Option to skip scanning opus files with existing headers output gain

[Tokariew]

Currently if someone use opus in mode a or t opus file will be scanned in each subsequent run after the first even when used with -S output. Can we get some option to skip opus files with existing gain headers?
Tested with:

➜ rsgain --version
rsgain 3.5.1 - using:
   libebur128     1.2.6
   libavformat    60.16.100
   libavcodec     60.31.102
   libavutil      58.29.100
   libswresample  4.12.100

Built with:
   TagLib         1.12

Compiler:         GCC 14.1
Build Date:       2024-06-08

[complexlogic]

That's an interesting idea that I hadn't considered before. I suppose the downside of skipping files with an existing header gain is that it could have a 'false positive'. The header gain set by another program may not actually correspond to ReplayGain; it could have been set for some other purpose.

Then again, I would expect new users to scan their entire library when using rsgain for the first time. So, the 'false positive' issue may not actually be a significant problem at all.

I will have to give this some further thought.

[Tokariew]

I was using r128gain in my podcast dowload and gain normalization and was thinking to switching to rsgain for it.

so i tested with firstly full run, and then hoping that -S option will provide option to skip scanned.
Not sure if it should be in existing -S option as header gain is not ReplaGain info.
And if it should work only in a or t mode for opus or for all modes