Lesson 22 · The commit object

The commit object

A commit ties a tree to an author, a committer, and a message. Assemble those lines in the right order, hash them, and you get a commit id that matches real Git exactly. Today you build your first commit object.

The goal

Build a commit object for a root tree with no parent and hash it to its exact id.

Start here - the target

TO DO

Scenario: A parentless commit hashes to its exact id

Giventhe root tree b7c8f173c30a232f001cc4d77c259e4c99afbbd8, author and committer Ada Lovelace <ada@example.com> 1700000000 +0000, and the message Initial commit with a trailing newline

WhenBuildCommit assembles the body as tree line, author line, committer line, a blank line, then the message, and hashes it as a commit

Thenthe body begins with tree b7c8f173c30a232f001cc4d77c259e4c99afbbd8 and a newline, has no parent line, and the author and committer lines follow

Andthe commit id is f18e98d326b190d096879422d56d4ac5cf572db1

Background

A commit object is just structured text. The lines come in a fixed order: tree and its id, then (later) any parent lines, then the author line, the committer line, a single blank line, and finally the commit message. That is the entire format. Wrap it as a commit object and hash it, exactly as with blobs and trees.

Because every part feeds the hash, the tree id, both identity lines, and the message all determine the commit id. With our fixed identity and timestamp, this initial commit over the root tree is always f18e98d3.... You can reproduce it in real Git with git commit-tree and the same dates, and get the same id back. A commit is thus a content-addressed snapshot-plus-metadata: it names a whole tree (the entire project state) with one id.

Make it work

// "tree <id>\n" + "author <ident>\n" + "committer <ident>\n" + "\n" + message
body := "tree " + tree + "\n"
body += "author " + ident + "\n"
body += "committer " + ident + "\n"
body += "\n" + message // message already ends in "\n"
id, _ := r.WriteObject("commit", []byte(body))

CheckpointDONE

Your commit id matches real Git. Commit and stop here. Confirm with git commit-tree.