// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd

package com.google.protobuf;

import static com.google.protobuf.UnsafeUtil.addressOffset;
import static com.google.protobuf.UnsafeUtil.hasUnsafeArrayOperations;
import static com.google.protobuf.UnsafeUtil.hasUnsafeByteBufferOperations;
import static java.lang.Character.MAX_SURROGATE;
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;
import static java.lang.Character.MIN_SURROGATE;
import static java.lang.Character.isSurrogatePair;
import static java.lang.Character.toCodePoint;

import java.nio.BufferOverflowException;
import java.nio.ByteBuffer;
import java.util.Arrays;

/**
 * A set of low-level, high-performance static utility methods related to the UTF-8 character
 * encoding. This class has no dependencies outside of the core JDK libraries.
 *
 * <p>There are several variants of UTF-8. The one implemented by this class is the restricted
 * definition of UTF-8 introduced in Unicode 3.1, which mandates the rejection of "overlong" byte
 * sequences as well as rejection of 3-byte surrogate codepoint byte sequences. Note that the UTF-8
 * decoder included in Oracle's JDK has been modified to also reject "overlong" byte sequences, but
 * (as of 2011) still accepts 3-byte surrogate codepoint byte sequences.
 *
 * <p>The byte sequences considered valid by this class are exactly those that can be roundtrip
 * converted to Strings and back to bytes using the UTF-8 charset, without loss:
 *
 * <pre>{@code
 * Arrays.equals(bytes, new String(bytes, Internal.UTF_8).getBytes(Internal.UTF_8))
 * }</pre>
 *
 * <p>See the Unicode Standard,</br> Table 3-6. <em>UTF-8 Bit Distribution</em>,</br> Table 3-7.
 * <em>Well Formed UTF-8 Byte Sequences</em>.
 *
 * @author martinrb@google.com (Martin Buchholz)
 */
final class Utf8 {

  /**
   * UTF-8 is a runtime hot spot so we attempt to provide heavily optimized implementations
   * depending on what is available on the platform. The processor is the platform-optimized
   * delegate for which all methods are delegated directly to.
   */
  private static final Processor processor =
      (UnsafeProcessor.isAvailable() && !Android.isOnAndroidDevice())
          ? new UnsafeProcessor()
          : new SafeProcessor();

  /**
   * A mask used when performing unsafe reads to determine if a long value contains any non-ASCII
   * characters (i.e. any byte >= 0x80).
   */
  private static final long ASCII_MASK_LONG = 0x8080808080808080L;

  /**
   * Maximum number of bytes per Java UTF-16 char in UTF-8.
   *
   * @see java.nio.charset.CharsetEncoder#maxBytesPerChar()
   */
  static final int MAX_BYTES_PER_CHAR = 3;

  /**
   * Used by {@code Unsafe} UTF-8 string validation logic to determine the minimum string length
   * above which to employ an optimized algorithm for counting ASCII characters. The reason for this
   * threshold is that for small strings, the optimization may not be beneficial or may even
   * negatively impact performance since it requires additional logic to avoid unaligned reads (when
   * calling {@code Unsafe.getLong}). This threshold guarantees that even if the initial offset is
   * unaligned, we're guaranteed to make at least one call to {@code Unsafe.getLong()} which
   * provides a performance improvement that entirely subsumes the cost of the additional logic.
   */
  private static final int UNSAFE_COUNT_ASCII_THRESHOLD = 16;

  /**
   * Returns {@code true} if the given byte array is a well-formed UTF-8 byte sequence.
   *
   * <p>This is a convenience method, equivalent to a call to {@code isValidUtf8(bytes, 0,
   * bytes.length)}.
   */
  static boolean isValidUtf8(byte[] bytes) {
    return processor.isValidUtf8(bytes, 0, bytes.length);
  }

  /**
   * Returns {@code true} if the given byte array slice is a well-formed UTF-8 byte sequence. The
   * range of bytes to be checked extends from index {@code index}, inclusive, to {@code limit},
   * exclusive.
   */
  static boolean isValidUtf8(byte[] bytes, int index, int limit) {
    return processor.isValidUtf8(bytes, index, limit);
  }

