tests/compiler/dart2js/helpers/text_helpers.dart

// Copyright (c) 2018, 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.

/// Compares [text1] and [text2] line-by-line. If a mismatch is found, a diff
/// of the [windowSize] lines before and after are printed and the mismatch line
/// number is returned. If identical, nothing is printed and `null` is returned.
int checkEqualContentAndShowDiff(String text1, String text2,
    {int windowSize: 20}) {
  List<String> lines1 = text1.split('\n');
  List<String> lines2 = text2.split('\n');
  for (int i = 0; i < lines1.length && i < lines2.length; i++) {
    if (i >= lines1.length || i >= lines2.length || lines1[i] != lines2[i]) {
      for (int j = i - windowSize; j < i + windowSize; j++) {
        if (j < 0) continue;
        String line1 = 0 <= j && j < lines1.length ? lines1[j] : null;
        String line2 = 0 <= j && j < lines2.length ? lines2[j] : null;
        if (line1 == line2) {
          print('  $j $line1');
        } else {
          String text = line1 == null ? '<eof>' : line1;
          String newText = line2 == null ? '<eof>' : line2;
          print('- $j ${text}');
          print('+ $j ${newText}');
          if (text.length > 80 && newText.length > 80) {
            assert(text != newText);
            StringBuffer diff = new StringBuffer();
            diff.write('  $j ');
            for (int k = 0; k < text.length && k < newText.length; k++) {
              int char1 = k < text.length ? text.codeUnitAt(k) : null;
              int char2 = k < newText.length ? newText.codeUnitAt(k) : null;
              if (char1 != char2) {
                diff.write('^');
              } else {
                diff.write(' ');
              }
            }
            print(diff);
          }
        }
      }
      return i;
    }
  }
  return null;
}