Lesson 11 · Subdirectories are entries pointing at trees

Subdirectories are entries pointing at trees

A directory inside a directory is just an entry whose mode is 40000 and whose id is another tree. Today you nest trees, turning the flat directory listing into a real tree of directories.

The goal

Build a tree that contains a subdirectory entry pointing at another tree.

Start here - the target

TO DO

Scenario: A tree references a subtree by id

Givena subtree for src holding main.go at 06ab7d0f9a35a7d1070711496d6ca1cb892a258f (mode 100644)

Whenthat subtree is written, then a root tree is built with entries hello.txt, README.md, and a src entry of mode 40000 pointing at the subtree id

Thenthe src subtree id is b9270df7070cc6a5e7dbdec610a7ce4f54c47b20

Andthe root tree id is b7c8f173c30a232f001cc4d77c259e4c99afbbd8

Background

Nothing new is needed to represent nested directories: a subdirectory is simply a tree entry whose mode is 40000 and whose id is the id of another tree. That other tree is written first (it is a normal object in the store), and the parent just references it by id. This is why Git can share unchanged subdirectories across commits for free: if src/ did not change, its subtree id is identical, so the parent points at the very same object.

So a repository’s whole directory structure is a tree of trees rooted at one top-level tree, with blobs at the leaves. The root tree id b7c8f173... is a single fingerprint for an entire directory hierarchy: change main.go and the src subtree id changes, which changes the root tree id. Next lesson we handle a subtle ordering rule that these mixed file-and-directory listings depend on.

Make it work

// a directory entry: mode "40000", name = dir name, id = subtree id
sub, _ := r.WriteTree([]Entry{{Mode: "100644", Name: "main.go", Id: mainId}})
root, _ := r.WriteTree([]Entry{
  {Mode: "100644", Name: "hello.txt", Id: helloId},
  {Mode: "100644", Name: "README.md", Id: readmeId},
  {Mode: "40000", Name: "src", Id: sub},
})

CheckpointDONE

You can build nested trees. Commit and stop here. These ids match git write-tree.