  /**
   * Determines if the given {@link ByteBuffer} is a valid UTF-8 string.
   *
   * <p>Selects an optimal algorithm based on the type of {@link ByteBuffer} (i.e. heap or direct)
   * and the capabilities of the platform.
   *
   * @param buffer the buffer to check.
   * @see Utf8#isValidUtf8(byte[], int, int)
   */
  static boolean isValidUtf8(ByteBuffer buffer) {
    int index = buffer.position();
    int limit = index + buffer.remaining();

    if (buffer.hasArray()) {
      final int offset = buffer.arrayOffset();
      return processor.isValidUtf8(buffer.array(), offset + index, offset + limit);
    } else if (buffer.isDirect()) {
      return processor.isValidUtf8BufferDirect(buffer, index, limit);
    } else {
      return processor.isValidUtf8BufferDefault(buffer, index, limit);
    }
  }

  // These UTF-8 handling methods are copied from Guava's Utf8 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.

  private static class UnpairedSurrogateException extends Exception {
    UnpairedSurrogateException(int index, int length) {
      super("Unpaired surrogate at index " + index + " of " + length);
    }
  }

  /**
   * 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.
   */
  static int encodedLength(String string) {
    // Warning to maintainers: this implementation is highly optimized.
    int utf16Length = string.length();
    int utf8Length = utf16Length;
    int i = 0;

    // This loop optimizes for pure ASCII.
    while (i < utf16Length && string.charAt(i) < 0x80) {
      i++;
    }

    // This loop optimizes for chars less than 0x800.
    for (; i < utf16Length; i++) {
      char c = string.charAt(i);
      if (c < 0x800) {
        utf8Length += ((0x7f - c) >>> 31); // branch free!
      } else {
        try {
          utf8Length += encodedLengthGeneral(string, i);
        } catch (UnpairedSurrogateException e) {
          // Our hand rolled loops don't handle unpaired surrogates here. This should be
          // exceptionally rare, so we fallback to the naive implementation to find out the
          // length that the Java internal implementation will return for this string after
          // replacement characters.
          return string.getBytes(Internal.UTF_8).length;
        }
        break;
      }
    }

    if (utf8Length < utf16Length) {
      // Necessary and sufficient condition for overflow because of maximum 3x expansion
      throw new IllegalArgumentException(
          "UTF-8 length does not fit in int: " + (utf8Length + (1L << 32)));
    }
    return utf8Length;
  }

  private static int encodedLengthGeneral(String string, int start)
      throws UnpairedSurrogateException {
    int utf16Length = string.length();
    int utf8Length = 0;
    for (int i = start; i < utf16Length; i++) {
      char c = string.charAt(i);
      if (c < 0x800) {
        utf8Length += (0x7f - c) >>> 31; // branch free!
      } else {
        utf8Length += 2;
        // jdk7+: if (Character.isSurrogate(c)) {
        if (Character.MIN_SURROGATE <= c && c <= Character.MAX_SURROGATE) {
          // Check that we have a well-formed surrogate pair.
          int cp = Character.codePointAt(string, i);
          if (cp < MIN_SUPPLEMENTARY_CODE_POINT) {
            throw new UnpairedSurrogateException(i, utf16Length);
          }
          i++;
        }
      }
    }
    return utf8Length;
  }

  static int encode(String in, byte[] out, int offset, int length) {
    return processor.encodeUtf8(in, out, offset, length);
  }

  // End Guava UTF-8 methods.

