deduplicator transformer

main.dart

// SO 29681639

import 'dart:convert';
import 'dart:async' show Future, Stream;
import 'dart:math' show Random;

final rnd = new Random();
main() {
  lineStream()
      .transform(new Utf8Decoder())
      .transform(new Deduplicator())
      .take(20).listen((string) => print(string));
}

Stream<List<int>> lineStream() async* {
  while(true) {
    yield intList().take(rnd.nextInt(100)).toList();
  }
}

Iterable<int> intList() sync* {
  final string = '0112333456777888999abcccddddddeeeefff';
  for(var c in string.codeUnits) {
    yield c;
  }
}

class Deduplicator extends Converter<String, List<String>> {

  const Deduplicator();

  List<String> convert(String data) {
    var lines = new List<String>();

    _DeduplicatorSink._addSlice(data, 0, data.length, true, null, lines.add);

    return lines;
  }

  StringConversionSink startChunkedConversion(Sink<String> sink) {
    if (sink is! StringConversionSink) {
      sink = new StringConversionSink.from(sink);
    }
    return new _DeduplicatorSink(sink);
  }
}

class _DeduplicatorSink extends StringConversionSinkBase {
  final StringConversionSink _sink;
  String _carry;

  _DeduplicatorSink(this._sink);

  void addSlice(String chunk, int start, int end, bool isLast) {
    _carry = _addSlice(chunk, start, end, isLast, _carry, _sink.add);
    if (isLast) _sink.close();
  }

  void close() {
    addSlice('', 0, 0, true);
  }

  static String _addSlice(String chunk, int start, int end, bool isLast, String carry,
                          void adder(String val)) {
    var buf = new StringBuffer();
    int pos = start;
    while(pos < end) {
      String char = chunk[pos];
      if(char != carry) {
        carry = char;
        buf.write(char);
      }
      pos++;
    }
    adder(buf.toString());
    return carry;
  }
}