gopls/diff/unified: remove redundant information

Sometimes the unified diff would show the same line removed and added.
That is avoided by this CL.

Fixes: golang/go#59232

Change-Id: Ie6562e3c922b8f0c4319eefdde1913b2d9bc7878
Reviewed-on: https://go-review.googlesource.com/c/tools/+/489695
Run-TryBot: Peter Weinberger <pjw@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/diff/diff.go

@@ -115,8 +115,8 @@ func lineEdits(src string, edits []Edit) ([]Edit, error) {
 	}
 
 	// Do all edits begin and end at the start of a line?
-	// TODO(adonovan): opt: is this fast path necessary?
-	// (Also, it complicates the result ownership.)
+	// TODO(adonovan, pjw): why does omitting this 'optimization'
+	// cause tests to fail? (TestDiff/insert-line,extra_newline)
 	for _, edit := range edits {
 		if edit.Start >= len(src) || // insertion at EOF
 			edit.Start > 0 && src[edit.Start-1] != '\n' || // not at line start

internal/diff/difftest/difftest.go

@@ -272,6 +272,13 @@ var TestCases = []struct {
 -
  A
 `,
+	}, {
+		Name:      "unified_lines",
+		In:        "aaa\nccc\n",
+		Out:       "aaa\nbbb\nccc\n",
+		Edits:     []diff.Edit{{Start: 3, End: 3, New: "\nbbb"}},
+		LineEdits: []diff.Edit{{Start: 0, End: 4, New: "aaa\nbbb\n"}},
+		Unified:   UnifiedPrefix + "@@ -1,2 +1,3 @@\n aaa\n+bbb\n ccc\n",
 	},
 }
 

internal/diff/unified.go

@@ -156,9 +156,18 @@ func toUnified(fromName, toName string, content string, edits []Edit) (unified,
 			last++
 		}
 		if edit.New != "" {
-			for _, content := range splitLines(edit.New) {
-				h.Lines = append(h.Lines, line{Kind: Insert, Content: content})
+			for i, content := range splitLines(edit.New) {
 				toLine++
+				// Merge identical Delete+Insert.
+				// This is an unwanted output of converting diffs to line diffs
+				// that is easiest to fix by postprocessing.
+				// e.g.  issue #59232: ("aaa\nccc\n", "aaa\nbbb\nccc")
+				// -> [Delete "aaa\n", Insert "aaa\n", Insert "bbb\n", ...].
+				if i == 0 && last > start && h.Lines[len(h.Lines)-1].Content == content {
+					h.Lines[len(h.Lines)-1].Kind = Equal
+					continue
+				}
+				h.Lines = append(h.Lines, line{Kind: Insert, Content: content})
 			}
 		}
 	}