[FEAT] Implement str.substr Expression

[danila-b]

State: DRAFT No tests and no docs yet until the implementation is finalized.

Adds Expressions.str.substr implementation. Resolves issue: https://github.com/Eventual-Inc/Daft/issues/1934

[samster25]

@kevinzwang can you take a look when you get a chance?

[danila-b]

The function turned out a bit lengthier than I expected, I'll see if I can shorten or refactor it but the general functionality should stay the same.

[kevinzwang]

@danila-b is this PR ready for review?

[danila-b]

@danila-b is this PR ready for review?

Yes, it is

[kevinzwang]

@danila-b are you still working on this? Your PR is almost in a mergeable state. Let me know if you'd like me to push it past the finish line

[danila-b]

@danila-b are you still working on this? Your PR is almost in a mergeable state. Let me know if you'd like me to push it past the finish line

Hey! Yeah I will try to implement the function with impl Iterator how you suggested one more time this weekend, and if it won't work out then feel free to take it over to get it across the finish line.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.17308% with 35 lines in your changes are missing coverage. Please review.

Please upload report for BASE (main@e836290). Learn more about missing BASE report. Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2269   +/-   ##
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  Coverage        ?   79.16%           
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  Files           ?      472           
  Lines           ?    54685           
  Branches        ?        0           
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  Hits            ?    43293           
  Misses          ?    11392           
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Files	Coverage Δ
daft/expressions/expressions.py	92.93% <100.00%> (ø)
src/daft-core/src/python/series.rs	94.43% <100.00%> (ø)
src/daft-dsl/src/functions/utf8/mod.rs	99.49% <100.00%> (ø)
src/daft-dsl/src/python.rs	91.57% <100.00%> (ø)
daft/series.py	91.88% <88.88%> (ø)
src/daft-core/src/series/ops/utf8.rs	93.10% <83.33%> (ø)
src/daft-dsl/src/functions/utf8/substr.rs	63.33% <63.33%> (ø)
src/daft-core/src/array/ops/utf8.rs	93.66% <84.55%> (ø)

[danila-b]

I realize there is no super clean solution, since you would need to box the iterators to not do the start_iter, start_repeat, length_iter, length_repeat matching, which comes at a performance cost. If you are happy with this, I'm ready to merge it!

Happy to merge it as it is :+1:

[kevinzwang]

Great, awesome work with this PR!