/*
 * Copyright (C) 2010 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.
 */

package com.google.zxing.common;

import java.util.Hashtable;

import com.google.zxing.DecodeHintType;

/**
 * Common string-related functions.
 *
 * @author Sean Owen
 */
public final class StringUtils {

  private static final String PLATFORM_DEFAULT_ENCODING =
      System.getProperty("file.encoding");
  public static final String SHIFT_JIS = "SJIS";
  public static final String GB2312 = "GB2312";
  private static final String EUC_JP = "EUC_JP";
  private static final String UTF8 = "UTF8";
  private static final String ISO88591 = "ISO8859_1";
  private static final boolean ASSUME_SHIFT_JIS =
      SHIFT_JIS.equalsIgnoreCase(PLATFORM_DEFAULT_ENCODING) ||
      EUC_JP.equalsIgnoreCase(PLATFORM_DEFAULT_ENCODING);

  private StringUtils() {}

  /**
   * @param bytes bytes encoding a string, whose encoding should be guessed
   * @param hints decode hints if applicable
   * @return name of guessed encoding; at the moment will only guess one of:
   *  {@link #SHIFT_JIS}, {@link #UTF8}, {@link #ISO88591}, or the platform
   *  default encoding if none of these can possibly be correct
   */
  public static String guessEncoding(byte[] bytes, Hashtable hints) {
    if (hints != null) {
      String characterSet = (String) hints.get(DecodeHintType.CHARACTER_SET);
      if (characterSet != null) {
        return characterSet;
      }
    }
    // Does it start with the UTF-8 byte order mark? then guess it's UTF-8
    if (bytes.length > 3 &&
        bytes[0] == (byte) 0xEF &&
        bytes[1] == (byte) 0xBB &&
        bytes[2] == (byte) 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.
    int length = bytes.length;
    boolean canBeISO88591 = true;
    boolean canBeShiftJIS = true;
    boolean canBeUTF8 = true;
    int utf8BytesLeft = 0;
    int maybeDoubleByteCount = 0;
    int maybeSingleByteKatakanaCount = 0;
    boolean sawLatin1Supplement = false;
    boolean sawUTF8Start = false;
    boolean 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 >= bytes.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;
  }

}