  /**
   * Decodes the given UTF-8 portion of the {@link ByteBuffer} into a {@link String}.
   *
   * @throws InvalidProtocolBufferException if the input is not valid UTF-8.
   */
  static String decodeUtf8(ByteBuffer buffer, int index, int size)
      throws InvalidProtocolBufferException {
    if (size == 0) {
      return "";
    }
    return processor.decodeUtf8(buffer, index, size);
  }

  /**
   * Decodes the given UTF-8 encoded byte array slice into a {@link String}.
   *
   * @throws InvalidProtocolBufferException if the input is not valid UTF-8.
   */
  static String decodeUtf8(byte[] bytes, int index, int size)
      throws InvalidProtocolBufferException {
    if (size == 0) {
      return "";
    }
    return processor.decodeUtf8(bytes, index, size);
  }

  /**
   * 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.
   * @see Utf8#encode(String, byte[], int, int)
   */
  static void encodeUtf8(String in, ByteBuffer out) {
    processor.encodeUtf8(in, out);
  }

  /**
   * Counts (approximately) the number of consecutive ASCII characters in the given buffer. The byte
   * order of the {@link ByteBuffer} does not matter, so performance can be improved if native byte
   * order is used (i.e. no byte-swapping in {@link ByteBuffer#getLong(int)}).
   *
   * @param buffer the buffer to be scanned for ASCII chars
   * @param index the starting index of the scan
   * @param limit the limit within buffer for the scan
   * @return the number of ASCII characters found. The stopping position will be at or before the
   *     first non-ASCII byte.
   */
  private static int estimateConsecutiveAscii(ByteBuffer buffer, int index, int limit) {
    int i = index;
    final int lim = limit - 7;
    // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
    // To speed things up further, we're reading longs instead of bytes so we use a mask to
    // determine if any byte in the current long is non-ASCII.
    for (; i < lim && (buffer.getLong(i) & ASCII_MASK_LONG) == 0; i += 8) {}
    return i - index;
  }

  /** A processor of UTF-8 strings, providing methods for checking validity and encoding. */
  abstract static class Processor {
    /**
     * Returns {@code true} if the given byte array slice is a well-formed UTF-8 byte sequence. The
     * range of bytes to be checked extends from index {@code index}, inclusive, to {@code limit},
     * exclusive.
     */
    abstract boolean isValidUtf8(byte[] bytes, int index, int limit);

    /**
     * Must only be called on Direct buffers. This exists as a separate method only so that the
     * UnsafeProcessor can optimize specially for that case.
     */
    protected boolean isValidUtf8BufferDirect(ByteBuffer buffer, int index, int limit) {
      return isValidUtf8BufferDefault(buffer, index, limit);
    }

    /**
     * Returns {@code true} if the given portion of the {@link ByteBuffer} is a well-formed UTF-8
     * byte sequence. The range of bytes to be checked extends from index {@code index}, inclusive,
     * to {@code limit}, exclusive.
     */
    protected boolean isValidUtf8BufferDefault(ByteBuffer buffer, int index, int limit) {
      index += estimateConsecutiveAscii(buffer, index, limit);

      for (; ; ) {
        // Optimize for interior runs of ASCII bytes.
        int byte1;
        do {
          if (index >= limit) {
            return true;
          }
        } while ((byte1 = buffer.get(index++)) >= 0);

        // If we're here byte1 is not ASCII. Only need to handle 2-4 byte forms.
        if (byte1 < (byte) 0xE0) {
          // Two-byte form (110xxxxx 10xxxxxx)
          if (index >= limit) {
            // Incomplete sequence
            return false;
          }

          // Simultaneously checks for illegal trailing-byte in
          // leading position and overlong 2-byte form.
          if (byte1 < (byte) 0xC2 || buffer.get(index) > (byte) 0xBF) {
            return false;
          }
          index++;
        } else if (byte1 < (byte) 0xF0) {
          // Three-byte form (1110xxxx 10xxxxxx 10xxxxxx)
          if (index >= limit - 1) {
            // Incomplete sequence
            return false;
          }

          byte byte2 = buffer.get(index++);
          if (byte2 > (byte) 0xBF
              // 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)
              // byte3 trailing-byte test
              || buffer.get(index) > (byte) 0xBF) {
            return false;
          }
          index++;
        } else {
          // Four-byte form (1110xxxx 10xxxxxx 10xxxxxx 10xxxxxx)
          if (index >= limit - 2) {
            // Incomplete sequence
            return false;
          }

