guessCharset static method

Encoding? guessCharset(Uint8List bytes, DecodeHint? hints)

@param bytes bytes encoding a string, whose encoding should be guessed @param hints decode hints if applicable @return Charset of guessed encoding; at the moment will only guess one of: {@link #SHIFT_JIS_CHARSET}, {@link StandardCharsets#UTF_8}, {@link StandardCharsets#ISO_8859_1}, {@link StandardCharsets#UTF_16}, or the platform default encoding if none of these can possibly be correct

Implementation

static Encoding? guessCharset(
  Uint8List bytes,
  DecodeHint? hints,
) {
  if (hints?.characterSet != null) {
    return CharacterSetECI.getCharacterSetECIByName(
      hints!.characterSet!,
    )?.charset;
  }

  // First try UTF-16, assuming anything with its BOM is UTF-16
  if (bytes.length > 2 &&
      ((bytes[0] == 0xFE && bytes[1] == 0xFF) ||
          (bytes[0] == 0xFF && bytes[1] == 0xFE))) {
    return utf16; //StandardCharsets.UTF_16;
  }

  // For now, merely tries to distinguish ISO-8859-1, UTF-8 and Shift_JIS,
  // which should be by far the most common encodings.
  final length = bytes.length;
  bool canBeISO88591 = true;
  bool canBeShiftJIS = true;
  bool canBeUTF8 = true;
  int utf8BytesLeft = 0;
  int utf2BytesChars = 0;
  int utf3BytesChars = 0;
  int utf4BytesChars = 0;
  int sjisBytesLeft = 0;
  int sjisKatakanaChars = 0;
  int sjisCurKatakanaWordLength = 0;
  int sjisCurDoubleBytesWordLength = 0;
  int sjisMaxKatakanaWordLength = 0;
  int sjisMaxDoubleBytesWordLength = 0;
  int isoHighOther = 0;

  final utf8bom = bytes.length > 3 &&
      bytes[0] == 0xEF &&
      bytes[1] == 0xBB &&
      bytes[2] == 0xBF;

  for (int i = 0;
      i < length && (canBeISO88591 || canBeShiftJIS || canBeUTF8);
      i++) {
    final value = bytes[i];

    // UTF-8 stuff
    if (canBeUTF8) {
      if (utf8BytesLeft > 0) {
        if ((value & 0x80) == 0) {
          canBeUTF8 = false;
        } else {
          utf8BytesLeft--;
        }
      } else if ((value & 0x80) != 0) {
        if ((value & 0x40) == 0) {
          canBeUTF8 = false;
        } else {
          utf8BytesLeft++;
          if ((value & 0x20) == 0) {
            utf2BytesChars++;
          } else {
            utf8BytesLeft++;
            if ((value & 0x10) == 0) {
              utf3BytesChars++;
            } else {
              utf8BytesLeft++;
              if ((value & 0x08) == 0) {
                utf4BytesChars++;
              } else {
                canBeUTF8 = false;
              }
            }
          }
        }
      }
    }

    // ISO-8859-1 stuff
    if (canBeISO88591) {
      if (value > 0x7F && value < 0xA0) {
        canBeISO88591 = false;
      } else if (value > 0x9F &&
          (value < 0xC0 || value == 0xD7 || value == 0xF7)) {
        isoHighOther++;
      }
    }

    // Shift_JIS stuff
    if (canBeShiftJIS) {
      if (sjisBytesLeft > 0) {
        if (value < 0x40 || value == 0x7F || value > 0xFC) {
          canBeShiftJIS = false;
        } else {
          sjisBytesLeft--;
        }
      } else if (value == 0x80 || value == 0xA0 || value > 0xEF) {
        canBeShiftJIS = false;
      } else if (value > 0xA0 && value < 0xE0) {
        sjisKatakanaChars++;
        sjisCurDoubleBytesWordLength = 0;
        sjisCurKatakanaWordLength++;
        if (sjisCurKatakanaWordLength > sjisMaxKatakanaWordLength) {
          sjisMaxKatakanaWordLength = sjisCurKatakanaWordLength;
        }
      } else if (value > 0x7F) {
        sjisBytesLeft++;
        //sjisDoubleBytesChars++;
        sjisCurKatakanaWordLength = 0;
        sjisCurDoubleBytesWordLength++;
        if (sjisCurDoubleBytesWordLength > sjisMaxDoubleBytesWordLength) {
          sjisMaxDoubleBytesWordLength = sjisCurDoubleBytesWordLength;
        }
      } else {
        //sjisLowChars++;
        sjisCurKatakanaWordLength = 0;
        sjisCurDoubleBytesWordLength = 0;
      }
    }
  }

  if (canBeUTF8 && utf8BytesLeft > 0) {
    canBeUTF8 = false;
  }
  if (canBeShiftJIS && sjisBytesLeft > 0) {
    canBeShiftJIS = false;
  }

  // Easy -- if there is BOM or at least 1 valid not-single byte character (and no evidence it can't be UTF-8), done
  if (canBeUTF8 &&
      (utf8bom || utf2BytesChars + utf3BytesChars + utf4BytesChars > 0)) {
    return utf8;
  }
  // Easy -- if assuming Shift_JIS or >= 3 valid consecutive not-ascii characters (and no evidence it can't be), done
  if (canBeShiftJIS &&
      (_assumeShiftJis ||
          sjisMaxKatakanaWordLength >= 3 ||
          sjisMaxDoubleBytesWordLength >= 3)) {
    return shiftJisCharset;
  }
  // Distinguishing Shift_JIS and ISO-8859-1 can be a little tough for short words. The crude heuristic is:
  // - If we saw
  //   - only two consecutive katakana chars in the whole text, or
  //   - at least 10% of bytes that could be "upper" not-alphanumeric Latin1,
  // - then we conclude Shift_JIS, else ISO-8859-1
  if (canBeISO88591 && canBeShiftJIS) {
    return (sjisMaxKatakanaWordLength == 2 && sjisKatakanaChars == 2) ||
            isoHighOther * 10 >= length
        ? shiftJisCharset
        : latin1;
  }

  // Otherwise, try in order ISO-8859-1, Shift JIS, UTF-8 and fall back to default platform encoding
  if (canBeISO88591) {
    return latin1;
  }
  if (canBeShiftJIS) {
    return shiftJisCharset;
  }
  if (canBeUTF8) {
    return utf8;
  }
  // Otherwise, we take a wild guess with platform encoding
  return _platformDefaultEncoding;
}