Tests : Created unit tests for _functions.py

[20086080]

Description

Add targeted unit tests for four utility functions in _functions.py that were previously flagged with # TEST comments.

These tests improve confidence in the mathematical utility layer of PteraSoftware by validating interpolation, centroid calculation, cosine spacing, and vector cross-product behavior using deterministic analytical cases.

Motivation

The flagged utility functions perform low-level numerical and geometric operations that are reused throughout the codebase. Although relatively compact, they contain mathematical logic where regressions or subtle numerical errors could propagate into higher-level aerodynamic calculations.

Adding focused unit tests improves maintainability and makes future refactoring safer by validating expected numerical behavior in isolation.

Relevant Issues

Closes Issue #146

Changes

New Tests

Added dedicated unit tests in test_functions.py for the following utility functions in _functions.py:

• cosspace
• numba_centroid_of_quadrilateral
• numba_1d_explicit_cross
• interp_between_points

Test Coverage

TestCosspace

• Verifies minimum and maximum endpoints are included by default.
• Verifies endpoint exclusion behavior when endpoint=False.
• Verifies correct number of output points.
• Verifies symmetry properties of cosine spacing.
• Verifies single-point edge-case behavior.

TestNumbaCentroidOfQuadrilateral

• Verifies centroid computation for a unit square.
• Verifies translation consistency.
• Verifies centroid calculation for nonplanar quadrilaterals.
• Verifies identical-point edge case behavior.

TestNumba1dExplicitCross

• Verifies analytically known cross-product results.
• Verifies parallel vectors produce zero cross products.
• Verifies agreement with numpy.cross.
• Verifies empty-input behavior.

TestInterpBetweenPoints

• Verifies linear interpolation between point pairs.
• Verifies interpolation across multiple point pairs.
• Verifies zero normalized spacing returns start points.
• Verifies unit normalized spacing returns end points.

Testing Methodology

Tests are deterministic and numerically validated using analytical expectations and invariant-based checks.

The testing approach focuses on:

• Known geometric and vector identities.
• Numerical correctness using numpy.testing.assert_allclose.
• Edge-case handling.
• Reproducible fixed-input validation rather than randomized testing.

Note - Tight tolerances (atol=1e-14) were used where appropriate to ensure numerical precision.

Validation

• Ran the full test suite locally to ensure no regressions.
• All existing tests pass.

Dependency Updates

None

Change Magnitude

Minor: Adds focused unit tests for existing utility functions without modifying runtime behavior.

Checklist (check each item when completed or not applicable)

• I am familiar with the current contribution guidelines.
• PR description links all relevant issues and follows this template.
• My branch is based on main and is up to date with the upstream main branch.
• All calculations use S.I. units.
• Code is formatted with black (line length = 88).
• Code is well documented with block comments where appropriate.
• Any external code, algorithms, or equations used have been cited in comments or docstrings.
• All new modules, classes, functions, and methods have docstrings in reStructuredText format, and are formatted using docformatter (--in-place --black). See the style guide for type hints and docstrings for more details.
• All new classes, functions, and methods in the pterasoftware package use type hints. See the style guide for type hints and docstrings for more details.
• If any major functionality was added or significantly changed, I have added or updated tests in the tests package.
• Code locally passes all tests in the tests package.
• This PR passes the ReadTheDocs build check (this runs automatically with the other workflows).
• This PR passes the black, codespell, and isort GitHub actions.
• This PR passes the mypy GitHub action.
• This PR passes all the tests GitHub actions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.69%. Comparing base (81c73b3) to head (7a10118).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   91.69%   91.69%           
=======================================
  Files          35       35           
  Lines        6828     6828           
=======================================
  Hits         6261     6261           
  Misses        567      567           

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