feat: add fft/base/fftpack/rffti

[gunjjoshi]

Resolves None.

Description

What is the purpose of this pull request?

This pull request:

• adds fft/base/fftpack/rffti, which is a part of fftpack.
• is an extension of feat: add initial implementation of fft/base/fftpack #4121.

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Used AI to understand how we generate C fixtures using FFTPACK.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
fft/base/fftpack/rffti	$\color{green}406/406$ $\color{green}+0.00%$	$\color{green}12/12$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}406/406$ $\color{green}+0.00%$

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

[kgryte]

Suggested change

	declare function rffti( N: number, workspace: Collection<number>, strideW: number, offsetW: number ): void;
	declare function rffti<T extends Collection<number>>( N: number, workspace: T, strideW: number, offsetW: number ): T;

I suggest going ahead and just returning the workspace array. Will require some minor edits here and elsewhere in this PR.

[gunjjoshi]

Thanks for making the updates @kgryte. Should we also update all the examples to show the usage of this as a returned workspace array? For example:

Changing this:

rffti( N, workspace, 1, 0 );

To this:

var out = rffti( N, workspace, 1, 0 );

[kgryte]

Yes, that works. And then you can also add

var bool = ( out === workspace);
// returns true

which is something we do elsewhere in similar situations.

[kgryte]

Not fully understanding why this file differs from what we did in https://github.com/gunjjoshi/stdlib/blob/20edeac7d326443708e922a8aa99dc6a93baa3ef/lib/node_modules/%40stdlib/fft/base/fftpack/rffti/lib/rffti1.js. What is the reason for the various discrepancies?

[kgryte]

I would revisit this file. IMO, the changes in https://github.com/gunjjoshi/stdlib/blob/20edeac7d326443708e922a8aa99dc6a93baa3ef/lib/node_modules/%40stdlib/fft/base/fftpack/rffti/lib/rffti1.js are arguably better, so unless there is some justification for the current implementation, I'd say go ahead and align with the current status in the draft FFT PR.

[kgryte]

Should add a test confirming that the workspace array is returned.

[kgryte]

Other than the implementation file, this PR is looking good.