          // TODO: Consider using getInt() to improve performance.
          int byte2 = buffer.get(index++);
          if (byte2 > (byte) 0xBF
              // Check that 1 <= plane <= 16.  Tricky optimized form of:
              // if (byte1 > (byte) 0xF4 ||
              //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
              //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
              || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
              // byte3 trailing-byte test
              || buffer.get(index++) > (byte) 0xBF
              // byte4 trailing-byte test
              || buffer.get(index++) > (byte) 0xBF) {
            return false;
          }
        }
      }
    }

    /**
     * Decodes the given byte array slice into a {@link String}.
     *
     * @throws InvalidProtocolBufferException if the byte array slice is not valid UTF-8
     */
    abstract String decodeUtf8(byte[] bytes, int index, int size)
        throws InvalidProtocolBufferException;

    /**
     * Decodes the given portion of the {@link ByteBuffer} into a {@link String}.
     *
     * @throws InvalidProtocolBufferException if the portion of the buffer is not valid UTF-8
     */
    final String decodeUtf8(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException {
      if (buffer.hasArray()) {
        final int offset = buffer.arrayOffset();
        return decodeUtf8(buffer.array(), offset + index, size);
      } else if (buffer.isDirect()) {
        return decodeUtf8Direct(buffer, index, size);
      }
      return decodeUtf8Default(buffer, index, size);
    }

    /** Decodes direct {@link ByteBuffer} instances into {@link String}. */
    abstract String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException;

    /**
     * Decodes {@link ByteBuffer} instances using the {@link ByteBuffer} API rather than potentially
     * faster approaches.
     */
    final String decodeUtf8Default(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException {
      // Bitwise OR combines the sign bits so any negative value fails the check.
      if ((index | size | buffer.limit() - index - size) < 0) {
        throw new ArrayIndexOutOfBoundsException(
            String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, size));
      }

      int offset = index;
      int limit = offset + size;

      // The longest possible resulting String is the same as the number of input bytes, when it is
      // all ASCII. For other cases, this over-allocates and we will truncate in the end.
      char[] resultArr = new char[size];
      int resultPos = 0;

      // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      while (offset < limit) {
        byte b = buffer.get(offset);
        if (!DecodeUtil.isOneByte(b)) {
          break;
        }
        offset++;
        DecodeUtil.handleOneByte(b, resultArr, resultPos++);
      }

      while (offset < limit) {
        byte byte1 = buffer.get(offset++);
        if (DecodeUtil.isOneByte(byte1)) {
          DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
          // It's common for there to be multiple ASCII characters in a run mixed in, so add an
          // extra optimized loop to take care of these runs.
          while (offset < limit) {
            byte b = buffer.get(offset);
            if (!DecodeUtil.isOneByte(b)) {
              break;
            }
            offset++;
            DecodeUtil.handleOneByte(b, resultArr, resultPos++);
          }
        } else if (DecodeUtil.isTwoBytes(byte1)) {
          if (offset >= limit) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleTwoBytes(
              byte1, /* byte2 */ buffer.get(offset++), resultArr, resultPos++);
        } else if (DecodeUtil.isThreeBytes(byte1)) {
          if (offset >= limit - 1) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleThreeBytes(
              byte1,
              /* byte2 */ buffer.get(offset++),
              /* byte3 */ buffer.get(offset++),
              resultArr,
              resultPos++);
        } else {
          if (offset >= limit - 2) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleFourBytes(
              byte1,
              /* byte2 */ buffer.get(offset++),
              /* byte3 */ buffer.get(offset++),
              /* byte4 */ buffer.get(offset++),
              resultArr,
              resultPos++);
          // 4-byte case requires two chars.
          resultPos++;
        }
      }

