Functions to compute OID

[jeffparsons]

I have this code in a project:

trait TreeExt {
    fn compute_oid(&self) -> ObjectId;
}

impl TreeExt for mutable::Tree {
    // Based on <https://git-scm.com/book/en/v2/Git-Internals-Git-Objects>.
    fn compute_oid(&self) -> ObjectId {
        let mut buf: Vec<u8> = Vec::new();
        self.write_to(&mut buf)
            .expect("Failed to write; shouldn't happen because we were writing to a Vec");
        let header = format!("tree {}\0", buf.len());
        let mut sha1 = sha1::Sha1::new();
        sha1.update(header.as_bytes());
        sha1.update(&buf);
        let sha1_digest = sha1.digest();
        ObjectId::from_borrowed_sha1(&sha1_digest.bytes())
    }
}

Would you be open to a PR that introduces an equivalent (but non-allocating) function to all relevant types in git-object? Seems like it might be generally useful.

[Byron]

I see how that could be useful.

In case you are looking for a place to put such a trait, git-repository has a place for that.

There is an in-memory object store already which is should be useful for this kind of functionality.