[bitmap] remove invariant that last chunk has room for next bit

[danlaine]

Follow up PR to #1516 based on this comment.

This PR removes the invariant that the bitmap has at least 1 chunk.

This requires corresponding changes in adb::current::Current, which uses the BitMap.

[roberto-bayardo]

Can you clarify why this is desirable? It seems to be adding complexity (albeit not much). The "this comment" link in the PR description doesn't work from me (perhaps because I'm using the "new" github UI?)

[roberto-bayardo]

this increase in invariant complexity makes me a little sad

[danlaine]

Can you clarify why this is desirable? It seems to be adding complexity (albeit not much). The "this comment" link in the PR description doesn't work from me (perhaps because I'm using the "new" github UI?)

It's a link to @BrendanChou's comment that reads:

I personally found it easier to not do this and make the number of chunks equal to len().div_ceil(N * 8)... wonder if that would simplify the code or not

Sounds like there's disagreement as to which invariant is simpler. I'm not especially opinionated. I suppose I slightly favor this PR's approach because it makes serialization more straightforward. Namely, you don't need to worry about serializing an empty chunk at the end of the bitmap.

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (danlaine/separate-bitmap@b1b4c49). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
storage/src/mmr/bitmap.rs	97.91%	1 Missing ⚠️
utils/src/bitmap/mod.rs	98.48%	1 Missing ⚠️

@@                     Coverage Diff                     @@
##             danlaine/separate-bitmap    #1589   +/-   ##
===========================================================
  Coverage                            ?   92.29%           
===========================================================
  Files                               ?      295           
  Lines                               ?    76250           
  Branches                            ?        0           
===========================================================
  Hits                                ?    70375           
  Misses                              ?     5875           
  Partials                            ?        0           

Files with missing lines	Coverage Δ
storage/src/adb/current.rs	96.12% <100.00%> (ø)
utils/src/bitmap/prunable.rs	99.13% <100.00%> (ø)
storage/src/mmr/bitmap.rs	95.36% <97.91%> (ø)
utils/src/bitmap/mod.rs	97.03% <98.48%> (ø)

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