feat: add `stats/base/dists/halfnormal/pdf` #9694

Om-A-osc · 2026-01-11T13:37:34Z

Progresses #9416.

Description

This pull request:

Adds the probability density function (PDF) implementation for the half-normal distribution.
Includes JavaScript and native implementations, along with tests, benchmarks, examples, and Julia fixtures for validation.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

[RFC]: Tracking issue for complete implementation of @stdlib/stats/base/dists/halfnormal package #9416

Questions

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Other

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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?

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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.

I used AI assistance to verify the mathematical formula for the half-normal PDF and to confirm expected numerical values.

@stdlib-js/reviewers

--- type: pre_commit_static_analysis_report description: Results of running static analysis checks when committing changes. report: - task: lint_filenames status: passed - task: lint_editorconfig status: passed - task: lint_markdown status: na - task: lint_package_json status: na - task: lint_repl_help status: na - task: lint_javascript_src status: passed - task: lint_javascript_cli status: na - task: lint_javascript_examples status: passed - task: lint_javascript_tests status: passed - task: lint_javascript_benchmarks status: passed - task: lint_python status: na - task: lint_r status: na - task: lint_c_src status: missing_dependencies - task: lint_c_examples status: missing_dependencies - task: lint_c_benchmarks status: missing_dependencies - task: lint_c_tests_fixtures status: na - task: lint_shell status: na - task: lint_typescript_declarations status: passed - task: lint_typescript_tests status: passed - task: lint_license_headers status: passed ---

stdlib-bot · 2026-01-11T13:37:45Z

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stdlib-bot · 2026-01-11T15:51:53Z

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/halfnormal/pdf	$\color{green}269/269$ $\color{green}+0.00%$	$\color{green}22/22$ $\color{green}+0.00%$	$\color{green}4/4$ $\color{green}+0.00%$	$\color{green}269/269$ $\color{green}+0.00%$

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

Planeshifter

Thanks for opening your first PR!

Please address the review comments and workflow failures.

Planeshifter · 2026-01-11T15:13:49Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/docs/repl.txt

+    > y = {{alias}}( 2.0, 1.0 )
+    ~0.108
+    > y = {{alias}}( 0.5, 1.0 )
+    ~0.352


The expected value here is incorrect. pdf(0.5, 1.0) returns approximately 0.704, not 0.352.

Suggested change

~0.352

~0.704

Fixed in latest commit.

Planeshifter · 2026-01-11T15:13:50Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/docs/repl.txt

+    > var y = myPDF( 0.0 )
+    ~0.399
+    > y = myPDF( 2.0 )
+    ~0.192


The expected value here is incorrect. myPDF(2.0) with sigma=2.0 returns approximately 0.242, not 0.192.

Suggested change

~0.192

~0.242

Planeshifter · 2026-01-11T15:13:51Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.factory.js

+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law of or agreed to in writing, software


There's a typo in the license header - "law of or agreed" should be "law or agreed". Curious how that happened.

Suggested change

* Unless required by applicable law of or agreed to in writing, software

* Unless required by applicable law or agreed to in writing, software

I’ve pushed two new commits addressing the review feedback. The first commit fixes the incorrect numeric values in the half-normal PDF documentation examples, and the second commit addresses the remaining style issues including the license header typo, indentation, and formatting as requested.

Kindly review when you have time, and please let me know if anything else needs adjustment. I’m happy to iterate further.

Planeshifter · 2026-01-11T15:13:53Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/src/pdf.c

+	if ( stdlib_base_is_nan( x ) || stdlib_base_is_nan( sigma ) || sigma <= 0.0 ) {
+		return 0.0/0.0; // NaN
+	}
+	if( x < 0.0 ) {


Missing space after if - should be if ( not if(

Suggested change

if( x < 0.0 ) {

if ( x < 0.0 ) {

Planeshifter · 2026-01-11T15:13:54Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/src/pdf.c

+	if( x < 0.0 ) {
+		return 0.0;
+	}
+	const double C = x / sigma;


In C89/C90, all variable declarations must appear at the beginning of a block, before any executable statements. This declaration should be moved to the top of the function body (right after the opening brace on line 36), like:

double stdlib_base_dists_halfnormal_pdf( const double x, const double sigma ) { double C; if ( stdlib_base_is_nan( x ) || ...

