Lesson 18 · write-tree for a flat index

write-tree for a flat index

write-tree turns the staging area into a tree object, the bridge from index to commit. Today you handle the flat case, an index with no subdirectories, and its id matches the tree you built by hand earlier.

The goal

Turn a flat index into one tree object.

Start here - the target

TO DO

Scenario: A flat index becomes a tree

Givenan index holding hello.txt (100644, ce013625030ba8dba906f756967f9e9ca394464a) and README.md (100644, 0805455a24b6c68fbc38d0fa5d121f735984285d), neither in a subdirectory

WhenWriteTreeFromIndex builds a tree from the index entries

Thenit returns 3aa9b583db8437a8dabb60b4b4c86ae87c17de85, the same tree those two files produced earlier

Andthat tree object is stored, so cat-file can read it back

Background

write-tree is the operation that snapshots the staging area into a real object. For a flat index (no subdirectories) it is almost trivial: every entry becomes a tree entry with the same mode, name, and id, and you hand them to the WriteTree you already built, which sorts and hashes them. The payoff is the point: an index staged from hello.txt and README.md produces the identical tree id you got by constructing those entries directly, 3aa9b583....

This closes the loop between the index and the object store: staged files become a content-addressed tree, ready to be committed. The next lesson handles the harder case, an index whose paths reach into subdirectories, which is where write-tree has to build nested trees.

Make it work

// each index entry with no slash in its path is one file entry
func (r *Repo) WriteTreeFromIndex(ix *Index) (string, error) {
  var entries []Entry
  for _, p := range ix.Paths() {
    e := ix.entries[p]
    entries = append(entries, Entry{Mode: e.Mode, Name: p, Id: e.Id})
  }
  return r.WriteTree(entries)
}

CheckpointDONE

You can turn a flat index into a tree. Commit and stop here.