[ajross]

"People" don't commonly use them, no. But it's a real and documented API to do this (see e.g. https://git-scm.com/book/en/v2/Git-Internals-Git-Objects).

And in any case you had a specific requirement above ("Given a collection of files, but not the git repo they're from, and libgit, I can't say if those files match a git tag hash"), and in fact this can be done!

[fragmede]

The git tag hash references a commit. Without the commit metadata, you don't have a tree object and thus don't know any hashes. You can take the files on disk and compute the hash and furthermore you can take that hash and make a tree object. but without the commit, all you can say is you have a tree object, you don't have a tree object for the commit in question to compare it to.

[ajross]

Read the link. You can extract the commit object and get the tree ref trivially. You can also enumerate commits (literally what "git log" is for). The only thing missing from the process is a fast reverse index going backwards from blob to tree to commit. But that can be generated in just a few seconds even for the largest repositories (16s to do a full git log of Linux on my box, for example).

I'm at a loss. You keep saying something can't be done, but it can, and it's not even hard.