Planeshifter · 2026-01-11T15:14:08Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.factory.js

+    y = pdf( -1.0 );
+	t.strictEqual( isnan(y), true, 'returns expected value' );


These lines use spaces for indentation instead of tabs. Per the stdlib style guide, JavaScript files should use TAB indentation. Also, isnan(y) should have spaces inside the parentheses: isnan( y ).

Suggested change

y = pdf( -1.0 );

t.strictEqual( isnan(y), true, 'returns expected value' );

y = pdf( -1.0 );

t.strictEqual( isnan( y ), true, 'returns expected value' );

Planeshifter · 2026-01-11T15:14:09Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.factory.js

+	t.strictEqual( isnan(y), true, 'returns expected value' );
+
+	y = pdf( PINF );
+	t.strictEqual( isnan(y), true, 'returns expected value' );


Missing spaces inside parentheses - should be isnan( y ) for consistency with the rest of the file.

Suggested change

t.strictEqual( isnan(y), true, 'returns expected value' );

t.strictEqual( isnan( y ), true, 'returns expected value' );

Planeshifter · 2026-01-11T15:14:10Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.factory.js

+	t.strictEqual( isnan(y), true, 'returns expected value' );
+
+	y = pdf( NINF );
+	t.strictEqual( isnan(y), true, 'returns expected value' );


Missing spaces inside parentheses - should be isnan( y ) for consistency.

Suggested change

t.strictEqual( isnan(y), true, 'returns expected value' );

t.strictEqual( isnan( y ), true, 'returns expected value' );

Planeshifter · 2026-01-11T15:14:11Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.pdf.js

+    y = pdf( -1.0, 0.0 );
+    t.strictEqual( isnan( y ), true, 'returns expected value' );


These lines use spaces for indentation instead of tabs. Per the stdlib style guide, JavaScript files should use TAB indentation.

Suggested change

y = pdf( -1.0, 0.0 );

t.strictEqual( isnan( y ), true, 'returns expected value' );

y = pdf( -1.0, 0.0 );

t.strictEqual( isnan( y ), true, 'returns expected value' );

--- type: pre_commit_static_analysis_report description: Results of running static analysis checks when committing changes. report: - task: lint_filenames status: passed - task: lint_editorconfig status: passed - task: lint_markdown status: na - task: lint_package_json status: na - task: lint_repl_help status: passed - task: lint_javascript_src status: na - task: lint_javascript_cli status: na - task: lint_javascript_examples status: na - task: lint_javascript_tests status: na - task: lint_javascript_benchmarks status: na - task: lint_python status: na - task: lint_r status: na - task: lint_c_src status: na - task: lint_c_examples status: na - task: lint_c_benchmarks status: na - task: lint_c_tests_fixtures status: na - task: lint_shell status: na - task: lint_typescript_declarations status: passed - task: lint_typescript_tests status: na - task: lint_license_headers status: passed ---

--- type: pre_commit_static_analysis_report description: Results of running static analysis checks when committing changes. report: - task: lint_filenames status: passed - task: lint_editorconfig status: passed - task: lint_markdown status: na - task: lint_package_json status: na - task: lint_repl_help status: na - task: lint_javascript_src status: na - task: lint_javascript_cli status: na - task: lint_javascript_examples status: na - task: lint_javascript_tests status: passed - task: lint_javascript_benchmarks status: na - task: lint_python status: na - task: lint_r status: na - task: lint_c_src status: missing_dependencies - task: lint_c_examples status: na - task: lint_c_benchmarks status: na - task: lint_c_tests_fixtures status: na - task: lint_shell status: na - task: lint_typescript_declarations status: passed - task: lint_typescript_tests status: na - task: lint_license_headers status: passed ---

Planeshifter

This PR should be getting close; left one more round of comments.

