Lesson 32 · log: walking the parent chain

log: walking the parent chain

log is the payoff of the commit graph: start at HEAD and follow parent links back through history. Today you walk the chain and list commits newest first.

The goal

Walk from HEAD through parent links, listing commit ids newest to oldest.

Start here - the target

TO DO

Scenario: log lists commits from HEAD to the root

Giventhe repository with two commits, HEAD resolving to 9bae7f0a71d91d794187cf9de39901bfbfbfc7b6 whose parent is f18e98d326b190d096879422d56d4ac5cf572db1

WhenLog() starts at the HEAD commit and follows the first parent until there is none

Thenit returns the ids in order 9bae7f0a71d91d794187cf9de39901bfbfbfc7b6 then f18e98d326b190d096879422d56d4ac5cf572db1

Andthe walk stops at the parentless first commit rather than looping or erroring

Background

log is where the graph you have been building becomes visible. Resolve HEAD to a commit, then walk: read the commit, emit its id, follow its first parent, and repeat until you reach a commit with no parent. That yields history newest first, which is how git log presents it. The traversal is pure graph-walking over objects you can already read.

Following only the first parent keeps this linear, which is all a simple history needs; a merge commit’s other parents would branch the walk, but our chain has none. The stopping condition is the parentless root commit, and getting that right means the walk terminates cleanly instead of dereferencing an empty parent. Two commits in, Log() returns 9bae7f0a... then f18e98d3..., the same order and ids git log --format=%H prints.

Make it work

// start at Resolve("HEAD"); ReadCommit; emit; follow parents[0]; repeat
func (r *Repo) Log() ([]string, error) {
  id, _ := r.Resolve("HEAD")
  for id != "" {
    // append id; c, _ := r.ReadCommit(id)
    // id = first parent, or "" if none
  }
}

CheckpointDONE

You can walk history with log. Commit and stop here. Your order matches git log --format=%H.