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feat: add fft/base/fftpack/rfftf#12002

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feat: add fft/base/fftpack/rfftf#12002
gunjjoshi wants to merge 4 commits into
stdlib-js:developfrom
gunjjoshi:rfftf-v2

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@gunjjoshi gunjjoshi commented May 8, 2026

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@gunjjoshi
feat: copy older implementation, add radf3
d2a3c99 
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gunjjoshi commented May 8, 2026

Opened this draft PR to discuss the implementation or ask questions, if any, and to track progress.

gunjjoshi
gunjjoshi commented May 8, 2026
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Comment thread lib/node_modules/@stdlib/fft/base/fftpack/rfftf/lib/radf3.js Outdated
Comment on lines +65 to +66
var TAUR = -0.5; // cos( 2π/3 )
var TAUI = 0.866025403784439; // sin( 2π/3 )
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@gunjjoshi gunjjoshi May 8, 2026

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I could have used math/base/special/sin and math/base/special/cos here, but instead I have hardcoded the exact constants used in pffft (Ref: https://github.com/marton78/pffft/blob/master/src/fftpack.c#L1308), to be as close as possible, to the actual results. Using math/base/special/sin and math/base/special/cos leads to slightly off results. If we are fine with that, I'll replace these hardcoded constants with stdlib functions.

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Not must difference for sin, but there is some difference for cos:

In [5]: var cos = require('@stdlib/math/base/special/cos')
Out[5]: [Function: cos]

In [6]: cos((2*pi)/3)
Out[6]: -0.4999999999999998

In [7]: var cos = require('@stdlib/math/base/special/cosf')
Out[7]: [Function: cosf]

In [8]: var pi = require('@stdlib/constants/float32/pi')
Out[8]: 3.1415927410125732

In [9]: cos((2*pi)/3)
Out[9]: -0.5000000596046448

In [10]:

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@kgryte kgryte May 11, 2026

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Yes, that seems reasonable to me. Better to use exact values. However, instead of cos and sin, for multiple of pi, one should use sinpi and cospi which will give you more accurate results.

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Thanks, I've now replaced the use of hardcoded constants by sinpi and cospi in 5f3fc63.

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@kgryte kgryte May 11, 2026

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Given that these are one time computations, it is probably preferable to just hard code the constants. My point about using sinpi and cospi is, if one were to perform computation, those should be used when what you are wanting to evaluate is a multiple of pi.

gunjjoshi added 3 commits May 11, 2026 00:53
@gunjjoshi
feat: add radf4
dd7b9bc 
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@gunjjoshi
feat: add radf5
b176292 
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@gunjjoshi
refactor: use sinpi and cospi
5f3fc63 
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