renamed cur_cat => cur_cat_idx and added some comments

[zyxue]

just for clarity and trying to make it easier to follow the code for those who aren't familiar with the code base.

unblock

[jameslamb]

Thanks for the changes.

This looks ok to me, but I'll just leave a "comment" review and let @guolinke or @shiyu1994 check that this new name correctly reflects what the code is doing.

Until then... can you please merge in the latest state of master? We just merged a fix for the failing CI jobs (#5510). Sorry for the inconvenience.

[guolinke]

LGTM

[jameslamb]

@zyxue please push a new commit addressing the linting errors (build link).

src/io/bin.cpp:355:  At least two spaces is best between code and comments  [whitespace/comments] [2]
src/io/bin.cpp:392:  At least two spaces is best between code and comments  [whitespace/comments] [2]
src/io/bin.cpp:454:  At least two spaces is best between code and comments  [whitespace/comments] [2]

[jameslamb]

Looks good, thanks! We will merge this when it builds.

[github-actions]

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