      return new String(resultArr, 0, resultPos);
    }

    protected int encodeUtf8Naive(String in, byte[] out, int offset, int length) {
      byte[] bytes = in.getBytes(Internal.UTF_8);
      if (bytes.length - offset > length) {
        throw new ArrayIndexOutOfBoundsException(
            "Not enough space in output buffer to encode UTF-8 string");
      }
      System.arraycopy(bytes, 0, out, offset, bytes.length);
      return offset + bytes.length;
    }

    protected void encodeUtf8Naive(String in, ByteBuffer out) {
      final byte[] bytes = in.getBytes(Internal.UTF_8);
      try {
        out.put(bytes);
      } catch (BufferOverflowException unused) {
        throw new ArrayIndexOutOfBoundsException(
            "Not enough space in output buffer to encode UTF-8 string");
      }
    }

    /**
     * Encodes an input character sequence ({@code in}) to UTF-8 in the target array ({@code out}).
     * For a string, this method is functionally identical to
     *
     * <pre>{@code
     * byte[] a = string.getBytes(UTF_8);
     * System.arraycopy(a, 0, bytes, offset, a.length);
     * return offset + a.length;
     * }</pre>
     *
     * but may be implemented differently for efficiency purposes.
     *
     * <p>Matching {@code String.getBytes(UTF_8)} this replaces unpaired surrogates with a
     * replacement character.
     *
     * <p>To ensure sufficient space in the output buffer, either call {@link #encodedLength} to
     * compute the exact amount needed, or leave room for {@code Utf8.MAX_BYTES_PER_CHAR *
     * sequence.length()}, which is the largest possible number of bytes that any input can be
     * encoded to.
     *
     * @param in the input character sequence to be encoded
     * @param out the target array
     * @param offset the starting offset in {@code bytes} to start writing at
     * @param length the length of the {@code bytes}, starting from {@code offset}
     * @throws ArrayIndexOutOfBoundsException if {@code sequence} encoded in UTF-8 is longer than
     *     {@code bytes.length - offset}
     * @return the new offset, equivalent to {@code offset + Utf8.encodedLength(sequence)}
     */
    abstract int encodeUtf8(String in, byte[] out, int offset, int length);

    /**
     * Encodes an input character sequence ({@code in}) to UTF-8 in the target buffer ({@code out}).
     * Upon returning from this method, the {@code out} position will point to the position after
     * the last encoded byte. This method requires paired surrogates, and therefore does not support
     * chunking.
     *
     * <p>To ensure sufficient space in the output buffer, either call {@link #encodedLength} to
     * compute the exact amount needed, or leave room for {@code Utf8.MAX_BYTES_PER_CHAR *
     * in.length()}, which is the largest possible number of bytes that any input can be encoded to.
     *
     * @param in the source character sequence to be encoded
     * @param out the target buffer
     * @throws ArrayIndexOutOfBoundsException if {@code in} encoded in UTF-8 is longer than {@code
     *     out.remaining()}
     */
    final void encodeUtf8(String in, ByteBuffer out) {
      if (out.hasArray()) {
        final int offset = out.arrayOffset();
        int endIndex = Utf8.encode(in, out.array(), offset + out.position(), out.remaining());
        Java8Compatibility.position(out, endIndex - offset);
      } else {
        encodeUtf8Internal(in, out);
      }
    }

    /** Encodes the input character sequence to a direct {@link ByteBuffer} instance. */
    protected abstract void encodeUtf8Internal(String in, ByteBuffer out);
  }

  /** {@link Processor} implementation that does not use any {@code sun.misc.Unsafe} methods. */
  static final class SafeProcessor extends Processor {
    @Override
    public boolean isValidUtf8(byte[] bytes, int index, int limit) {
      // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      while (index < limit && bytes[index] >= 0) {
        index++;
      }

      if (index >= limit) {
        return true;
      }

      return isValidUtf8NonAscii(bytes, index, limit);
    }

    @Override
    String decodeUtf8(byte[] bytes, int index, int size) throws InvalidProtocolBufferException {
      // Bitwise OR combines the sign bits so any negative value fails the check.
      if ((index | size | bytes.length - index - size) < 0) {
        throw new ArrayIndexOutOfBoundsException(
            String.format("buffer length=%d, index=%d, size=%d", bytes.length, index, size));
      }

      int offset = index;
      final int limit = offset + size;

