gopls/internal/fake: sort edits by both start and end position

When processing edits, they must be non-overlapping but they may be
adjacent: we must allow for insertions adjacent to replacements, e.g. an
edit of 3:1-3:1 followed by an edit of 3:1-3:15. Achieve this in the
fake editor by sorting edits by end position after start position.

Longer term, we should eliminate this ad-hoc editing in favor of
diff.ApplyEdit.

Change-Id: I72a252952585d0a652d97287486aa61c167db485
Reviewed-on: https://go-review.googlesource.com/c/tools/+/431219
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dylan Le <dungtuanle@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

gopls/internal/lsp/fake/edit.go

@@ -110,17 +110,23 @@ func inText(p Pos, content []string) bool {
 // invalid for the current content.
 //
 // TODO(rfindley): this function does not handle non-ascii text correctly.
+// TODO(rfindley): replace this with diff.ApplyEdits: we should not be
+// maintaining an additional representation of edits.
 func editContent(content []string, edits []Edit) ([]string, error) {
 	newEdits := make([]Edit, len(edits))
 	copy(newEdits, edits)
-	sort.Slice(newEdits, func(i, j int) bool {
-		if newEdits[i].Start.Line < newEdits[j].Start.Line {
-			return true
+	sort.SliceStable(newEdits, func(i, j int) bool {
+		ei := newEdits[i]
+		ej := newEdits[j]
+
+		// Sort by edit start position followed by end position. Given an edit
+		// 3:1-3:1 followed by an edit 3:1-3:15, we must process the empty edit
+		// first.
+		if cmp := comparePos(ei.Start, ej.Start); cmp != 0 {
+			return cmp < 0
 		}
-		if newEdits[i].Start.Line > newEdits[j].Start.Line {
-			return false
-		}
-		return newEdits[i].Start.Column < newEdits[j].Start.Column
+
+		return comparePos(ei.End, ej.End) < 0
 	})
 
 	// Validate edits.
@@ -157,3 +163,20 @@ func editContent(content []string, edits []Edit) ([]string, error) {
 	advance(len(content)-1, len([]rune(content[len(content)-1])))
 	return strings.Split(b.String(), "\n"), nil
 }
+
+// comparePos returns -1 if left < right, 0 if left == right, and 1 if left > right.
+func comparePos(left, right Pos) int {
+	if left.Line < right.Line {
+		return -1
+	}
+	if left.Line > right.Line {
+		return 1
+	}
+	if left.Column < right.Column {
+		return -1
+	}
+	if left.Column > right.Column {
+		return 1
+	}
+	return 0
+}