Lesson 20 · Rendering a hunk - Build a Diff Tool

Rendering a hunk

Now assemble a whole hunk as text - the @@ header with both ranges, then the body lines with their space, minus, and plus prefixes. This is the unified diff format, one hunk at a time.

The goal

Render a hunk as its @@ header line followed by its prefixed body lines.

Start here - the target

TO DO

Scenario: A single-change hunk renders in unified format

Giventhe hunk from diffing ["line1","line2","line3","line4","line5"] against the same list with "line3" changed to "CHANGED", context 3

Whenthe hunk is rendered

Thenthe header line is "@@ -1,5 +1,5 @@" (old length 5 counts Keep and Delete lines, new length 5 counts Keep and Insert lines)

Andthe body is the six lines " line1", " line2", "-line3", "+CHANGED", " line4", " line5", each ending in a newline

Background

A hunk header states, in one line, exactly what the hunk spans: @@ -oldStart,oldLen +newStart,newLen @@. The old length counts every line present in the old file - the Keeps and Deletes - and the new length counts every line present in the new file - the Keeps and Inserts. The starts are the 1-based line numbers where the hunk begins on each side, which the grouping already recorded. Feed each start and length through formatRange and you have the header.

The body is exactly the prefixed rendering you built at the end of chapter one: a space for kept lines, a minus for deleted, a plus for inserted, in edit-script order (deletes before their paired inserts). So a hunk is its header plus Format(ops) for its operations. Notice the old and new lengths differ from the number of body lines whenever a line changes - here five old and five new lines are shown across six body lines, because the changed line appears once as - and once as +. Next you wrap one or more hunks with the file header to produce a complete unified diff.

Make it work

oldLen, newLen := 0, 0
for _, op := range h.Ops {
  if op.Kind != Insert { oldLen++ } // Keep or Delete
  if op.Kind != Delete { newLen++ } // Keep or Insert
}
header := "@@ -" + formatRange(h.OldStart, oldLen) +
          " +" + formatRange(h.NewStart, newLen) + " @@\n"
// then Format(h.Ops) for the body, reusing the space/-/+ prefixes

CheckpointDONE

You can render one hunk in the unified diff format. Commit and stop here.