Lesson 25 · Reading a commit

Reading a commit

To walk history we need to read a commit back into its parts: the tree it points at, its parents, and its message. Today you parse a commit object, the inverse of building one.

The goal

Parse a commit object into its tree, parents, and message.

Start here - the target

TO DO

Scenario: A commit object parses into its fields

Giventhe stored commits f18e98d326b190d096879422d56d4ac5cf572db1 and 9bae7f0a71d91d794187cf9de39901bfbfbfc7b6

WhenReadCommit(id) parses each

Thenthe first has tree b7c8f173c30a232f001cc4d77c259e4c99afbbd8, no parents, and message Initial commit

Andthe second has tree 4ab82d44e36709a311a91646e9fb09c1f35af3d4, one parent f18e98d326b190d096879422d56d4ac5cf572db1, and message Update greeting

Background

Parsing a commit mirrors parsing a tree, but it is line-oriented text. Split the body at the first blank line: everything before it is headers, everything after is the message. Each header line is a keyword and a value: tree with one id, zero or more parent lines each with an id, and the author and committer lines. Collect the parents into a list in the order they appear.

This gives us everything we need to traverse the commit graph. A commit knows its tree (so we can inspect the snapshot) and its parents (so we can walk backward through history). With reading in hand, log becomes a simple loop: start at a commit, print it, follow the first parent, repeat. We build that once we have refs to tell us where to start.

Make it work

// header lines until the blank line: "tree X", "parent Y", "author ...";
// everything after the blank line is the message
func (r *Repo) ReadCommit(id string) (*Commit, error) {
  _, _, body, _ := r.CatFile(id)
  // split on the first blank line; parse "key value" header lines
}

CheckpointDONE

You can read a commit's tree, parents, and message. Commit and stop here.