      // The longest possible resulting String is the same as the number of input bytes, when it is
      // all ASCII. For other cases, this over-allocates and we will truncate in the end.
      char[] resultArr = new char[size];
      int resultPos = 0;

      // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      while (offset < limit) {
        byte b = bytes[offset];
        if (!DecodeUtil.isOneByte(b)) {
          break;
        }
        offset++;
        DecodeUtil.handleOneByte(b, resultArr, resultPos++);
      }

      while (offset < limit) {
        byte byte1 = bytes[offset++];
        if (DecodeUtil.isOneByte(byte1)) {
          DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
          // It's common for there to be multiple ASCII characters in a run mixed in, so add an
          // extra optimized loop to take care of these runs.
          while (offset < limit) {
            byte b = bytes[offset];
            if (!DecodeUtil.isOneByte(b)) {
              break;
            }
            offset++;
            DecodeUtil.handleOneByte(b, resultArr, resultPos++);
          }
        } else if (DecodeUtil.isTwoBytes(byte1)) {
          if (offset >= limit) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleTwoBytes(byte1, /* byte2 */ bytes[offset++], resultArr, resultPos++);
        } else if (DecodeUtil.isThreeBytes(byte1)) {
          if (offset >= limit - 1) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleThreeBytes(
              byte1,
              /* byte2 */ bytes[offset++],
              /* byte3 */ bytes[offset++],
              resultArr,
              resultPos++);
        } else {
          if (offset >= limit - 2) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleFourBytes(
              byte1,
              /* byte2 */ bytes[offset++],
              /* byte3 */ bytes[offset++],
              /* byte4 */ bytes[offset++],
              resultArr,
              resultPos++);
          // 4-byte case requires two chars.
          resultPos++;
        }
      }

      return new String(resultArr, 0, resultPos);
    }

    @Override
    String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException {
      // For safe processing, we have to use the ByteBufferAPI.
      return decodeUtf8Default(buffer, index, size);
    }

    @Override
    int encodeUtf8(String in, byte[] out, int offset, int length) {
      int utf16Length = in.length();
      int j = offset;
      int i = 0;
      int limit = offset + length;
      // Designed to take advantage of
      // https://wiki.openjdk.java.net/display/HotSpotInternals/RangeCheckElimination
      for (char c; i < utf16Length && i + j < limit && (c = in.charAt(i)) < 0x80; i++) {
        out[j + i] = (byte) c;
      }
      if (i == utf16Length) {
        return j + utf16Length;
      }
      j += i;
      for (char c; i < utf16Length; i++) {
        c = in.charAt(i);
        if (c < 0x80 && j < limit) {
          out[j++] = (byte) c;
        } else if (c < 0x800 && j <= limit - 2) { // 11 bits, two UTF-8 bytes
          out[j++] = (byte) ((0xF << 6) | (c >>> 6));
          out[j++] = (byte) (0x80 | (0x3F & c));
        } else if ((c < Character.MIN_SURROGATE || Character.MAX_SURROGATE < c) && j <= limit - 3) {
          // Maximum single-char code point is 0xFFFF, 16 bits, three UTF-8 bytes
          out[j++] = (byte) ((0xF << 5) | (c >>> 12));
          out[j++] = (byte) (0x80 | (0x3F & (c >>> 6)));
          out[j++] = (byte) (0x80 | (0x3F & c));
        } else if (j <= limit - 4) {
          // Minimum code point represented by a surrogate pair is 0x10000, 17 bits,
          // four UTF-8 bytes
          final char low;
          if (i + 1 == in.length() || !Character.isSurrogatePair(c, (low = in.charAt(++i)))) {
            return encodeUtf8Naive(in, out, offset, length);
          }
          int codePoint = Character.toCodePoint(c, low);
          out[j++] = (byte) ((0xF << 4) | (codePoint >>> 18));
          out[j++] = (byte) (0x80 | (0x3F & (codePoint >>> 12)));
          out[j++] = (byte) (0x80 | (0x3F & (codePoint >>> 6)));
          out[j++] = (byte) (0x80 | (0x3F & codePoint));
        } else {
          // If we are surrogates and we're not a surrogate pair, retry with the replacement
          // characters encoder instead of an ArrayOutOfBoundsException.
          if ((Character.MIN_SURROGATE <= c && c <= Character.MAX_SURROGATE)
              && (i + 1 == in.length() || !Character.isSurrogatePair(c, in.charAt(i + 1)))) {
            return encodeUtf8Naive(in, out, offset, length);
          }
          throw new ArrayIndexOutOfBoundsException(
              "Not enough space in output buffer to encode UTF-8 string");
        }
      }
      return j;
    }

