simplegit

simplegit is git reimplementation from bottom-up

To install dependencies:

bun install

Link the Bun CLI:

bun link
bun link simplegit

Now just run 🚀

simplegit
simplegit init testgit
simplegit cat-file blob <blob_sha>
simplegit hash-object <file_path>
simplegit log <valid_sha>
simplegit ls-tree <valid_sha>

Note: To get any sha just run git log on any git repo and you can plug it in simplegit commands💡

Understanding and building simple git

Important files where our internal git data gets stored

1. .git/objects/ : the object store, more on this as we build it.
2. .git/refs/ the reference store, which we’ll discuss a bit later. It contains two subdirectories, heads and tags.
3. .git/HEAD, a reference to the current HEAD (more on that later!)
4. .git/config, the repository’s configuration file.
5. .git/description, holds a free-form description of this repository’s contents, for humans, and is rarely used.

Understanding Git object storage system

1. Git storage format - It uses SHA-1 hash!!!

   <header><ascii_space><size_of_object_in_bytes><null_byte_0x00><content>

   header - specifies the type (blob, commit, tag or tree)
   ascii_space - ASCII space (0x20)
   size_of_object_in_bytes - size of the content stored in the object
   null_byte_0x00 - Null byte 0x00
   content - contents of object
   

   Fun tip 💡: Checkout playground/zlib.ts if you provide it any valid .git/objects/<hash>/<hash> location it will fetch the content from the hash it's really cool

Understanding Git commit format and writing a basic parser for it

1. Format of commits history:

   tree 29ff16c9c14e2652b22f8b78bb08a5a07930c147
   parent 206941306e8a8af65b66eaaaea388a7ae24d49a0
   author Thibault Polge <thibault@thb.lt> 1527025023 +0200
   committer Thibault Polge <thibault@thb.lt> 1527025044 +0200
   gpgsig -----BEGIN PGP SIGNATURE-----
   
       iQIzBAABCAAdFiEExwXquOM8bWb4Q2zVGxM2FxoLkGQFAlsEjZQACgkQGxM2FxoL
       kGQdcBAAqPP+ln4nGDd2gETXjvOpOxLzIMEw4A9gU6CzWzm+oB8mEIKyaH0UFIPh
       rNUZ1j7/ZGFNeBDtT55LPdPIQw4KKlcf6kC8MPWP3qSu3xHqx12C5zyai2duFZUU
       wqOt9iCFCscFQYqKs3xsHI+ncQb+PGjVZA8+jPw7nrPIkeSXQV2aZb1E68wa2YIL
       3eYgTUKz34cB6tAq9YwHnZpyPx8UJCZGkshpJmgtZ3mCbtQaO17LoihnqPn4UOMr
       V75R/7FjSuPLS8NaZF4wfi52btXMSxO/u7GuoJkzJscP3p4qtwe6Rl9dc1XC8P7k
       NIbGZ5Yg5cEPcfmhgXFOhQZkD0yxcJqBUcoFpnp2vu5XJl2E5I/quIyVxUXi6O6c
       /obspcvace4wy8uO0bdVhc4nJ+Rla4InVSJaUaBeiHTW8kReSFYyMmDCzLjGIu1q
       doU61OM3Zv1ptsLu3gUE6GU27iWYj2RWN3e3HE4Sbd89IFwLXNdSuM0ifDLZk7AQ
       WBhRhipCCgZhkj9g2NEk7jRVslti1NdN5zoQLaJNqSwO1MtxTmJ15Ksk3QP6kfLB
       Q52UWybBzpaP9HEd4XnR+HuQ4k2K0ns2KgNImsNvIyFwbpMUyUWLMPimaV1DWUXo
       5SBjDB/V/W2JBFR+XKHFJeFwYhj7DD/ocsGr4ZMx/lgc8rjIBkI=
       =lgTX
       -----END PGP SIGNATURE-----
   
       Create first draft
   

   Example commit format

• tree - A tree maps blobs IDs to filesystem locations, and descibes a state of the work tree. Put simply, it is the actual content of the commit: file contents, and where they go

• parent - is a reference to the parent of this commit. It may be repeated: merge commits, for example, have multiple parents. It may also be absent: the very first commit in a repository obviously doesn't have a parent

• author and committer - these can be different because author of commit is not necessarily the person who can commit it. (little bit wierd but earlier git was done through email)

• gpgsig - is the PGP signature of this object

Checkout playground/commit.ts to checkout the basic parser for this

2. Reading commit data

   Understaning Trees format:

   [mode] space [path] 0x00 [sha-1]
   

   • mode = upto 6 bytes and is an octal representation of a file mode (Buffer)
   • space = space character (32 ASCII)
   • path = its a single filesystem entry
   • 0X00 = null character (Buffer)
   • sha1 = object sha in binary encoding (Buffer)