[0].operation.params with an approximate comparison (e.g., use
pytest.approx or math.isclose for each element) so floating-point precision
won't cause flaky failures; update assertions referencing
qc.data[0].operation.params accordingly to compare each parameter with an
approximate tolerance.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 3f19cc14-6cfb-43bc-b9de-c41095c7af81

📥 Commits

Reviewing files that changed from the base of the PR and between 4797e4c and 09563bc.

📒 Files selected for processing (5)

• CHANGELOG.md
• src/mqt/predictor/rl/parsing.py
• src/mqt/predictor/rl/predictorenv.py
• tests/compilation/test_helper_rl.py
• tests/compilation/test_integration_further_SDKs.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[burgholzer]

LGTM 👍🏻
Would be nice to contribute an upstream fix for the conversion to BQSKit at some point.