internal/lsp/diff: rewrite ApplyEdits to work with sub-line diffs

This replaces the definition of ApplyEdits to be more like that in
go vet -fix, so that we can apply the results of suggested fixes.

Change-Id: Ib5724139464954e3790bc51ed1edc3ce4b2115ff
Reviewed-on: https://go-review.googlesource.com/c/tools/+/193959
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/diff/apply_edits.go

@@ -0,0 +1,58 @@
+package diff
+
+import (
+	"bytes"
+	"sort"
+
+	"golang.org/x/tools/internal/span"
+)
+
+func init() {
+	ApplyEdits = applyEdits
+}
+
+func applyEdits(before string, edits []TextEdit) string {
+	// Preconditions:
+	//   - all of the edits apply to before
+	//   - and all the spans for each TextEdit have the same URI
+
+	// copy edits so we don't make a mess of the caller's slice
+	s := make([]TextEdit, len(edits))
+	copy(s, edits)
+	edits = s
+
+	// TODO(matloob): Initialize the Converter Once?
+	var conv span.Converter = span.NewContentConverter("", []byte(before))
+	offset := func(point span.Point) int {
+		if point.HasOffset() {
+			return point.Offset()
+		}
+		offset, err := conv.ToOffset(point.Line(), point.Column())
+		if err != nil {
+			panic(err)
+		}
+		return offset
+	}
+
+	// sort the copy
+	sort.Slice(edits, func(i, j int) bool { return offset(edits[i].Span.Start()) < offset(edits[j].Span.Start()) })
+
+	var after bytes.Buffer
+	beforeOffset := 0
+	for _, edit := range edits {
+		if offset(edit.Span.Start()) < beforeOffset {
+			panic("overlapping edits") // TODO(matloob): ApplyEdits doesn't return an error. What do we do?
+		} else if offset(edit.Span.Start()) > beforeOffset {
+			after.WriteString(before[beforeOffset:offset(edit.Span.Start())])
+			beforeOffset = offset(edit.Span.Start())
+		}
+		// offset(edit.Span.Start) is now equal to beforeOffset
+		after.WriteString(edit.NewText)
+		beforeOffset += offset(edit.Span.End()) - offset(edit.Span.Start())
+	}
+	if beforeOffset < len(before) {
+		after.WriteString(before[beforeOffset:])
+		beforeOffset = len(before[beforeOffset:]) // just to preserve invariants
+	}
+	return after.String()
+}

internal/lsp/diff/apply_edits_test.go

@@ -0,0 +1,29 @@
+package diff
+
+import (
+	"testing"
+
+	"golang.org/x/tools/internal/span"
+)
+
+func TestApplyEdits(t *testing.T) {
+	var testCases = []struct {
+		before string
+		edits  []TextEdit
+		want   string
+	}{
+		{"", nil, ""},
+		{"X", []TextEdit{{newSpan(0, 1), "Y"}}, "Y"},
+		{" X ", []TextEdit{{newSpan(1, 2), "Y"}}, " Y "},
+		{" X X ", []TextEdit{{newSpan(1, 2), "Y"}, {newSpan(3, 4), "Z"}}, " Y Z "},
+	}
+	for _, tc := range testCases {
+		if got := applyEdits(tc.before, tc.edits); got != tc.want {
+			t.Errorf("applyEdits(%v, %v): got %v, want %v", tc.before, tc.edits, got, tc.want)
+		}
+	}
+}
+
+func newSpan(start, end int) span.Span {
+	return span.New("", span.NewPoint(0, 0, start), span.NewPoint(0, 0, end))
+}

internal/lsp/diff/myers.go

@@ -14,7 +14,6 @@ import (
 
 func init() {
 	ComputeEdits = myersComputeEdits
-	ApplyEdits = myersApplyEdits
 	ToUnified = myersToUnified
 }
 
@@ -24,11 +23,6 @@ func myersComputeEdits(uri span.URI, before, after string) []TextEdit {
 	return myersDiffToEdits(uri, myers.Operations(u, f))
 }
 
-func myersApplyEdits(before string, edits []TextEdit) string {
-	ops := myersEditsToDiff(edits)
-	return strings.Join(myers.ApplyEdits(myers.SplitLines(before), ops), "")
-}
-
 func myersToUnified(from, to string, before string, edits []TextEdit) string {
 	u := myers.SplitLines(before)
 	ops := myersEditsToDiff(edits)