feat: add implementation of halfnormal/mean

[shubham220420]

Progresses: #9416

Description

What is the purpose of this pull request?

This pull request:

• This PR adds the mean calculation for half-normal distribution function.

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

Any questions for reviewers of this pull request?

No.

Other

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No.

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

Yes I took help of AI for creating test cases under the fixtures/julia directory.

{{TODO: add disclosure if applicable}}

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/halfnormal/mean	$\color{green}166/166$ $\color{green}+0.00%$	$\color{green}8/8$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}166/166$ $\color{green}+0.00%$

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

[Planeshifter]

Flagged a bunch of issues.

[shubham220420]

thankyou for your reviews sir @Planeshifter , I have applied the requested changes.
Incase any lint error occurs I'll solve it, I've been working on this module from quite few hours now, and will look further tomorrow

[Planeshifter]

Thanks for your continued efforts! Should be ready to land soon.

[shubham220420]

I have applied the requested suggestion sir.

[Planeshifter]

A few last comments.

[Planeshifter]

Can we add "mean", "average", "avg", and "expected" in its keywords?

[Planeshifter]

Today, we prefer to use logEachMap instead of a manual loop with logEach. This simplifies the code:

Suggested change

	var sigma = uniform( 10, 0.1, 20.0, opts );
	var mu = new Float64Array( sigma.length );
	var i;
	for ( i = 0; i < sigma.length; i++ ) {
	mu[ i ] = mean( sigma[ i ] );
	}
	logEach( 'σ: %0.4f, E(X;σ): %0.4f', sigma, mu );
	var sigma = uniform( 10, 0.1, 20.0, opts );
	logEachMap( 'σ: %0.4f, E(X;σ): %0.4f', sigma, mean );

You'd also need to change the import from logEach to logEachMap and remove the Float64Array import.

[Planeshifter]

The error message here has a bug - it says σ: '+v but v is the computed mean, not the sigma value. Also, the message is missing the other debugging info like expected value and delta. Compare with normal/mean/test/test.js for the full format:

Suggested change

	t.ok( delta <= tol, 'within tolerance. σ: '+v );
	t.ok( delta <= tol, 'within tolerance. σ: '+sigma[i]+'. y: '+v+'. E: '+expected[i]+'. Δ: '+delta+'. tol: '+tol+'.' );