code: /*
   * Copyright 2014 Google Inc. All rights reserved.
   *
   * 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.flatbuffers;
  
  import java.nio.ByteBuffer;
  
  import static java.lang.Character.MIN_HIGH_SURROGATE;
  import static java.lang.Character.MIN_LOW_SURROGATE;
  import static java.lang.Character.MIN_SUPPLEMENTARY_CODE_POINT;
  
  public abstract class Utf8 {
  
    /**
     * Returns the number of bytes in the UTF-8-encoded form of {@code sequence}. For a string,
     * this method is equivalent to {@code string.getBytes(UTF_8).length}, but is more efficient in
     * both time and space.
     *
     * @throws IllegalArgumentException if {@code sequence} contains ill-formed UTF-16 (unpaired
     *     surrogates)
     */
    public abstract int encodedLength(CharSequence sequence);
  
    /**
     * Encodes the given characters to the target {@link ByteBuffer} using UTF-8 encoding.
     *
     * <p>Selects an optimal algorithm based on the type of {@link ByteBuffer} (i.e. heap or direct)
     * and the capabilities of the platform.
     *
     * @param in the source string to be encoded
     * @param out the target buffer to receive the encoded string.
     */
    public abstract void encodeUtf8(CharSequence in, ByteBuffer out);
  
    /**
     * Decodes the given UTF-8 portion of the {@link ByteBuffer} into a {@link String}.
     *
     * @throws IllegalArgumentException if the input is not valid UTF-8.
     */
    public abstract String decodeUtf8(ByteBuffer buffer, int offset, int length);
  
    private static Utf8 DEFAULT;
  
    /**
     * Get the default UTF-8 processor.
     * @return the default processor
     */
    public static Utf8 getDefault() {
      if (DEFAULT == null) {
        DEFAULT = new Utf8Safe();
      }
      return DEFAULT;
    }
  
    /**
     * Set the default instance of the UTF-8 processor.
     * @param instance the new instance to use
     */
    public static void setDefault(Utf8 instance) {
      DEFAULT = instance;
    }
  
    /**
     * Utility methods for decoding bytes into {@link String}. Callers are responsible for extracting
     * bytes (possibly using Unsafe methods), and checking remaining bytes. All other UTF-8 validity
     * checks and codepoint conversion happen in this class.
     */
    static class DecodeUtil {
  
      /**
       * Returns whether this is a single-byte codepoint (i.e., ASCII) with the form '0XXXXXXX'.
       */
      static boolean isOneByte(byte b) {
        return b >= 0;
      }
  
      /**
       * Returns whether this is a two-byte codepoint with the form '10XXXXXX'.
       */
      static boolean isTwoBytes(byte b) {
        return b < (byte) 0xE0;
      }
  
      /**
       * Returns whether this is a three-byte codepoint with the form '110XXXXX'.
       */
      static boolean isThreeBytes(byte b) {
        return b < (byte) 0xF0;
      }
  
      static void handleOneByte(byte byte1, char[] resultArr, int resultPos) {
        resultArr[resultPos] = (char) byte1;
      }
  
      static void handleTwoBytes(
          byte byte1, byte byte2, char[] resultArr, int resultPos)
          throws IllegalArgumentException {
        // Simultaneously checks for illegal trailing-byte in leading position (<= '11000000') and
        // overlong 2-byte, '11000001'.
        if (byte1 < (byte) 0xC2) {
          throw new IllegalArgumentException("Invalid UTF-8: Illegal leading byte in 2 bytes utf");
        }
        if (isNotTrailingByte(byte2)) {
          throw new IllegalArgumentException("Invalid UTF-8: Illegal trailing byte in 2 bytes utf");
        }
        resultArr[resultPos] = (char) (((byte1 & 0x1F) << 6) | trailingByteValue(byte2));
      }
  
      static void handleThreeBytes(
          byte byte1, byte byte2, byte byte3, char[] resultArr, int resultPos)
          throws IllegalArgumentException {
        if (isNotTrailingByte(byte2)
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // check for illegal surrogate codepoints
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                || isNotTrailingByte(byte3)) {
          throw new IllegalArgumentException("Invalid UTF-8");
        }
        resultArr[resultPos] = (char)
                                   (((byte1 & 0x0F) << 12) | (trailingByteValue(byte2) << 6) | trailingByteValue(byte3));
      }
  
      static void handleFourBytes(
          byte byte1, byte byte2, byte byte3, byte byte4, char[] resultArr, int resultPos)
          throws IllegalArgumentException{
        if (isNotTrailingByte(byte2)
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                //   valid 4-byte leading byte?
                // if (byte1 > (byte) 0xF4 ||
                //   overlong? 4 most significant bits must not all be zero
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //   codepoint larger than the highest code point (U+10FFFF)?
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                || isNotTrailingByte(byte3)
                || isNotTrailingByte(byte4)) {
          throw new IllegalArgumentException("Invalid UTF-8");
        }
        int codepoint = ((byte1 & 0x07) << 18)
                            | (trailingByteValue(byte2) << 12)
                            | (trailingByteValue(byte3) << 6)
                            | trailingByteValue(byte4);
        resultArr[resultPos] = DecodeUtil.highSurrogate(codepoint);
        resultArr[resultPos + 1] = DecodeUtil.lowSurrogate(codepoint);
      }
  
      /**
       * Returns whether the byte is not a valid continuation of the form '10XXXXXX'.
       */
      private static boolean isNotTrailingByte(byte b) {
        return b > (byte) 0xBF;
      }
  
      /**
       * Returns the actual value of the trailing byte (removes the prefix '10') for composition.
       */
      private static int trailingByteValue(byte b) {
        return b & 0x3F;
      }
  
      private static char highSurrogate(int codePoint) {
        return (char) ((MIN_HIGH_SURROGATE - (MIN_SUPPLEMENTARY_CODE_POINT >>> 10))
                           + (codePoint >>> 10));
      }
  
      private static char lowSurrogate(int codePoint) {
        return (char) (MIN_LOW_SURROGATE + (codePoint & 0x3ff));
      }
    }
  
    // These UTF-8 handling methods are copied from Guava's Utf8Unsafe class with a modification to throw
    // a protocol buffer local exception. This exception is then caught in CodedOutputStream so it can
    // fallback to more lenient behavior.
    static class UnpairedSurrogateException extends IllegalArgumentException {
      UnpairedSurrogateException(int index, int length) {
        super("Unpaired surrogate at index " + index + " of " + length);
      }
    }
  }
  