Planeshifter · 2026-01-11T19:18:17Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/README.md

+
+@license Apache-2.0
+
+Copyright (c) 2018 The Stdlib Authors.


This is a new file, so the copyright year should be 2026 rather than 2018.

Suggested change

Copyright (c) 2018 The Stdlib Authors.

Copyright (c) 2026 The Stdlib Authors.

Planeshifter · 2026-01-11T19:18:18Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/README.md

+
+[halfnormal-distribution]: https://en.wikipedia.org/wiki/Half-normal_distribution
+
+[degenerate-distribution]: https://en.wikipedia.org/wiki/Degenerate_distribution


This link reference isn't used anywhere in the document - looks like copy-paste residue that can be removed.

Suggested change

[degenerate-distribution]: https://en.wikipedia.org/wiki/Degenerate_distribution

Planeshifter · 2026-01-11T19:18:19Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/README.md

+var y = pdf( -1.0, 1.0 );
+// returns 0.0
+```
+


The README is missing the #### pdf.factory( sigma ) documentation section that should appear here to document the factory method. See other similar distribution packages like levy/pdf/README.md for the pattern - it should include a brief description and example usage of the factory function.

Planeshifter · 2026-01-11T19:18:20Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.factory.js

+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2018 The Stdlib Authors.


New file - copyright year should be 2026.

Suggested change

* Copyright (c) 2018 The Stdlib Authors.

* Copyright (c) 2026 The Stdlib Authors.

Planeshifter · 2026-01-11T19:18:22Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/test/test.pdf.js

+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2018 The Stdlib Authors.


New file - copyright year should be 2026.

Suggested change

* Copyright (c) 2018 The Stdlib Authors.

* Copyright (c) 2026 The Stdlib Authors.

Planeshifter · 2026-01-11T19:18:23Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/docs/types/index.d.ts

+* var y = myPDF( 0.5 );
+* // returns ~0.387
+*/
+declare const pdf: PDF;


For consistency with other stdlib PDF packages, this should use declare var instead of declare const.

Suggested change

declare const pdf: PDF;

declare var pdf: PDF;

Planeshifter · 2026-01-11T19:18:24Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/src/pdf.c

+* @return         evaluated PDF
+*
+* @example
+* double y = stdlib_stats_base_dists_halfnormal_pdf( 2.0, 1.0 );


The function name in the example doesn't match the actual function name. It says stdlib_stats_base_dists_halfnormal_pdf but should be stdlib_base_dists_halfnormal_pdf.

Suggested change

* double y = stdlib_stats_base_dists_halfnormal_pdf( 2.0, 1.0 );

* double y = stdlib_base_dists_halfnormal_pdf( 2.0, 1.0 );

Planeshifter · 2026-01-11T19:18:25Z

lib/node_modules/@stdlib/stats/base/dists/halfnormal/pdf/lib/factory.js

+/**
+* Returns a function for evaluating the probability density function (PDF) for a half-normal distribution.
+*
+* @param {NonNegativeNumber} sigma - scale parameter


Since sigma <= 0.0 returns NaN (not just sigma < 0), this should be {PositiveNumber} rather than {NonNegativeNumber}.

Suggested change

* @param {NonNegativeNumber} sigma - scale parameter

* @param {PositiveNumber} sigma - scale parameter

I’ve pushed two new commits to address the latest review feedback.
The first commit updates the documentation in README.md (including the copyright year and factory documentation), and the second commit fixes the remaining issues across the source and type files.

Kindly take another look when you have time, and please let me know if anything else needs to be adjusted.

