Abstrace parent class converts encoded bytes to codepoints.

class Utf32BytesDecoder implements _ListRangeIterator {
  final _ListRangeIterator utf32EncodedBytesIterator;
  final int replacementCodepoint;

  Utf32BytesDecoder._fromListRangeIterator(
      this.utf32EncodedBytesIterator, this.replacementCodepoint);

  factory Utf32BytesDecoder(List<int> utf32EncodedBytes, [
      int offset = 0, int length,
      int replacementCodepoint = UNICODE_REPLACEMENT_CHARACTER_CODEPOINT]) {
    if (length == null) {
      length = utf32EncodedBytes.length - offset;
    }
    if (hasUtf32beBom(utf32EncodedBytes, offset, length)) {
      return new Utf32beBytesDecoder(utf32EncodedBytes, offset + 4, length - 4,
          false, replacementCodepoint);
    } else if (hasUtf32leBom(utf32EncodedBytes, offset, length)) {
      return new Utf32leBytesDecoder(utf32EncodedBytes, offset + 4, length - 4,
          false, replacementCodepoint);
    } else {
      return new Utf32beBytesDecoder(utf32EncodedBytes, offset, length, false,
          replacementCodepoint);
    }
  }

  List<int> decodeRest() {
    List<int> codeunits = new List<int>(remaining);
    int i = 0;
    while (hasNext()) {
      codeunits[i++] = next();
    }
    return codeunits;
  }

  bool hasNext() => utf32EncodedBytesIterator.hasNext();

  int next() {
    if (utf32EncodedBytesIterator.remaining < 4) {
      utf32EncodedBytesIterator.skip(utf32EncodedBytesIterator.remaining);
      if (replacementCodepoint != null) {
          return replacementCodepoint;
      } else {
        throw new ArgumentError(
            "Invalid UTF32 at ${utf32EncodedBytesIterator.position}");
      }
    } else {
      int codepoint = decode();
      if (_validCodepoint(codepoint)) {
        return codepoint;
      } else if (replacementCodepoint != null) {
          return replacementCodepoint;
      } else {
        throw new ArgumentError(
            "Invalid UTF32 at ${utf32EncodedBytesIterator.position}");
      }
    }
  }

  int get position => utf32EncodedBytesIterator.position ~/ 4;

  void backup([int by = 1]) {
    utf32EncodedBytesIterator.backup(4 * by);
  }

  int get remaining => (utf32EncodedBytesIterator.remaining + 3) ~/ 4;

  void skip([int count = 1]) {
    utf32EncodedBytesIterator.skip(4 * count);
  }

  abstract int decode();
}