Improve the performance of applying edits

I wrote a test that generates 10,000 lines of code, each containing one
violation of `prefer_single_quotes`, computes the fixes for all of the
diagnostics, and applies the 20,000 edits, measuring how long the
application takes.

Before this change the average was 3355.0 ms.

After this change the average is 3.8 ms.

The change was inspired by Jens.

Change-Id: Icd54a81ca3848fd5f0168934f713a7be8caec359
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425601
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analysis_server/lib/src/protocol/protocol_internal.dart

@@ -23,11 +23,41 @@ final Map<String, RefactoringKind> REQUEST_ID_REFACTORING_KINDS =
 /// Get the result of applying a set of [edits] to the given [code].  Edits
 /// are applied in the order they appear in [edits].  Access via
 /// SourceEdit.applySequence().
-String applySequenceOfEdits(String code, Iterable<SourceEdit> edits) {
-  for (var edit in edits) {
-    code = edit.apply(code);
+String applySequenceOfEdits(String code, List<SourceEdit> edits) {
+  var buffer = StringBuffer();
+  var start = 0;
+  for (var i = edits.length - 1; i >= 0; i--) {
+    var edit = edits[i];
+    var offset = edit.offset;
+    var length = edit.length;
+    if (length < 0) {
+      throw RangeError('The edit length is negative.');
+    }
+    if (offset + length > code.length) {
+      throw RangeError('The edit extends past the end of the code.');
+    }
+    if (start > offset) {
+      // One of the edits overlaps with another, requiring that they be applied
+      // from largest offset to smallest. This should only be possible in code
+      // that creates source edits without using the `ChangeBuilder` to do so.
+      //
+      // We should consider fixing the places where overlapping edits are
+      // produced so that this branch can be removed. One such place is
+      // exhibited by `_DoCompletionTest.test_noBody`.
+      for (var edit in edits) {
+        code = edit.apply(code);
+      }
+      return code;
+    } else if (start < offset) {
+      buffer.write(code.substring(start, offset));
+    }
+    buffer.write(edit.replacement);
+    start = offset + length;
+  }
+  if (start < code.length) {
+    buffer.write(code.substring(start));
   }
-  return code;
+  return buffer.toString();
 }
 
 /// Compare the lists [listA] and [listB], using [itemEqual] to compare

pkg/analysis_server/lib/src/services/correction/dart/flutter_convert_to_stateful_widget.dart

@@ -113,7 +113,7 @@ class FlutterConvertToStatefulWidget extends ResolvedCorrectionProducer {
         linesRange,
       );
       movedNode.accept(visitor);
-      return SourceEdit.applySequence(text, visitor.edits.reversed);
+      return SourceEdit.applySequence(text, visitor.edits.reversed.toList());
     }
 
     var statefulWidgetClass = await sessionHelper.getFlutterClass(

pkg/analysis_server/lib/src/services/correction/dart/flutter_convert_to_stateless_widget.dart

@@ -135,7 +135,7 @@ class FlutterConvertToStatelessWidget extends ResolvedCorrectionProducer {
         linesRange,
       );
       movedNode.accept(visitor);
-      return SourceEdit.applySequence(text, visitor.edits.reversed);
+      return SourceEdit.applySequence(text, visitor.edits.reversed.toList());
     }
 
     var statelessWidgetClass = await sessionHelper.getFlutterClass(

pkg/analysis_server/tool/spec/codegen_dart_protocol.dart

@@ -903,7 +903,7 @@ class CodegenProtocolVisitor extends DartCodegenVisitor with CodeGenerator {
           ),
         ]);
         writeln(
-          'static String applySequence(String code, Iterable<SourceEdit> edits) =>',
+          'static String applySequence(String code, List<SourceEdit> edits) =>',
         );
         writeln('    applySequenceOfEdits(code, edits);');
         return true;

pkg/analysis_server_client/lib/src/protocol/protocol_common.dart

@@ -3489,7 +3489,7 @@ class SourceChange implements HasToJson {
 class SourceEdit implements HasToJson {
   /// Get the result of applying a set of [edits] to the given [code]. Edits
   /// are applied in the order they appear in [edits].
-  static String applySequence(String code, Iterable<SourceEdit> edits) =>
+  static String applySequence(String code, List<SourceEdit> edits) =>
       applySequenceOfEdits(code, edits);
 
   /// The offset of the region to be modified.

