R23-T058: Refactor resistor_color_code.py to share colour lookup and validation helpers - #68

Description

Refactors electronics/resistor_color_code.py without changing behaviour: the repeated "look the colour up in this band's table or raise ValueError" code in four functions is replaced by one shared helper, and the nested validation / resistance code is flattened. An equivalence test compares the original and refactored code to prove nothing changed.

Related Issue

Fixes: #58

Type of Change

• Bug Fix
• New Feature
• UI/UX Improvement
• Performance Improvement
• Tests
• Documentation
• Refactoring / Maintenance
• CI/CD

Changes Made

• Added get_color_value(color, color_values, band_name) (with 2 doctests) and used it in get_significant_digits, get_multiplier, get_tolerance, get_temperature_coeffecient. Error messages are unchanged.
• check_validity: guard clauses instead of nested if/else (nesting depth 4 -> 2).
• calculate_resistance: guard clause, removed None placeholders, fixed local typo resisitance.
• Original 24 doctests unchanged (26 total); all 10 raise statements kept.
• Added work/R23-T058/REFACTOR.md, equivalence_test.py, and before/after complexity and coverage output.

Screenshots / Recordings

Not applicable (no UI changes).

Testing

• python -m pytest --doctest-modules electronics/resistor_color_code.py -> 26 passed
• python -m pytest work/R23-T058/equivalence_test.py -> 60 passed (original vs refactored: same results and same exception type and message on thousands of inputs and every raise path)
• ruff check electronics/resistor_color_code.py -> all checks passed
• McCabe complexity per function: calculate_resistance 5 -> 3, get_significant_digits 3 -> 1, get_multiplier/get_tolerance/get_temperature_coeffecient 2 -> 1, check_validity 5 -> 5 (flatter), total 22 -> 17

Checklist

• I have tested my changes locally.
• I have added or updated tests, if needed.
• I have updated documentation, if needed.
• I have followed the project's coding guidelines.
• I have run the relevant tests and checks.
• I have addressed previous review feedback, if applicable.

Additional Notes

check_validity stays at complexity 5 (its three error conditions and the colour loop are inherent), so the worst score is unchanged; calculate_resistance, the longest function, drops from 5 to 3. The raise ValueError("Input is invalid") at the end of calculate_resistance is unreachable (check_validity raises first); it was kept so the 10 raise paths stay identical. Details in work/R23-T058/REFACTOR.md.