--- type: pre_commit_static_analysis_report description: Results of running static analysis checks when committing changes. report: - task: lint_filenames status: passed - task: lint_editorconfig status: passed - task: lint_markdown status: passed - task: lint_package_json status: na - task: lint_repl_help status: na - task: lint_javascript_src status: na - task: lint_javascript_cli status: na - task: lint_javascript_examples status: na - task: lint_javascript_tests status: na - task: lint_javascript_benchmarks status: na - task: lint_python status: na - task: lint_r status: na - task: lint_c_src status: na - task: lint_c_examples status: na - task: lint_c_benchmarks status: na - task: lint_c_tests_fixtures status: na - task: lint_shell status: na - task: lint_typescript_declarations status: passed - task: lint_typescript_tests status: na - task: lint_license_headers status: passed ---

--- type: pre_commit_static_analysis_report description: Results of running static analysis checks when committing changes. report: - task: lint_filenames status: passed - task: lint_editorconfig status: passed - task: lint_markdown status: na - task: lint_package_json status: na - task: lint_repl_help status: na - task: lint_javascript_src status: passed - task: lint_javascript_cli status: na - task: lint_javascript_examples status: na - task: lint_javascript_tests status: passed - task: lint_javascript_benchmarks status: na - task: lint_python status: na - task: lint_r status: na - task: lint_c_src status: missing_dependencies - task: lint_c_examples status: na - task: lint_c_benchmarks status: na - task: lint_c_tests_fixtures status: na - task: lint_shell status: na - task: lint_typescript_declarations status: passed - task: lint_typescript_tests status: passed - task: lint_license_headers status: passed ---

Om-A-osc added 5 commits January 11, 2026 15:41

halfnormal/pdf init

2c80926

examples working

2d81b5e

all test + benchmark clearing -native tests

3202b45

types added

0c58e9c

stdlib-bot added the Statistics Issue or pull request related to statistical functionality. label Jan 11, 2026

stdlib-bot added First-time Contributor A pull request from a contributor who has never previously committed to the project repository. Needs Review A pull request which needs code review. labels Jan 11, 2026

Planeshifter requested changes Jan 11, 2026

View reviewed changes

Om-A-osc added 2 commits January 11, 2026 23:41

Planeshifter changed the title ~~feat: add implementation of halfnormal/pdf~~ feat: add stats/base/dists/halfnormal/pdf Jan 11, 2026

Planeshifter requested changes Jan 11, 2026

View reviewed changes

Om-A-osc added 2 commits January 12, 2026 09:41

	* Unless required by applicable law of or agreed to in writing, software
	* Unless required by applicable law or agreed to in writing, software

		y = pdf( -1.0 );
		t.strictEqual( isnan(y), true, 'returns expected value' );

	t.strictEqual( isnan(y), true, 'returns expected value' );
	t.strictEqual( isnan( y ), true, 'returns expected value' );

		y = pdf( -1.0, 0.0 );
		t.strictEqual( isnan( y ), true, 'returns expected value' );

	Copyright (c) 2018 The Stdlib Authors.
	Copyright (c) 2026 The Stdlib Authors.


		[halfnormal-distribution]: https://en.wikipedia.org/wiki/Half-normal_distribution

		[degenerate-distribution]: https://en.wikipedia.org/wiki/Degenerate_distribution

	* Copyright (c) 2018 The Stdlib Authors.
	* Copyright (c) 2026 The Stdlib Authors.

	* double y = stdlib_stats_base_dists_halfnormal_pdf( 2.0, 1.0 );
	* double y = stdlib_base_dists_halfnormal_pdf( 2.0, 1.0 );

	* @param {NonNegativeNumber} sigma - scale parameter
	* @param {PositiveNumber} sigma - scale parameter

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feat: add stats/base/dists/halfnormal/pdf #9694

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feat: add stats/base/dists/halfnormal/pdf #9694

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Om-A-osc commented Jan 11, 2026

Description

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stdlib-bot commented Jan 11, 2026

Getting Started

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Running Tests Locally

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feat: add `stats/base/dists/halfnormal/pdf` #9694

feat: add `stats/base/dists/halfnormal/pdf` #9694

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