Lesson 13 · Pretty-printing a tree

Pretty-printing a tree

To end the chapter with something you can read, we format a tree the way git cat-file -p does: mode, type, id, and name, one entry per line. This also surfaces where Git pads the mode for display.

The goal

Format a tree's entries as human-readable lines matching git cat-file -p.

Start here - the target

TO DO

Scenario: A tree prints one readable line per entry

Giventhe root tree b7c8f173c30a232f001cc4d77c259e4c99afbbd8

WhenPrintTree(id) formats each entry as the mode padded to 6 digits, a space, the type, a space, the id, a tab, then the name

Thenthe first line is 100644 space blob space 0805455a24b6c68fbc38d0fa5d121f735984285d, a tab, then README.md

Andthe src line shows the padded mode 040000 and type tree (040000 tree b9270df7070cc6a5e7dbdec610a7ce4f54c47b20, tab, src), the type being tree when the mode is 40000 and blob otherwise

Background

Now the trees are readable. For display, Git pads the mode to six digits, so the directory mode we store as 40000 prints as 040000, while 100644 is already six digits and is unchanged. Note the split: the mode on disk (inside the hashed tree body) has no leading zero, but the mode Git shows you does. Mixing these up is a classic source of confusion, and pinning both keeps them straight.

The type is derived, not stored: an entry with mode 40000 points at a tree, anything else at a blob. Lay it out as mode, type, id, a tab, then the name, and you have reproduced git cat-file -p for trees. Run that command against your root tree id and the output matches line for line, entries in the same sorted order. That is the whole chapter proven at once: your trees are real Git trees.

Make it work

// mode padded left with zeros to width 6; type from mode
typ := "blob"
if e.Mode == "40000" { typ = "tree" }
line := fmt.Sprintf("%06s %s %s\t%s", e.Mode, typ, e.Id, e.Name)

CheckpointDONE

You can pretty-print a tree exactly like git cat-file -p. Commit and stop here. Compare your output to git cat-file -p on the same id.