“Git gets easier once you get the basic idea that branches are homeomorphic endofunctors mapping submanifolds of a Hilbert space.”
— Isaac Wolkerstorfer
— Isaac Wolkerstorfer
We have files, which are inert objects and form a "file space". We have diffs. Diffs can "act" on a file to produce a new file or a conflict --- we call this as "applying a patch". Mathematicians would call it a "group(oid) action". Diffs are a groupoid because (1) there's an identity diff that does nothing, (2) diffs can be smashed together, (3) if the smashing of diffs succeeds, the operation of concatenation is associative, (4) all diffs are invertible. If we were to delete added lines and add the deleted lines, this inverts the diff.
Finally, we have a rooted DAG where each node is a diff, and the root is the empty diff. Git queries enable to manipulate paths in the DAG, which corresponds to smashing together diffs along a path to produce a file.
If one groks this, the rest of git is a pretty poor interface on top of this nice mathematical structure.
For more physics-y applications of torsors, check out the baez article: https://math.ucr.edu/home/baez/torsors.html
What do you think is the difference between a "commit" and a "diff"?
> You have two nearly identical 22K objects on your disk (each compressed to approximately 7K). Wouldn’t it be nice if Git could store one of them in full but then the second object only as the delta between it and the first?
> It turns out that it can. The initial format in which Git saves objects on disk is called a “loose” object format. However, occasionally Git packs up several of these objects into a single binary file called a “packfile” in order to save space and be more efficient. Git does this if you have too many loose objects around, if you run the git gc command manually, or if you push to a remote server.
> When Git packs objects, it looks for files that are named and sized similarly, and stores just the deltas from one version of the file to the next. You can look into the packfile and see what Git did to save space.
If a commit references a "blob", and the "blob" that it references is, in fact, a diff, why would we say that the commit "does not reference a diff"?
There're optimizations on the storage level, compression etc, but on logic level those are transparent
The "diff" you're referring to is an implementation detail of the compression. It's not even always there; it depends on which commits are present in your clone. It's also not even the same "diff" you work with when you use git to generate or apply patches. Using the same word only leads to confusion.
There is a disconnect somewhere. The linked answer says:
> Now, git is different. Git stores references to complete blobs and this means that with git, only one commit is sufficient to recreate the codebase at that point in time. Git does not need to look up information from past revisions to create a snapshot.
> So if that is the case, then where does the delta compression that git uses come in?
> Well, it is nothing but a compression concept - there is no point storing the same information twice, if only a tiny amount has changed. Therefore, represent what has changed, but store a reference to it, so that the commit that it belongs to, which is in effect a tree of references, can still be re-created without looking at past commits.
You can recreate a file that is stored as a root blob plus some series of diffs without looking at information from past commits. But you can't recreate it without doing the diffs! You have to look at the root blob. This is, internally, tracked separately from the commit which created it. But your conclusion:
> when you checkout stuff, git doesn't do diffs to give you the working directory at that point.
cannot be true. If the working directory at that point corresponds to a blob which has only diff information stored, git must apply that diff to a separate blob in order to give you the working directory.
But when you create a new commit, that commit object is stored as a loose object, with any new file blobs and tree objects. This represents a complete snapshot of your working tree.
But git does not need to make a complete copy of the working tree on each commit. Because objects are referred to by the hash of their contents (with a git specific header), git only needs to store each version of a file once.
Another way to think of it, if it was diffs, if you had 1000 commits, getting to the 'head' would take forever because it had to replay all the commits diffs just to get there.
Yes, you could combine diffs & snapshots, but that in itself is a tricky complexity in an already very complex system.
[0] https://tom.preston-werner.com/2009/05/19/the-git-parable.ht...
There is also the obvious performance problem that you would have to replay all diffs to get to switch between commits.
Yes, it does.
> Every commit that deletes the whitespace from an otherwise empty line in a 30KB file is another 30KB of hard drive space
Yes, it is.
