Lesson 02 · The edit operation

The edit operation

A diff is a list of edit operations that turn one document into another - keep a line, delete a line, insert a line. Today you define that operation type and write the first, trivial diff - two identical documents need no edits at all, just keeps.

The goal

Define an edit operation and make Diff return an all-keep script when the two inputs are identical.

Start here - the target

TO DO

Scenario: Diffing a document against itself keeps every line

Giventwo identical documents ["a", "b", "c"] and ["a", "b", "c"]

WhenDiff is called on them

Thenit returns three operations, each a Keep: Keep "a", Keep "b", Keep "c"

Anddiffing two empty documents returns an empty edit script

Background

A diff is best understood as a program that edits one document into another. That program is a sequence of three kinds of operation: Keep a line that appears in both, Delete a line that is in the old document but not the new, and Insert a line that is in the new but not the old. Read the operations top to bottom and you walk from the old document to the new one. This []Op edit script is the central value our whole library produces, formats, and applies.

Today’s Diff only handles the easy case - two identical documents, where the answer is simply “keep everything.” That is the walking skeleton: a real public Diff you can call and test right now, even though it is not yet correct for documents that actually differ. The next lessons make it handle deletions, then insertions, then arbitrary changes. Define Op with room for all three kinds now, even though we only emit Keep today.

Make it work

type Kind int
const (
  Keep Kind = iota
  Insert
  Delete
)
type Op struct {
  Kind Kind
  Line string
}
// for now, assume the inputs are identical
func Diff(a, b []string) []Op {
  ops := make([]Op, 0, len(a))
  for _, line := range a {
    ops = append(ops, Op{Keep, line})
  }
  return ops
}

CheckpointDONE

You have an Op type and a Diff that handles identical inputs. Commit and stop here.