    @Override
    protected void encodeUtf8Internal(String in, ByteBuffer out) {
      final int inLength = in.length();
      int outIx = out.position();
      int inIx = 0;

      // Since ByteBuffer.putXXX() already checks boundaries for us, no need to explicitly check
      // access. Assume the buffer is big enough and let it handle the out of bounds exception
      // if it occurs.
      try {
        // Designed to take advantage of
        // https://wiki.openjdk.java.net/display/HotSpotInternals/RangeCheckElimination
        for (char c; inIx < inLength && (c = in.charAt(inIx)) < 0x80; ++inIx) {
          out.put(outIx + inIx, (byte) c);
        }
        if (inIx == inLength) {
          // Successfully encoded the entire string.
          Java8Compatibility.position(out, outIx + inIx);
          return;
        }

        outIx += inIx;
        for (char c; inIx < inLength; ++inIx, ++outIx) {
          c = in.charAt(inIx);
          if (c < 0x80) {
            // One byte (0xxx xxxx)
            out.put(outIx, (byte) c);
          } else if (c < 0x800) {
            // Two bytes (110x xxxx 10xx xxxx)

            // Benchmarks show put performs better than putShort here (for HotSpot).
            out.put(outIx++, (byte) (0xC0 | (c >>> 6)));
            out.put(outIx, (byte) (0x80 | (0x3F & c)));
          } else if (c < MIN_SURROGATE || MAX_SURROGATE < c) {
            // Three bytes (1110 xxxx 10xx xxxx 10xx xxxx)
            // Maximum single-char code point is 0xFFFF, 16 bits.

            // Benchmarks show put performs better than putShort here (for HotSpot).
            out.put(outIx++, (byte) (0xE0 | (c >>> 12)));
            out.put(outIx++, (byte) (0x80 | (0x3F & (c >>> 6))));
            out.put(outIx, (byte) (0x80 | (0x3F & c)));
          } else {
            // Four bytes (1111 xxxx 10xx xxxx 10xx xxxx 10xx xxxx)

            // Minimum code point represented by a surrogate pair is 0x10000, 17 bits, four UTF-8
            // bytes
            final char low;
            if (inIx + 1 == inLength || !isSurrogatePair(c, (low = in.charAt(++inIx)))) {
              // Unpaired surrogate, fall back to the naive encoder that will do replacement
              // characters.
              encodeUtf8Naive(in, out);
              return;
            }
            // TODO: Consider using putInt() to improve performance.
            int codePoint = toCodePoint(c, low);
            out.put(outIx++, (byte) ((0xF << 4) | (codePoint >>> 18)));
            out.put(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 12))));
            out.put(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 6))));
            out.put(outIx, (byte) (0x80 | (0x3F & codePoint)));
          }
        }

        // Successfully encoded the entire string.
        Java8Compatibility.position(out, outIx);
      } catch (IndexOutOfBoundsException unused) {
        // TODO: Consider making the API throw IndexOutOfBoundsException instead.
        throw new ArrayIndexOutOfBoundsException(
            "Not enough space in output buffer to encode UTF-8 string");
      }
    }

    private static boolean isValidUtf8NonAscii(byte[] bytes, int index, int limit) {
      for (; ; ) {
        int byte1;
        int byte2;

