Mog - My Own Git Implementation

Table of Contents

• Introduction
• Implementation Details
• Git Internals

Introduction

Mog is my own implementation of Git, built from scratch in C++. 🚀

There are broadly two types of commands in git:

1. Porcelain Commands: High-level commands that users interact with directly (e.g., git add, git commit, git push)
2. Plumbing Commands: Low-level commands that interact with the Git database (e.g., git hash-object, git cat-file, git update-ref)

Mog focuses on the plumbing commands and aims to provide a basic understanding of how Git works under the hood. It supports the following plumbing commands:

• mog init: Initialize a new Git repository
• mog hash-object <file>: Compute the hash of a file and store it in the Git database
• mog cat-file <hash>: Retrieve and display the contents of a Git object
• mog write-tree: Create a tree object from the current working directory
• mog read-tree <tree-hash>: Read the contents of a tree object into the working directory
• mog commit-tree <tree-hash> [-p <parent-hash>]: Create a commit object from a tree object

Implementation Details

I wanted to understand how Git works under the hood, and I figured the best way to do that was to build my own implementation from the ground up. Along the way, I also deepened my knowledge of C++ and how to use it to develop a full-fledged application.

Git Internals

Git operates as a key-value store based on three primary object types:

Object Types Overview

1. Blob Objects - Store file contents
2. Tree Objects - Represent directories
3. Commit Objects - Capture repository snapshots

Blob (Binary Large Object)

A blob stores the contents of a file. Its format is:

blob <content-length>\0<content>

• <content-length>: The length of the file content in bytes
• <content>: The actual file content

Tree Object

A tree stores information about directories and the files they contain. Its format is:

tree <content-length>\0<entry>

Each entry in a tree object follows this structure:

<mode> <filename>\0<hash>

• <mode>: File mode (permissions)
• <filename>: Name of the file or directory
• <hash>: SHA-1 hash of the corresponding blob or tree object

Commit Object

A commit represents a snapshot of the repository at a specific point in time. Its format is:

commit <content-length>\0<commit-content>

The commit content includes:

tree <tree-hash>
parent <parent-hash>
author <author>
committer <committer>
<commit-message>

• <tree-hash>: Reference to the tree object representing the state of the repository
• <parent-hash>: Reference to the previous commit (optional for the first commit)
• <author>: The person who created the commit
• <committer>: The person who committed the changes
• <commit-message>: A message describing the commit