fix: detect reference cycles

[dzhalaevd]

Summary

Detect reference cycles during structural validation and report the complete cycle path through CyclicReferenceError

Related issue

Closes #20

Summary by CodeRabbit

• Bug Fixes

  • Added structural detection for direct, indirect, and cross-sheet reference cycles.
  • Validation now reports complete semantic cycle paths and affected columns.
  • Duplicate cycle reports are suppressed, and issues are detected before data processing or compilation.

• Documentation

  • Documented CyclicReferenceError and clarified its distinction from runtime cycle errors.

• Public API

  • Exposed CyclicReferenceError through the package’s public error interfaces.

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Reviewing files that changed from the base of the PR and between 09f66b6 and 3201803.

📒 Files selected for processing (2)

• src/caxton/_internal/validation/cycles.py
• tests/test_validation_cycles.py

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📝 Walkthrough

Walkthrough

The change adds structural cycle detection for formula and worksheet references. It introduces CyclicReferenceError, reports complete semantic cycle paths, exports the error publicly, and adds coverage for direct, indirect, cross-sheet, duplicate, and large cycles.

Changes

Reference cycle validation

Layer / File(s)	Summary
Validation error contract and public documentation src/caxton/core/errors/validation.py, src/caxton/core/errors/__init__.py, src/caxton/errors.py, src/caxton/__init__.py, docs/guides/errors.md, docs/reference/errors.md, ARCHITECTURE.md, changelog.d/20.bugfix.md	CyclicReferenceError records the affected column and complete cycle path. Public exports and documentation distinguish structural validation from runtime cycle errors.
Shared dependency cycle detector src/caxton/_internal/validation/cycles.py, src/caxton/_internal/validation/expressions.py, src/caxton/_internal/validation/formulas.py	An iterative detector deduplicates repeated dependencies and reports dependency cycles. Formula and expression validation pass dependency mappings, source paths, and column labels to the shared helper.
Cycle scenarios and integration coverage tests/test_errors.py, tests/test_validation_cycles.py, tests/test_formulas.py, tests/test_spreadsheet_compiler.py	Tests cover error context, self-loops, duplicate dependencies, independent and large cycles, formula chains, cross-worksheet references, and validation before compilation.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 32018

The change is merge-ready after normal checks and review; no actionable merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant StructuralValidation
  participant report_reference_cycles
  participant Notification
  StructuralValidation->>report_reference_cycles: pass formula and reference dependencies
  report_reference_cycles->>Notification: emit CyclicReferenceError with semantic cycle path
  Notification-->>StructuralValidation: return aggregated validation issues

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 11 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the primary change: detecting reference cycles.
Description check	✅ Passed	The description includes the required Summary and Related issue sections. It explains the behavior change and links issue #20 with the required closing syntax.
Linked Issues check	✅ Passed	The implementation satisfies issue #20 by detecting direct, indirect, duplicate, and cross-sheet reference cycles during structural validation, reporting complete cycle paths through CyclicReferenceEr…
Out of Scope Changes check	✅ Passed	The changes remain within scope. Documentation, exports, changelog updates, shared validation logic, and tests directly support the reference-cycle detection feature.

Full details: Linked Issues check

Explanation

The implementation satisfies issue #20 by detecting direct, indirect, duplicate, and cross-sheet reference cycles during structural validation, reporting complete cycle paths through CyclicReferenceError, and adding focused tests.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.08%. Comparing base (f955d08) to head (3201803).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   87.01%   87.08%   +0.07%     
==========================================
  Files         125      126       +1     
  Lines        6429     6449      +20     
  Branches      780      779       -1     
==========================================
+ Hits         5594     5616      +22     
+ Misses        573      572       -1     
+ Partials      262      261       -1     

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Actionable comments posted: 1

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Inline comments:
In `@src/caxton/_internal/validation/cycles.py`:
- Around line 52-71: Update the dependency traversal around validate_columns to
suppress repeated dependencies within each pending frame, so duplicate
references to an active node produce only one cycle report. Preserve traversal
of distinct dependencies and add a regression test covering repeated
ref("value") self-references.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 97f4407c-e6c3-4f8a-a842-fe0d5a81122c

📥 Commits

Reviewing files that changed from the base of the PR and between f955d08 and 09f66b6.

📒 Files selected for processing (15)

• ARCHITECTURE.md
• changelog.d/20.bugfix.md
• docs/guides/errors.md
• docs/reference/errors.md
• src/caxton/__init__.py
• src/caxton/_internal/validation/cycles.py
• src/caxton/_internal/validation/expressions.py
• src/caxton/_internal/validation/formulas.py
• src/caxton/core/errors/__init__.py
• src/caxton/core/errors/validation.py
• src/caxton/errors.py
• tests/test_errors.py
• tests/test_formulas.py
• tests/test_spreadsheet_compiler.py
• tests/test_validation_cycles.py

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.