🐛 Fix illegal number of qubits input

[flowerthrower]

Description

This PR fixes (#811) illegal number of qubits in qpeexact and harmonizes docstrings that specify num_qubits.

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.
• I have added migration instructions to the upgrade guide (if needed).
• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved input validation across quantum benchmarks with explicit error messages replacing runtime assertions.
  • Invalid qubit counts now raise clear, descriptive errors before circuit construction.

• Documentation

  • Updated parameter documentation to specify exact qubit count requirements for each benchmark (e.g., minimum values, divisibility constraints).
  • Enhanced clarity on input constraints for better usability.

Walkthrough

Added explicit input validation and clarified docstrings across multiple benchmark circuit creators; several assertions were replaced with ValueError raises to prevent invalid circuit construction. Tests were updated to cover the new invalid-input behavior and one dedicated AE test was removed.

Changes

Cohort / File(s)	Summary
Validation + Early Errors src/mqt/bench/benchmarks/ae.py, src/mqt/bench/benchmarks/bmw_quark_copula.py, src/mqt/bench/benchmarks/hhl.py, src/mqt/bench/benchmarks/qpeexact.py	Replaced assertions with explicit ValueError raises or added input checks enforcing minimum qubits/divisibility before circuit construction (AE: ≥2, BMW quark copula: divisible by 2, HHL: ≥3, QPE exact: ≥2). Docstrings updated to reflect requirements.
Docstring clarifications (no runtime changes) src/mqt/bench/benchmarks/bv.py, src/mqt/bench/benchmarks/cdkm_ripple_carry_adder.py, src/mqt/bench/benchmarks/draper_qft_adder.py, src/mqt/bench/benchmarks/full_adder.py, src/mqt/bench/benchmarks/half_adder.py, src/mqt/bench/benchmarks/hrs_cumulative_multiplier.py, src/mqt/bench/benchmarks/modular_adder.py, src/mqt/bench/benchmarks/multiplier.py, src/mqt/bench/benchmarks/rg_qft_multiplier.py, src/mqt/bench/benchmarks/shor.py	Reworded parameter docs to clarify qubit count constraints (minimums, parity, divisibility, and length requirements). No changes to logic, signatures, or error handling.
Tests tests/test_bench.py	Expanded invalid-input cases in test_wrong_circuit_size for hhl, qpeexact, bmw_quark_copula, and ae with updated expected error messages. Removed test_create_ae_circuit_with_invalid_qubit_number.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐇 I nibble at docs and tidy the gate,
I hop in to guard an invalid state,
With ValueErrors firm and docstrings bright,
Circuits now sleep safe through day and night. 🥕✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fix illegal number of qubits input' directly describes the main purpose of the PR: fixing invalid qubit count validation across multiple benchmark files.
Description check	✅ Passed	The PR description includes a summary of changes (fixes #811, harmonizes docstrings), references the specific issue, and includes all checklist items marked as completed.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix-qpe-min-qubits

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[coderabbitai]

Actionable comments posted: 4

🤖 Fix all issues with AI agents

In `@src/mqt/bench/benchmarks/full_adder.py`:
- Around line 24-25: The docstring for the function that documents the parameter
num_qubits is inconsistent with the runtime guard (the guard checks `num_qubits
< 4` and allows `num_qubits == 4`), so update the docstring sentence that
currently says "bigger than 4" to "at least 4" or "≥ 4" to match the validation
for the parameter `num_qubits`; locate the docstring referencing `num_qubits` in
the full adder function (the parameter named `num_qubits`) and change the
wording accordingly so it aligns with the existing guard `num_qubits < 4`.

In `@src/mqt/bench/benchmarks/half_adder.py`:
- Around line 24-25: The docstring for the function that builds the half-adder
circuit describes num_qubits as "bigger than 3" but the code validation allows
num_qubits == 3 (guard checks num_qubits < 3 and error text says "≥ 3"); update
the docstring wording for the num_qubits parameter to "at least 3" (or "≥ 3") to
match the validation and error message for the num_qubits parameter so the
documentation and guard (num_qubits) are consistent.

In `@src/mqt/bench/benchmarks/hhl.py`:
- Line 28: Update the docstring for the parameter num_qubits in hhl.py to state
it is the total number of qubits (including 1 system qubit, num_qpe_qubits =
num_qubits - 2 phase-estimation qubits, and 1 ancilla), not just the
phase-estimation register size; clarify the minimum requirement (at least 3) and
the relationship with num_qpe_qubits so callers understand how num_qubits maps
to phase-estimation precision.

In `@src/mqt/bench/benchmarks/modular_adder.py`:
- Around line 24-25: The docstring for the parameter num_qubits incorrectly says
"bigger than 2" while the validation allows num_qubits == 2; update the
docstring in modular_adder (the num_qubits parameter description) to say "at
least 2" or "≥ 2" to match the guard that checks num_qubits < 2 and the error
message.

[Copilot]

Pull request overview

This PR fixes input validation issues across multiple quantum benchmark circuits by replacing assertions with proper ValueError exceptions and adding comprehensive test coverage.

Changes:

• Replaced assert statements with proper ValueError exceptions in hhl.py and bmw_quark_copula.py
• Added input validation to qpeexact.py (previously had no validation)
• Consolidated validation tests into the existing parameterized test_wrong_circuit_size test and removed redundant standalone test
• Updated docstrings across 13 benchmark files to clarify input requirements

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_bench.py	Added test cases for hhl, qpeexact, bmw_quark_copula, and ae validation; removed redundant standalone ae test
src/mqt/bench/benchmarks/hhl.py	Replaced assertion with ValueError for num_qubits < 3 validation; updated docstring
src/mqt/bench/benchmarks/qpeexact.py	Added new validation for num_qubits <= 1; updated docstring
src/mqt/bench/benchmarks/bmw_quark_copula.py	Replaced assertion with ValueError for odd num_qubits validation; updated docstring
src/mqt/bench/benchmarks/ae.py	Updated docstring to clarify minimum qubit requirement
src/mqt/bench/benchmarks/shor.py	Minor docstring formatting improvement
src/mqt/bench/benchmarks/rg_qft_multiplier.py	Enhanced docstring with specific validation requirements
src/mqt/bench/benchmarks/multiplier.py	Enhanced docstring with specific validation requirements
src/mqt/bench/benchmarks/modular_adder.py	Minor docstring formatting improvement
src/mqt/bench/benchmarks/hrs_cumulative_multiplier.py	Enhanced docstring with specific validation requirements
src/mqt/bench/benchmarks/half_adder.py	Minor docstring formatting improvement
src/mqt/bench/benchmarks/full_adder.py	Minor docstring formatting improvement
src/mqt/bench/benchmarks/draper_qft_adder.py	Enhanced docstring with specific validation requirements
src/mqt/bench/benchmarks/cdkm_ripple_carry_adder.py	Enhanced docstring with specific validation requirements
src/mqt/bench/benchmarks/bv.py	Enhanced docstring to clarify hidden_string length requirement

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[burgholzer]

Nice. Great to have more consistency across the documentation and better error messages.
Looking forward to getting a release out with these changes. Probably after the dynamic circuits have been merged.

[burgholzer]

Ah, a changelog entry could have been nice.. maybe if you remember in your next PR, you could add one for this PR as well 🙂