internal/lsp: add tests at the diff hook layer

this makes sure that any diff implementation obeys the semantics we expect
at higher layers

Change-Id: Iae8842cfb9fece94ea71c04ec146d825eff0cbeb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/191017
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/diff/diff_test.go

@@ -0,0 +1,15 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package diff_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/internal/lsp/diff/difftest"
+)
+
+func TestDiff(t *testing.T) {
+	difftest.DiffTest(t)
+}

internal/lsp/diff/difftest/difftest.go

@@ -0,0 +1,51 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package difftest supplies a set of tests that will operate on any
+// implementation of a diff algorithm as exposed by
+// "golang.org/x/tools/internal/lsp/diff"
+package difftest
+
+import (
+	"testing"
+
+	"golang.org/x/tools/internal/lsp/diff"
+	"golang.org/x/tools/internal/span"
+)
+
+func DiffTest(t *testing.T) {
+	t.Helper()
+	for _, test := range []struct{ name, in, out, unified string }{{
+		name: "empty",
+		in:   "",
+		out:  "",
+	}, {
+		name: "no_diff",
+		in:   "gargantuan",
+		out:  "gargantuan",
+	}, {
+		name: "insert_rune",
+		in:   "gord",
+		out:  "gourd",
+	}, {
+		name: "delete_rune",
+		in:   "groat",
+		out:  "goat",
+	}, {
+		name: "replace_rune",
+		in:   "loud",
+		out:  "lord",
+	}, {
+		name: "insert_line",
+		in:   "one\nthree\n",
+		out:  "one\ntwo\nthree\n",
+	}} {
+		edits := diff.ComputeEdits(span.FileURI("/"+test.name), test.in, test.out)
+		got := diff.ApplyEdits(test.in, edits)
+		if got != test.out {
+			t.Logf("test %v had diff:%v\n", test.name, diff.ToUnified(test.name+".orig", test.name, test.in, edits))
+			t.Errorf("diff %v got:\n%v\nexpected:\n%v", test.name, got, test.out)
+		}
+	}
+}

internal/lsp/diff/myers/diff.go

@@ -67,6 +67,10 @@ func ApplyEdits(a []string, operations []*Op) []string {
 // Operations returns the list of operations to convert a into b, consolidating
 // operations for multiple lines and not including equal lines.
 func Operations(a, b []string) []*Op {
+	if len(a) == 0 && len(b) == 0 {
+		return nil
+	}
+
 	trace, offset := shortestEditSequence(a, b)
 	snakes := backtrack(trace, len(a), len(b), offset)