Lesson 20 · write-tree with subdirectories

write-tree with subdirectories

The full write-tree groups index paths by their directory, recursively builds a subtree for each, and links them into the root tree. Today you complete it, turning a flat list of paths into a real tree of directories.

The goal

Turn an index with subdirectory paths into nested tree objects and a root tree.

Start here - the target

TO DO

Scenario: A nested index becomes a tree of trees

Givenan index holding hello.txt, README.md, and src/main.go (main.go at 06ab7d0f9a35a7d1070711496d6ca1cb892a258f)

WhenWriteTreeFromIndex groups paths by their first path segment, recursively builds a tree for the src group, and links it as a mode-40000 entry named src

Thenthe src subtree id is b9270df7070cc6a5e7dbdec610a7ce4f54c47b20 and the root tree id is b7c8f173c30a232f001cc4d77c259e4c99afbbd8

Andevery tree it builds (root and each subtree) is written to the store

Background

This is the heart of write-tree. Group the index entries by their first path segment: a path with no slash (hello.txt) is a file entry in the current tree, while a path like src/main.go belongs to the src group with the remaining part (main.go) as its path inside that group. Recurse on each group to build its subtree, then add a 40000 entry naming the directory and pointing at the subtree’s id. The recursion bottoms out at directories containing only files.

Because every subtree is written to the store as it is built, the whole hierarchy is persisted, and the root tree id is a single fingerprint for the entire staged snapshot. Stage hello.txt, README.md, and src/main.go in a real Git repository and run git write-tree: it returns the same b7c8f173.... Your staging area now produces byte-identical trees to Git, which is exactly what a commit needs.

Make it work

// group entries by first segment: no slash -> a file entry here;
// "dir/rest" -> collect under "dir", recurse on the rest, add a 40000 entry
func (r *Repo) buildTree(entries map[string]IndexEntry) (string, error) {
  // files: Entry{mode,name,id}; dirs: recurse -> subId, Entry{"40000",dir,subId}
}

CheckpointDONE

write-tree builds the full nested tree from the index. Commit and stop here. Stage the same files in real Git and git write-tree gives the identical id.