Make Nyquist effects more memory efficient

[petersampsonaudacity]

Describe the bug
Many Nyquist plug-ins will fail or may be sluggish if the track selection is too long, or could cause Audacity to crash. This is because the maximum selection length is limited by the amount of available ram, up to a maximum of 2 GB. '''Workaround:''' Unless you know that a particular Nyquist plug-in is safe to use with long selections, do not select more than about 1 hour of audio (44100 Hz sample rate) at a time. For higher sample rates the limit is proportionally less. If necessary, apply the effect to sections of 1 hour or less.

Originally logged in 2015 on Bugzilla as 1285
https://bugzilla.audacityteam.org/show_bug.cgi?id=1285

To Reproduce
Steps to reproduce the behavior:

1. Select 2 hrs of Audio
2. Apply a Nyquist effect.
3. Observe: Audacity gets very slow and uses lots of RAM.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional information (please complete the following information):

• OS: all OS
• Version: all Audacity

Additional context
@SteveDaulton wrote on the Bugzilla thread

Many Nyquist effects load the entire selection into ram, which is limited to a maximum of 2 GB. This is the old "can't normalize big files" problem that has been discussed many times over the years.

The only reason that I don't consider it to be P1 is that there are no reports of this bug affecting plug-ins that are shipped as standard (but it does affect many plug-ins that are hosted on the Audacity wiki).

[petersampsonaudacity]

@SteveDaulton later wrote in the Bugzilla thread

A side effect of this issue is that after processing a long selection, Audacity may appear to be frozen while the large amount of memory is released.

Example:
To find the value of the final sample of a 1 hour mono selection:
(sref track (- 1 (/ 1.0 len)))

On my machine, processing takes about 36 seconds, then a message box pops up with the value of the final sample.
On clicking "OK", Audacity appears to be frozen for about 7 seconds (about 600 MB of ram is being released).
(Some plug-ins "in the wild" may take over 40 seconds to release ram.)

Ideally, Nyquist should be able to request the audio data that it needs (in this case, the final sample of the selection), which would make finding the value of the final sample virtually instant with negligible ram requirements.

[werame]

FYI, Nyquist.cpp makes a copy even of the input buffers it passes to nyx.c. #2396 (comment)

More precisely at

audacity/src/effects/nyquist/Nyquist.cpp

Line 2465 in 672f192

// C++20

And by "even" I mean despite the fact that the output buffers are entirely separate. But the present design permits clean undo recovery in the case where nyx.c were to corrupt its input buffers somehow.

There's also this compile-time hardcoded limitation on how much Nyquist is allowed to process

audacity/src/effects/nyquist/Nyquist.cpp

Line 884 in b58d771

"Selection too long for Nyquist code.\nMaximum allowed selection is %ld samples\n(about %.1f hours at 44100 Hz sample rate).")

I'm a bit surprised you didn't run into that. Ok, you actually did and filed that separately #1607

But one indeed usually runs into memory limits with Nyquist before running into the 13.5 hrs limit. Mind you, at higher sampling rate, one runs into the limit sooner. (Aside: still it's way better than SuperCollider's 6 minute limit in its BufRd before it starts making popping noises,
due to fp phasor imprecision 🙄 )