pkg/analysis_server_client/lib/src/protocol/protocol_internal.dart

@@ -76,11 +76,41 @@ String applyEdit(String code, SourceEdit edit) {
 /// Get the result of applying a set of [edits] to the given [code]. Edits
 /// are applied in the order they appear in [edits]. Access via
 /// SourceEdit.applySequence().
-String applySequenceOfEdits(String code, Iterable<SourceEdit> edits) {
-  for (var edit in edits) {
-    code = edit.apply(code);
+String applySequenceOfEdits(String code, List<SourceEdit> edits) {
+  var buffer = StringBuffer();
+  var start = 0;
+  for (var i = edits.length - 1; i >= 0; i--) {
+    var edit = edits[i];
+    var offset = edit.offset;
+    var length = edit.length;
+    if (length < 0) {
+      throw RangeError('The edit length is negative.');
+    }
+    if (offset + length > code.length) {
+      throw RangeError('The edit extends past the end of the code.');
+    }
+    if (start > offset) {
+      // One of the edits overlaps with another, requiring that they be applied
+      // from largest offset to smallest. This should only be possible in code
+      // that creates source edits without using the `ChangeBuilder` to do so.
+      //
+      // We should consider fixing the places where overlapping edits are
+      // produced so that this branch can be removed. One such place is
+      // exhibited by `_DoCompletionTest.test_noBody`.
+      for (var edit in edits) {
+        code = edit.apply(code);
+      }
+      return code;
+    } else if (start < offset) {
+      buffer.write(code.substring(start, offset));
+    }
+    buffer.write(edit.replacement);
+    start = offset + length;
+  }
+  if (start < code.length) {
+    buffer.write(code.substring(start));
   }
-  return code;
+  return buffer.toString();
 }
 
 /// Returns the [FileEdit] for the given [file], maybe `null`.

pkg/analyzer_plugin/lib/protocol/protocol_common.dart

@@ -3569,7 +3569,7 @@ class SourceChange implements HasToJson {
 class SourceEdit implements HasToJson {
   /// Get the result of applying a set of [edits] to the given [code]. Edits
   /// are applied in the order they appear in [edits].
-  static String applySequence(String code, Iterable<SourceEdit> edits) =>
+  static String applySequence(String code, List<SourceEdit> edits) =>
       applySequenceOfEdits(code, edits);
 
   /// The offset of the region to be modified.

pkg/analyzer_plugin/lib/src/protocol/protocol_internal.dart

@@ -109,11 +109,41 @@ String applyEdit(String code, SourceEdit edit) {
 /// Get the result of applying a set of [edits] to the given [code]. Edits
 /// are applied in the order they appear in [edits]. Access via
 /// SourceEdit.applySequence().
-String applySequenceOfEdits(String code, Iterable<SourceEdit> edits) {
-  for (var edit in edits) {
-    code = edit.apply(code);
+String applySequenceOfEdits(String code, List<SourceEdit> edits) {
+  var buffer = StringBuffer();
+  var start = 0;
+  for (var i = edits.length - 1; i >= 0; i--) {
+    var edit = edits[i];
+    var offset = edit.offset;
+    var length = edit.length;
+    if (length < 0) {
+      throw RangeError('The edit length is negative.');
+    }
+    if (offset + length > code.length) {
+      throw RangeError('The edit extends past the end of the code.');
+    }
+    if (start > offset) {
+      // One of the edits overlaps with another, requiring that they be applied
+      // from largest offset to smallest. This should only be possible in code
+      // that creates source edits without using the `ChangeBuilder` to do so.
+      //
+      // We should consider fixing the places where overlapping edits are
+      // produced so that this branch can be removed. One such place is
+      // exhibited by `_DoCompletionTest.test_noBody`.
+      for (var edit in edits) {
+        code = edit.apply(code);
+      }
+      return code;
+    } else if (start < offset) {
+      buffer.write(code.substring(start, offset));
+    }
+    buffer.write(edit.replacement);
+    start = offset + length;
+  }
+  if (start < code.length) {
+    buffer.write(code.substring(start));
   }
-  return code;
+  return buffer.toString();
 }
 
 /// Returns the [FileEdit] for the given [file], maybe `null`.

pkg/analyzer_plugin/tool/spec/codegen_dart_protocol.dart

@@ -839,7 +839,7 @@ class CodegenProtocolVisitor extends DartCodegenVisitor with CodeGenerator {
               '[code]. Edits are applied in the order they appear in [edits].')
         ]);
         writeln(
-            'static String applySequence(String code, Iterable<SourceEdit> edits) =>');
+            'static String applySequence(String code, List<SourceEdit> edits) =>');
         writeln('    applySequenceOfEdits(code, edits);');
         return true;
       default: