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Zero git - the imitation stupid content tracker
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README small comments, public domain license May 9, 2019
add support detached head, symlinks, executable bit; annotate optional code Apr 21, 2019
branch small comments, public domain license May 9, 2019
cat-file rev-parse: dedup, support ^ and ~ Apr 21, 2019
checkout support detached head, symlinks, executable bit; annotate optional code Apr 21, 2019
checkout-index small comments, public domain license May 9, 2019
commands.dot add README Apr 23, 2019
commands.png add README Apr 23, 2019
commit support detached head, symlinks, executable bit; annotate optional code Apr 21, 2019
commit-tree support detached head, symlinks, executable bit; annotate optional code Apr 21, 2019
get-sha1-basic add README Apr 23, 2019
hash-object hash-object Apr 12, 2019
index.py big cleanup, refactor, fill some missing functionality Apr 21, 2019
init compatibility with git-hydra and old macos bash Apr 23, 2019
read-tree
reset reset May 9, 2019
rev-parse support detached head, symlinks, executable bit; annotate optional code Apr 21, 2019
symbolic-ref checkout: checkout-index, improve rev-parse, symbolic-ref Apr 19, 2019
test small comments, public domain license May 9, 2019
update-ref big cleanup, refactor, fill some missing functionality Apr 21, 2019
write-tree support directories Apr 22, 2019
zit big cleanup, refactor, fill some missing functionality Apr 21, 2019

README

THIS WILL EAT YOUR DATA AND NOT APOLOGIZE FOR TALKING WHILE CHEWING.
Probably not but this is not a robust implementation of git so don't let it out
on your important code.

This code is released into the public domain.

////////////////////////////////////////////////////////////////

	ZIT - the imitation stupid content tracker

////////////////////////////////////////////////////////////////
"zit" can mean anything, depending on your mood.

 - random three-letter combination that is pronounceable, and not
   actually used by any common UNIX command.
 - embarassing. ugly and greasy. adolescence. Take your pick from the
   dictionary of slang.
 - "zero git": you're in a good mood, and it actually helps you
   understand git. Angels sing, and a light suddenly fills the room.
 - "zuuuuuuuugh, idiotic truckload of sh*t": when it breaks

How to use this
~~~~~~~~~~~~~~~

zit is not intended to be "well written" code. It intentionally avoids any
significant refactoring (like functions) or error handling. It tries in many
ways to be like git: imperfect but simple. Instead zit is meant to be a terse
implementation to help demystify git.

Poke around the code and even try writing it yourself. One strategy is to
delete all the files except for test and try to implement files one-by-one to
get `zit test` to pass. The test script can run against your implementation of
zit; for example if you are writing it in your current directory:

    $ /path/to/jmou/zit/test ./zit

Most of the files are named after actual git commands, so you can use git help
to read relevant documentation.

The zit code is structured similarly to git v1.0.
https://github.com/git/git/tree/v1.0.13

Also it may be helpful to reference commands.png which diagrams the
dependencies between zit subcommands.

git storytime
~~~~~~~~~~~~~

To understand git is to understand the circumstances that gave it rise.

https://jmou.github.io/rc-git-storytime/ (press `p` for rough notes)

    "Git is a weekend hack which looks like a weekend hack." --Bram Cohen
    https://wincent.com/blog/a-look-back-bram-cohen-vs-linus-torvalds

git cared a lot about performance, and it had to be developed quickly.
Usability, and even being useful for version control, were far out of mind.

Learning from git
~~~~~~~~~~~~~~~~~

Why should we bother learning about git or how it works?

    "Study your successes . . . not only yours but other people's. Why did
    Galileo do what he did? How did Newton do it? Try as best you can to study
    other people, how they succeed, what were the elements of their success,
    which elements of that can you adapt to your personality. You can't be
    everybody but you have to find your own method, and studying success is a
    very good way of forming your own style." --Richard Hamming
    "You and Your Research" https://www.youtube.com/watch?v=a1zDuOPkMSw

git has an inconsistent UI and competing tools are better implemented.

    "The worse-is-better software first will gain acceptance, second will
    condition its users to expect less, and third will be improved to a point
    that is almost the right thing." --Richard Gabriel
    "Worse is Better" https://www.jwz.org/doc/worse-is-better.html

    (Most "clever" git features were implemented /primarily/ because they were
    easy to write: bisect, staging, reflog, add -p, rebase -i)

git design points
~~~~~~~~~~~~~~~~~

- The filesystem is a database. .git/objects is a simple key-value store where
  the filename is the key. The first two hex characters are the directory name,
  effectively splitting the keyspace into 256 buckets. This simple optimization
  benefits any programs that perform poorly on directories with many files.
- Plumbing vs porcelain. git started as a tool chest of low level commands,
  or plumbing. Higher level commands were eventually written building on top of
  plumbing; the commands that users were expected to use were called porcelain.
  The only difference between plumbing and porcelain was convention, and it was
  easy to develop git organically. Since git was composed of many distinct
  commands, it naturally became polyglot (mostly a mix of C, bash, and perl);
  today most of it has been rewritten in C for performance and portability.
- The index (sometimes called staging or the directory cache) conceptually
  intermediates between git's object storage and your checked out files.
  Namely:
  - The index can be used to generate a complete tree object without reading
    any other files.
  - The index can be efficiently compared with files as you modify them to find
    what is out of date.

git personality quirks
~~~~~~~~~~~~~~~~~~~~~~

    "It's not an SCM, it's a distribution and archival mechanism. I bet you
    could make a reasonable SCM on top of it, though." --Linus Torvalds
    http://lkml.iu.edu/hypermail/linux/kernel/0504.0/2022.html

(SCM is "source code management"; today we might say VCS or version control.)

git was created for a very narrow problem that was specific to Linus' workflow
of managing hundreds of patch sets at a time. It was built from the ground up,
starting with efficient data representations, and as features were added it
came to look more and more like a full version control system for developers.

git is not shy about its implementation details. For example, the index was
implemented very early on in git as a performance optimization, so that
generating tree objects would not need to re-read every source file. While
other distributed version control systems like Mercurial chose not to expose
any such representation to users, git instead embraced the index and rebranded
it from its inception as a "directory cache" into a "staging area."

Other things to check out
~~~~~~~~~~~~~~~~~~~~~~~~~

From Recurse Center folks!
- git-hydra (delightful repo visualization): https://github.com/blinry/git-hydra
- In-depth git explanation: https://maryrosecook.com/blog/post/git-from-the-inside-out

Many features are not implemented but can be interesting to learn more about.
- tags
- packfiles
- reflog (TODO implement this?)
- fetch/pull/push and remote tracking branches (TODO implement this?)
- stash (TODO implement this?)
- diffing and merging
- rebase

The "Building Git" book takes a similar approach to learning git much further.
https://shop.jcoglan.com/building-git/

Mercurial is another distributed version control system that places much more
value on being intuitive. Read about the differences from their perspective.
https://www.mercurial-scm.org/wiki/GitConcepts

git's documentation is surprisingly approachable and informal (if a bit
disorganized). Leaf through and read about anything that sounds interesting.
https://github.com/git/git/tree/master/Documentation/technical

A dozen reads of "Git for Computer Scientists" is when git "clicked" for me.
https://eagain.net/articles/git-for-computer-scientists/

Well-written history and technical explanation of git from "The Architecture
of Open Source Applications": https://www.aosabook.org/en/git.html

Early article on git written well before v1.0: https://lwn.net/Articles/131657/
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