pkg/analysis_server/lib/src/edit/nnbd_migration/offset_mapper.dart

// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analyzer_plugin/protocol/protocol_common.dart';

/// An object that can map the offsets before a sequence of edits to the offsets
/// after applying the edits.
abstract class OffsetMapper {
  /// A mapper used for files that were not modified.
  static OffsetMapper identity = _IdentityMapper();

  /// Return a mapper representing the file modified by the given [edits].
  factory OffsetMapper.forEdits(List<SourceEdit> edits) => _EditMapper(edits);

  /// Return the post-edit offset that corresponds to the given pre-edit
  /// [offset].
  int map(int offset);
}

/// A mapper used for files that were modified by a set of edits.
class _EditMapper implements OffsetMapper {
  /// A list whose elements are the highest pre-edit offset for which the
  /// corresponding element of [_deltas] should be applied.
  List<int> _offsets = [];

  /// A list whose elements are the deltas to be applied for all pre-edit
  /// offsets that are less than or equal to the corresponding element of
  /// [_offsets].
  List<int> _deltas = [];

  /// Initialize a newly created mapper based on the given set of [edits].
  _EditMapper(List<SourceEdit> edits) {
    _initializeDeltas(edits);
  }

  @override
  int map(int offset) => offset + _deltaFor(offset);

  /// Return the delta to be added to the pre-edit [offset] to produce the
  /// post-edit offset.
  int _deltaFor(int offset) {
    for (int i = 0; i < _offsets.length; i++) {
      int currentOffset = _offsets[i];
      if (currentOffset >= offset || currentOffset < 0) {
        return _deltas[i];
      }
    }
    // We should never get here because [_initializeDeltas] always adds an
    // offset/delta pair at the end of the list whose offset is less than zero.
    return 0;
  }

  /// Initialize the list of old offsets and deltas used by [_deltaFor].
  void _initializeDeltas(List<SourceEdit> edits) {
    int previousDelta = 0;
    for (SourceEdit edit in edits) {
      int offset = edit.offset;
      int length = edit.length;
      _offsets.add(offset);
      _deltas.add(previousDelta);
      previousDelta += (edit.replacement.length - length);
    }
    _offsets.add(-1);
    _deltas.add(previousDelta);
  }
}

/// A mapper used for files that were not modified.
class _IdentityMapper implements OffsetMapper {
  @override
  int map(int offset) => offset;
}