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Analogies for Git
November 13, 2016

I've done some development on Git. I'm pretty proud of it, because
it's a tool that powers so much of modern software development.

At Practice, I was asked to describe the difference between SVN and
Git, and also between Perforce and Git.

The answer I gave goes like this:

"A Guide to SF Chronophysics" describes four types of time travel
plots. Type 1 is deterministic -- whatever happens, was what was
destined to happen. There's only one timeline. Type 3 is the one
where someone steps on a butterfly and Trump is elected president.
Type 2 is half way in between -- you can change things, but they tend
to converge back to the original timeline. And finally, type 4
involves multiple universes -- every change (including time travel)
creates a new timeline.

SVN is type 1. Git is type 4. When you "amend" a commit in Git, you
actually create a whole new commit, forking off from the same parent
as the previous one. You can use your time machine's "reflog"
functionality to see the old one. Similarly, rebase creates a new
timeline from some point in the past.

Perforce, I'm told, is somewhat like git, but it treats changesets
rather than snapshots ("commits" in gitspeak, although in ordinary
usage the term commit often refers to a changeset) as fundamental.

This is an instance of the mathematical notion of duality. The first
example of duality I learned was polyhedra: if you swap the faces of
a polyhedron with vertices, you get a different polyhedron. The dual
of a cube is an octahedron (known by gamers as a d8). Instead of six
faces and eight vertices, it's got eight faces and six vertices.. The
dual of a dodecahedron (d12) is an icosahedron (d20). The dual of a
tetrahedron (d4) is itself. The Japanese addressing system is almost
a dual of the US addressing system. In the US, we give addresses in
terms of strees. In the Japanese system, blocks are the fundamental
unit. I have been meaning for some time to design a game around the
concept of duality, but I have not yet figured out quite how to do
it.

Anyway, the graph of changesets is just the graph of snapshots with
the vertices and edges swapped. Duality.

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