Lesson 19 · The empty tree

The empty tree

An empty index should still produce a valid tree, and it produces one of Git's most recognisable constants. Today you pin that boundary, the tree with no entries.

The goal

Confirm that write-tree of an empty index produces the empty tree id.

Start here - the target

TO DO

Scenario: An empty index produces the empty tree

Givenan empty index with no entries

WhenWriteTreeFromIndex builds a tree

Thenthe tree body is empty, so the object is a tree of size 0

Andits id is 4b825dc642cb6eb9a060e54bf8d69288fbee4904, the well-known empty tree

Background

A tree with no entries has an empty body, so its serialized form is just the header tree 0 and a NUL. Hash that and you get 4b825dc6..., a constant that appears throughout Git internals (it is the parent-tree of an initial commit’s diff, among other uses). There is no special case in your code: the same write-tree path that handles files handles zero files, because an empty list concatenates to empty bytes.

This is a boundary worth pinning precisely because it is easy to get subtly wrong, for example by writing nothing at all instead of a real (if empty) object, or by adding a stray byte. If your empty tree matches 4b825dc6..., your object serialization is correct right down to the degenerate case. Next lesson, subdirectories.

Make it work

// no entries -> empty body -> HashObject("tree", []byte{})
// this must equal the pinned empty-tree id
id, _ := r.WriteTreeFromIndex(NewIndex())
// id == "4b825dc642cb6eb9a060e54bf8d69288fbee4904"

CheckpointDONE

The empty tree hashes correctly. Commit and stop here.