Lesson 26 · Applying a patch - Build a Diff Tool

Applying a patch

Now close the loop - take a parsed patch and an original document and produce the target. Applying walks the hunk against the original, copying context, dropping deletions, and splicing in insertions.

The goal

Apply parsed hunks to the original document to reconstruct the target.

Start here - the target

TO DO

Scenario: Applying a diff reconstructs the target

Giventhe original ["line1","line2","line3","line4","line5"] and the patch parsed from its unified diff against the same list with "line3" changed to "CHANGED"

WhenApply runs the patch on the original

Thenit returns ["line1","line2","CHANGED","line4","line5"], the target

Andmore generally, for any documents a and b, Apply(a, Parse(UnifiedDiff("a","b", aText, bText, 3))) equals the lines of b

Background

Applying a patch replays the edit script against the original document. Walk each hunk in order: first copy the untouched lines before it (up to the hunk’s OldStart), then process its operations - a Keep copies the current old line and advances, a Delete skips the current old line without emitting it, and an Insert emits the new line without touching the old cursor. After the last hunk, copy whatever unchanged lines remain. The result is the target document, rebuilt from the original plus the patch.

This is the property the whole library has been building toward: apply(a, diff(a, b)) == b. A diff is a faithful, replayable recipe, and applying it is deterministic - no searching, no guessing, because your own diff’s line numbers line up exactly with the original. The only thing that could go wrong is applying a patch to the wrong original, where the context no longer matches - and detecting exactly that is the next lesson, the difference between a toy and a real patch tool.

Make it work

result := []string{}
oldPos := 0 // 0-based cursor into a
for _, h := range hunks {
  for oldPos < h.OldStart-1 { // copy lines before the hunk
    result = append(result, a[oldPos]); oldPos++
  }
  for _, op := range h.Ops {
    switch op.Kind {
    case Keep:   result = append(result, a[oldPos]); oldPos++
    case Delete: oldPos++            // skip the deleted line
    case Insert: result = append(result, op.Line)
    }
  }
}
result = append(result, a[oldPos:]...) // trailing unchanged lines

CheckpointDONE

Applying your own diff reproduces the target. Commit and stop here.