"It's worth repeating that git stores every revision of an object separately in the database, addressed by the SHA checksum of its contents. There is no obvious connection between two versions of a file; that connection is made by following the commit objects and looking at what objects were contained in the relevant trees. Git might thus be expected to consume a fair amount of disk space; unlike many source code management systems, it stores whole files, rather than the differences between revisions. It is, however, quite fast, and disk space is considered to be cheap." -- https://lwn.net/Articles/131657/
One of the insights of the git design was that, nowadays, disk space is cheap. The first releases of git always stored each object separately in its own file in the object database. Git still does so nowadays, but once the number of files gets over a certain threshold, newer releases of git run an "automatic GC" which combines these "loose objects" into a "pack file"; and within that "pack file", it uses a binary diff (a xdelta) between similar objects to reduce the total size. But that's just a physical storage optimization; in the logical model, whenever you ask for an object, you always get its full contents, not a delta against some other object.
Isn't it the case then that git doesn't store in full every version of a tracked file?
The question was does git's implementation store - in full - every version of a tracked file? The answer is that it doesn't. git has a sophisticated storage engine precisely to avoid the inefficiencies of the naive approach.
Ie. I can develop a fix for a issue for version 1.2.30 of some software, generate a the diff using the diff tool and then apply this diff using the patch tool to version 1.1.15 of the software. This might fail (or result in something undesired), but there no principal problem in moving the diff around and applying it somewhere else.
A git commit however is a particular version, so git is not really good at applying a commit somewhere else.
That only one is how it's implemented is besides the point really, until you get _quite_ low level.
I really hope something like pijul takes off.
In algebraic geometry, given a smooth algebraic group G, a G-torsor or a principal G-bundle P over a scheme X is a scheme (or even algebraic space) with an action of G that is locally trivial in the given Grothendieck topology in the sense that the base change Y × X P {\displaystyle Y\times _{X}P} Y\times _{X}P along "some" covering map Y → X {\displaystyle Y\to X} Y\to X is the trivial torsor Y × G → Y {\displaystyle Y\times G\to Y} Y\times G\to Y (G acts only on the second factor).[1] Equivalently, a G-torsor P on X is a principal homogeneous space for the group scheme G X = X × G {\displaystyle G_{X}=X\times G} G_{X}=X\times G (i.e., G X {\displaystyle G_{X}} G_{X} acts simply transitively on P {\displaystyle P} P.)
Look, there's even links to the sub-terms:
("algebraic group" article): In algebraic geometry, an algebraic group (or group variety) is a group that is an algebraic variety, such that the multiplication and inversion operations are given by regular maps on the variety.
("action" article): In algebraic geometry, an action of a group scheme is a generalization of a group action to a group scheme. Precisely, given a group S-scheme G, a left action of G on an S-scheme X is an S-morphism
("principal homogeneous space" article): In mathematics, a principal homogeneous space,[1] or torsor, for a group G is a homogeneous space X for G in which the stabilizer subgroup of every point is trivial. Equivalently, a principal homogeneous space for a group G is a non-empty set X on which G acts freely and transitively (meaning that, for any x, y in X, there exists a unique g in G such that x·g = y, where · denotes the (right) action of G on X). An analogous definition holds in other categories, where, for example,
Really, all you have to do is read the article carefully and follow the links to unknown terminology if you can't immediately intuit its meaning. However, it's only just basic category theory, really.
(yes, guys, /s).
- You have a branch `master`.
- You have a branch `feature` which contains commit C, which conflicts with `master`.
- You merge `feature` into `master`, fixing conflicts.
- You log the commits of `master`.
* First is the merge commit, whose diff contains code added by commit C (including the conflict resolution).
* Next you see commit C, whose diff contains code added by commit C (obviously)
Well why didn't you just say so!
But isn't the core idea of Git, everyone has their own repository and they share branches, kind of complicated and tricky? In my experience the developers I see having problems with Git are having trouble with the hard stuff, not the easy bits like committing and pushing a change.
Git's--and Mercurial's--secret sauce is the directed acyclic graph of commits.
Everything else is window dressing. It's complicated if you're used to a linear sequence of commits, but it's not too hard once you grasp the tree structure.
All operations are operations that manipulate (or share) the DAG, so it now becomes a matter of mapping how you want to manipulate the DAG onto the command set the tool gives you.
Git's toolset was designed by a dozen madmen that didn't talk to each other, so it's a loosely connected set of tools with conflicting syntax and meaning. Mercurial's toolset was designed to be used, so if you know the right terminology, you can easily figure out what command to write. Want to commit something? It's probably hg commit. Want to rebase? It's probably hg rebase. Etc.
Git exposes unnecessary complexity. But it won the DVCS popularity contest, so we're stuck with it. I wish something else had won. Oh well...
But the first impression is strong, it's still a good joke.