qzxing/source/zxing/common/StringUtils.cpp

// -*- mode:c++; tab-width:2; indent-tabs-mode:nil; c-basic-offset:2 -*-

/*
 * Copyright (C) 2010-2011 ZXing authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <zxing/common/StringUtils.h>
#include <zxing/DecodeHints.h>

using namespace std;
using namespace zxing;
using namespace zxing::common;

// N.B.: these are the iconv strings for at least some versions of iconv

char const* const StringUtils::PLATFORM_DEFAULT_ENCODING = "UTF-8";
char const* const StringUtils::ASCII = "ASCII";
char const* const StringUtils::SHIFT_JIS = "SHIFT_JIS";
char const* const StringUtils::GB2312 = "GBK";
char const* const StringUtils::EUC_JP = "EUC-JP";
char const* const StringUtils::UTF8 = "UTF-8";
char const* const StringUtils::ISO88591 = "ISO8859-1";
const bool StringUtils::ASSUME_SHIFT_JIS = false;

string
StringUtils::guessEncoding(unsigned char* bytes, int length, Hashtable const& hints) {
  Hashtable::const_iterator i = hints.find(DecodeHints::CHARACTER_SET);
  if (i != hints.end()) {
    return i->second;
  }
  // Does it start with the UTF-8 byte order mark? then guess it's UTF-8
  if (length > 3 &&
      bytes[0] == (unsigned char) 0xEF &&
      bytes[1] == (unsigned char) 0xBB &&
      bytes[2] == (unsigned char) 0xBF) {
    return UTF8;
  }
  // For now, merely tries to distinguish ISO-8859-1, UTF-8 and Shift_JIS,
  // which should be by far the most common encodings. ISO-8859-1
  // should not have bytes in the 0x80 - 0x9F range, while Shift_JIS
  // uses this as a first byte of a two-byte character. If we see this
  // followed by a valid second byte in Shift_JIS, assume it is Shift_JIS.
  // If we see something else in that second byte, we'll make the risky guess
  // that it's UTF-8.
  bool canBeISO88591 = true;
  bool canBeShiftJIS = true;
  bool canBeUTF8 = true;
  int utf8BytesLeft = 0;
  int maybeDoubleByteCount = 0;
  int maybeSingleByteKatakanaCount = 0;
  bool sawLatin1Supplement = false;
  bool sawUTF8Start = false;
  bool lastWasPossibleDoubleByteStart = false;

  for (int i = 0;
       i < length && (canBeISO88591 || canBeShiftJIS || canBeUTF8);
       i++) {

    int value = bytes[i] & 0xFF;

    // UTF-8 stuff
    if (value >= 0x80 && value <= 0xBF) {
      if (utf8BytesLeft > 0) {
        utf8BytesLeft--;
      }
    } else {
      if (utf8BytesLeft > 0) {
        canBeUTF8 = false;
      }
      if (value >= 0xC0 && value <= 0xFD) {
        sawUTF8Start = true;
        int valueCopy = value;
        while ((valueCopy & 0x40) != 0) {
          utf8BytesLeft++;
          valueCopy <<= 1;
        }
      }
    }

    // ISO-8859-1 stuff

    if ((value == 0xC2 || value == 0xC3) && i < length - 1) {
      // This is really a poor hack. The slightly more exotic characters people might want to put in
      // a QR Code, by which I mean the Latin-1 supplement characters (e.g. u-umlaut) have encodings
      // that start with 0xC2 followed by [0xA0,0xBF], or start with 0xC3 followed by [0x80,0xBF].
      int nextValue = bytes[i + 1] & 0xFF;
      if (nextValue <= 0xBF &&
          ((value == 0xC2 && nextValue >= 0xA0) || (value == 0xC3 && nextValue >= 0x80))) {
        sawLatin1Supplement = true;
      }
    }
    if (value >= 0x7F && value <= 0x9F) {
      canBeISO88591 = false;
    }

    // Shift_JIS stuff

    if (value >= 0xA1 && value <= 0xDF) {
      // count the number of characters that might be a Shift_JIS single-byte Katakana character
      if (!lastWasPossibleDoubleByteStart) {
        maybeSingleByteKatakanaCount++;
      }
    }
    if (!lastWasPossibleDoubleByteStart &&
        ((value >= 0xF0 && value <= 0xFF) || value == 0x80 || value == 0xA0)) {
      canBeShiftJIS = false;
    }
    if ((value >= 0x81 && value <= 0x9F) || (value >= 0xE0 && value <= 0xEF)) {
      // These start double-byte characters in Shift_JIS. Let's see if it's followed by a valid
      // second byte.
      if (lastWasPossibleDoubleByteStart) {
        // If we just checked this and the last byte for being a valid double-byte
        // char, don't check starting on this byte. If this and the last byte
        // formed a valid pair, then this shouldn't be checked to see if it starts
        // a double byte pair of course.
        lastWasPossibleDoubleByteStart = false;
      } else {
        // ... otherwise do check to see if this plus the next byte form a valid
        // double byte pair encoding a character.
        lastWasPossibleDoubleByteStart = true;
        if (i >= length - 1) {
          canBeShiftJIS = false;
        } else {
          int nextValue = bytes[i + 1] & 0xFF;
          if (nextValue < 0x40 || nextValue > 0xFC) {
            canBeShiftJIS = false;
          } else {
            maybeDoubleByteCount++;
          }
          // There is some conflicting information out there about which bytes can follow which in
          // double-byte Shift_JIS characters. The rule above seems to be the one that matches practice.
        }
      }
    } else {
      lastWasPossibleDoubleByteStart = false;
    }
  }
  if (utf8BytesLeft > 0) {
    canBeUTF8 = false;
  }

  // Easy -- if assuming Shift_JIS and no evidence it can't be, done
  if (canBeShiftJIS && ASSUME_SHIFT_JIS) {
    return SHIFT_JIS;
  }
  if (canBeUTF8 && sawUTF8Start) {
    return UTF8;
  }
  // Distinguishing Shift_JIS and ISO-8859-1 can be a little tough. The crude heuristic is:
  // - If we saw
  //   - at least 3 bytes that starts a double-byte value (bytes that are rare in ISO-8859-1), or
  //   - over 5% of bytes could be single-byte Katakana (also rare in ISO-8859-1),
  // - and, saw no sequences that are invalid in Shift_JIS, then we conclude Shift_JIS
  if (canBeShiftJIS && (maybeDoubleByteCount >= 3 || 20 * maybeSingleByteKatakanaCount > length)) {
    return SHIFT_JIS;
  }
  // Otherwise, we default to ISO-8859-1 unless we know it can't be
  if (!sawLatin1Supplement && canBeISO88591) {
    return ISO88591;
  }
  // Otherwise, we take a wild guess with platform encoding
  return PLATFORM_DEFAULT_ENCODING;
}