Fix average bandwidth value from decaying when the system bandwidth being constant

[KVPasupuleti]

Description

In the movingAverageBandwidthSelector method, we are using this line to calculate the average.

average = decay * this.systemBandwidth + (1 - decay) * average;

I see a problem here.

To explain with an example,

I have an average value of 1000 currently and a new chunk got downloaded with a bandwidth of 500. And the next chunk is downloaded after 1 second.

The systemBandwidth is updated after each chunk gets downloaded.

So, my expectation is this 500 should be contributed to the average only once until the next chunk gets downloaded.

But, as the checkABR method is getting called for every 250ms, this 500 is getting contributed 4 times until the next chunk gets downloaded, by which the average value is getting decreased gradually because the systemBandwidth is being constant until then.

You can check the logs attached below.

Video-js-ABR.log

Specific Changes proposed

• Change the average value only if the systemBandwidth changes and is greater than zero.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

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[codecov]

Codecov Report

Merging #1137 (fd3bd54) into main (0be51eb) will increase coverage by 0.26%.
The diff coverage is 94.03%.

@@            Coverage Diff             @@
##             main    #1137      +/-   ##
==========================================
+ Coverage   86.20%   86.47%   +0.26%     
==========================================
  Files          39       39              
  Lines        9289     9501     +212     
  Branches     2127     2191      +64     
==========================================
+ Hits         8008     8216     +208     
- Misses       1281     1285       +4     

Impacted Files	Coverage Δ
src/manifest.js	93.06% <37.50%> (-4.79%)	⬇️
src/videojs-http-streaming.js	90.30% <66.66%> (-0.29%)	⬇️
src/playlist.js	94.11% <89.74%> (-0.39%)	⬇️
src/playlist-loader.js	93.73% <91.11%> (+2.09%)	⬆️
src/media-segment-request.js	95.72% <98.52%> (+0.99%)	⬆️
src/master-playlist-controller.js	94.51% <100.00%> (+0.20%)	⬆️
src/media-groups.js	98.66% <100.00%> (-0.32%)	⬇️
src/playlist-selectors.js	86.07% <100.00%> (+0.84%)	⬆️
src/segment-loader.js	96.18% <100.00%> (+0.36%)	⬆️
src/sync-controller.js	96.89% <100.00%> (+0.16%)	⬆️
... and 4 more

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[gkatsev]

Wrote a test for this #1141
Once the other PR passes review (I'm doing something I'm unsure about) I'll get all the stuff merged in.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