Lesson 03 · Diffing against an empty document

Diffing against an empty document

Before the general algorithm, pin the two extreme cases every diff must get right - when one side is empty. All-delete when the new document is empty, all-insert when the old one is. These edges anchor everything.

The goal

Make Diff emit all deletions when the new document is empty, and all insertions when the old document is empty.

Start here - the target

TO DO

Scenario: An empty side yields a pure-delete or pure-insert script

Giventhe old document ["a", "b"] and an empty new document []

WhenDiff is called

Thenit returns Delete "a", Delete "b"

Anddiffing an empty old document [] against ["x", "y"] returns Insert "x", Insert "y"

Background

The two documents can differ in only so many ways, and the simplest are the extremes: the new document is empty (delete everything), or the old document is empty (insert everything). Getting these right now does two things - it makes Diff correct on a whole class of real inputs (clearing a file, creating a file), and it forces the Delete and Insert operations into use so they are exercised before the harder cases arrive.

Notice the asymmetry: a Delete carries a line from the old document, an Insert carries a line from the new one. A Keep, when we get to mixed diffs, carries a line that is in both. Keep this rule straight - it is what makes an edit script replayable: reading only the Keep and Insert lines reconstructs the new document, reading only the Keep and Delete lines reconstructs the old one.

Make it work

// handle the empty-side cases before the general algorithm
if len(b) == 0 {
  // every old line must be deleted
  ...
}
if len(a) == 0 {
  // every new line must be inserted
  ...
}

CheckpointDONE

Diff handles both empty-side extremes. Commit and stop here.