Improve size tracking table

[jprochazk]

What

• The change diff had the opposite sign
• Previous/Current headers now use the base/PR branch names instead
• Changed some variable names and re-ordered some code
• Added tracking of latest commit hash via an index file that's only updated on main

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

The units here confuse me. What unit does threshold have? If it is in percents, why not just compare abs(change_pct) >= threshold ? Again: proper names help a lot.

I prefer to not use percentages except for ui:s (i.e. multiply with 100 during the formatting). It usually ends up with far less 100 in the code (and less bugs). _ratio or _fraction is a pretty good suffix for such numbers. Percentages in arguments have the problem of the unit being invisible. --threshold 0.05 is usually clearer than --threshold 5 (5 kiB?)

[jprochazk]

If you're about to use the script, you're going to run sizes.py --help first, and the help text for threshold says N%, so --threshold 5 means 5%. As a user, that seems marginally better than 0.05, because the table also outputs percentages. I tell the script I want to see differences of more than 5% and I see all the differences which were more than 5%, no mental math required, however little it may be.

[emilk]

That's much clearer to read now - thanks 🙏

[emilk]

I understand that sizes.py --help explains the unit of threshold, but that helps very little when reading this file 🤷‍♂️
Not a blocker, just trying to explain my reasoning a bit further.

[jprochazk]

I agree, but I also think that 0.05 is as ambigous as 5 if there's no unit :/

[emilk]

Maybe - but 0.05 is less likely to be of unit Bytes at least :)

[emilk]

--threshold-percent would be very clear in any case

[jprochazk]

We can do crimes instead:

comparison=$(
  python3 scripts/ci/sizes.py compare \
    --threshold=5% \
    "prev.json" "data.json"
)

The % is optional

[emilk]

Haha, now we're talking 😆