Why isn't the index a tree?

I expected the index to be implemented something like a ref to a tree object (per stage) plus some stat()/assume-unchanged/etc. metadata. Instead, it appears to be a (sorted?) flat list of full paths with their associated SHA1s and metadata.

Is there a reason why each stage in the index isn't implemented as a tree?

If the index was a tree, I believe it would make several tasks easier:
* you could easily get the diff between stage 1 and some arbitrary
revision (e.g., git diff HEAD^^ :1:)
* you could commit and checkout empty directories
* you could use update-index --cacheinfo to directly insert/replace
an entire subdirectory
* etc.

Thanks,
Richard

I think the answer is that there is meta data in the index (particularly timestamps) needed for efficiently tracking changes to the filesystem that isn't needed in a tree - forcing everything into a tree early would necessitate creating SHA1 hashes for lots of trees that will eventually not be needed.

So, the index is a data structure tuned for performance in ways that a tree cannot be.

jon

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