test: migrate stats/base/dists/gumbel/pdf to ULP-based assertions

[kgryte]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates the tests for stats/base/dists/gumbel/pdf from relative tolerance testing (delta <= EPS * abs( expected )) to ULP difference testing using @stdlib/assert/is-almost-same-value.

The following test files were updated:

• test/test.pdf.js
• test/test.factory.js
• test/test.native.js

In each file, the @stdlib/math/base/special/abs and @stdlib/constants/float64/eps imports were removed, the @stdlib/assert/is-almost-same-value import was added, and each if ( y === expected[i] ) { ... } else { ... } tolerance block was replaced with a single t.strictEqual( isAlmostSameValue( y, expected[ i ], N ), true, 'returns expected value' ); assertion. The pre-existing expected[i] !== null fixture guards were left intact.

ULP bounds

The ULP bound was tightened to the measured minimum over the full fixture set. For every fixture (positive_mean, negative_mean, large_variance) and for both the main and factory entry points, the maximum observed ULP difference is 1, so 1 ULP is used throughout:

Fixture	Fixture size	Measured minimum ULP
positive_mean	1000	1
negative_mean	1000	1
large_variance	1000	1

At 0 ULPs, 548 of the 3015 assertions in test/test.pdf.js and 548 of the 3018 assertions in test/test.factory.js fail, confirming that 1 is the tightest bound which still passes. The suite was run twice at the final bound with identical results.

test/test.native.js uses the same 1 ULP bound, matching the convention in previously migrated packages in this family (e.g., stats/base/dists/gumbel/cdf, stats/base/dists/pareto-type1/pdf). The C implementation in src/main.c is a direct transliteration of lib/main.js and delegates to the same exp routine.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Migrate math/base/special packages from relative tolerance testing to ULP difference testing (tracking issue) #11352

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Only test files are changed; no implementation, documentation, or fixture files were touched.

Note on verification: in the environment used to author this PR, make install-node-modules could not complete (es-object-atoms@^1.1.2 is not resolvable from the registry), so the native add-on could not be built and test/test.native.js was skipped locally, as it is whenever the add-on is absent. The JavaScript test files were run directly (twice, green) and linted with the repository's own etc/eslint/.eslintrc.tests.js configuration via the pre-commit hook, which reported no issues.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

This PR was authored by Claude Code, which studied previously merged migrations in this family, applied the mechanical conversion, and measured the minimum ULP bound over the full fixture set.

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/gumbel/pdf	$\\color{green}294/294$ $\\color{green}+100.00\\%$	$\\color{green}23/23$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}294/294$ $\\color{green}+100.00\\%$

The above coverage report was generated for the changes in this PR.