Merge pull request git#550 from derrickstolee/gsoc-sparse-index

Soc-2022-Ideas: Sparse Index Integrations

Author: chriscool

SoC-2022-Ideas.md

@@ -17,5 +17,57 @@ about refactoring projects versus projects that implement new
 features" in the [general application information](https://git.github.io/General-Application-Information)
 page though.
 
-### Idea X
-_TBD: Ideas to be added based on discussion with community_
+### More Sparse Index Integrations
+
+The [spare index feature](https://github.blog/2021-11-10-make-your-monorepo-feel-small-with-gits-sparse-index/)
+accelerates Git commands when using
+[sparse-checkout in cone mode](https://github.blog/2020-01-17-bring-your-monorepo-down-to-size-with-sparse-checkout/).
+This works by modifying the on-disk index file in a way that includes
+"sparse directory" entries instead of only file entries. This requires
+care when enabling the sparse index for different commands, as custom
+logic might be necessary. At minimum, interaction with the sparse index
+needs to be carefully tested in the Git test suite when enabling it.
+
+The most-used commands have already been integrated with the sparse
+index feature. This process usually takes a few steps:
+
+1. Add tests to [`t1092-sparse-checkout-compatibility.sh`](https://github.com/git/git/blob/master/t/t1092-sparse-checkout-compatibility.sh)
+ for the builtin, with a focus on what happens for paths outside of the
+ sparse-checkout cone.
+2. Disable [the `command_requires_full_index` setting](https://github.com/git/git/blob/master/repository.h#L35)
+ in the builtin and ensure the tests pass.
+3. If the tests do not pass, then alter the logic to work with the sparse index.
+4. Add tests to [check that a sparse index stays sparse](https://github.com/git/git/blob/38062e73e009f27ea192d50481fcb5e7b0e9d6eb/t/t1092-sparse-checkout-compatibility.sh#L873-L939).
+5. Add [performance tests](https://github.com/git/git/blob/master/t/perf/p2000-sparse-operations.sh)
+ to demonstrate speedup.
+
+Here is a list of builtins that could be integrated with the sparse index.
+They are generally organized in order of least-difficult to most-difficult.
+This allows the student to gain partial success early in the project and
+the student can complete as many as possible in the timeframe (without
+expectation that _all_ will be completed during the project).
+
+*`git mv`
+*`git rm`
+*`git grep`
+*`git rev-parse`
+*`git fsck`
+*`git check-attr`
+*`git describe`
+*`git diff-files`
+*`git diff-index`
+*`git diff-tree`
+*`git worktree`
+*`git write-tree`
+*`git apply`
+*`git am`
+*`git checkout--worker` (for parallel checkout)
+*`git merge-index`
+*`git rerere`
+
+Project Size: Medium
+Languages: C, shell(bash)
+
+Co-Mentors:
+* Derrick Stolee `<[email protected]>`
+* Victoria Dye `<[email protected]>`