packages/barback/example/lazy_transformer/lib/transformer.dart

// Copyright (c) 2014, 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:barback/barback.dart';

import 'dart:async';

class CodedMessageConverter extends Transformer implements LazyTransformer {
  // A constructor named "asPlugin" is required. It can be empty, but
  // it must be present.
  CodedMessageConverter.asPlugin();

  Future<bool> isPrimary(AssetId id) async => id.extension == '.txt';

  void declareOutputs(DeclaringTransform transform) {
    transform.declareOutput(transform.primaryId.changeExtension('.shhhhh'));
  }

  Future apply(Transform transform) async {
    var content = await transform.primaryInput.readAsString();

    // The output file is created with the '.shhhhh' extension.
    var id = transform.primaryInput.id.changeExtension('.shhhhh');

    var newContent = new StringBuffer();
    for (var i = 0; i < content.length; i++) {
      newContent.write(rot13(content[i]));
    }
    transform.addOutput(new Asset.fromString(id, newContent.toString()));
  }

  rot13(var ch) {
    var c = ch.codeUnitAt(0);
    if (c >= 'a'.codeUnitAt(0) && c <= 'm'.codeUnitAt(0)) {
      c += 13;
    } else if (c >= 'A'.codeUnitAt(0) && c <= 'M'.codeUnitAt(0)) {
      c += 13;
    } else if (c >= 'n'.codeUnitAt(0) && c <= 'z'.codeUnitAt(0)) {
      c -= 13;
    } else if (c >= 'N'.codeUnitAt(0) && c <= 'Z'.codeUnitAt(0)) {
      c -= 13;
    }
    return new String.fromCharCode(c);
  }
}