        // Optimize for interior runs of ASCII bytes.
        do {
          if (index >= limit) {
            return true;
          }
        } while ((byte1 = bytes[index++]) >= 0);

        if (byte1 < (byte) 0xE0) {
          // two-byte form

          if (index >= limit) {
            // Incomplete sequence
            return false;
          }

          // Simultaneously checks for illegal trailing-byte in
          // leading position and overlong 2-byte form.
          if (byte1 < (byte) 0xC2 || bytes[index++] > (byte) 0xBF) {
            return false;
          }
        } else if (byte1 < (byte) 0xF0) {
          // three-byte form

          if (index >= limit - 1) { // incomplete sequence
            return false;
          }
          if ((byte2 = bytes[index++]) > (byte) 0xBF
              // 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)
              // byte3 trailing-byte test
              || bytes[index++] > (byte) 0xBF) {
            return false;
          }
        } else {
          // four-byte form

          if (index >= limit - 2) { // incomplete sequence
            return false;
          }
          if ((byte2 = bytes[index++]) > (byte) 0xBF
              // Check that 1 <= plane <= 16.  Tricky optimized form of:
              // if (byte1 > (byte) 0xF4 ||
              //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
              //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
              || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
              // byte3 trailing-byte test
              || bytes[index++] > (byte) 0xBF
              // byte4 trailing-byte test
              || bytes[index++] > (byte) 0xBF) {
            return false;
          }
        }
      }
    }
  }

  /** {@link Processor} that uses {@code sun.misc.Unsafe} where possible to improve performance. */
  static final class UnsafeProcessor extends Processor {
    /** Indicates whether or not all required unsafe operations are supported on this platform. */
    static boolean isAvailable() {
      return hasUnsafeArrayOperations() && hasUnsafeByteBufferOperations();
    }

    @Override
    public boolean isValidUtf8(byte[] bytes, final int index, final int limit) {
      // Bitwise OR combines the sign bits so any negative value fails the check.
      if ((index | limit | bytes.length - limit) < 0) {
        throw new ArrayIndexOutOfBoundsException(
            String.format("Array length=%d, index=%d, limit=%d", bytes.length, index, limit));
      }
      long offset = index;
      int remaining = limit - index;

      // Skip past ASCII characters as quickly as possible.
      final int skipped = unsafeEstimateConsecutiveAscii(bytes, offset, remaining);
      remaining -= skipped;
      offset += skipped;

      for (; ; ) {
        // Optimize for interior runs of ASCII bytes.
        // TODO: Consider checking 8 bytes at a time after some threshold?
        // Maybe after seeing a few in a row that are ASCII, go back to fast mode?
        int byte1 = 0;
        for (; remaining > 0 && (byte1 = UnsafeUtil.getByte(bytes, offset++)) >= 0; --remaining) {}
        if (remaining == 0) {
          return true;
        }
        remaining--;

        // If we're here byte1 is not ASCII. Only need to handle 2-4 byte forms.
        if (byte1 < (byte) 0xE0) {
          // Two-byte form (110xxxxx 10xxxxxx)
          if (remaining == 0) {
            // Incomplete sequence
            return false;
          }
          remaining--;

          // Simultaneously checks for illegal trailing-byte in
          // leading position and overlong 2-byte form.
          if (byte1 < (byte) 0xC2 || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
            return false;
          }
        } else if (byte1 < (byte) 0xF0) {
          // Three-byte form (1110xxxx 10xxxxxx 10xxxxxx)
          if (remaining < 2) {
            // Incomplete sequence
            return false;
          }
          remaining -= 2;

          final int byte2;
          if ((byte2 = UnsafeUtil.getByte(bytes, offset++)) > (byte) 0xBF
              // 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)
              // byte3 trailing-byte test
              || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
            return false;
          }
        } else {
          // Four-byte form (1110xxxx 10xxxxxx 10xxxxxx 10xxxxxx)
          if (remaining < 3) {
            // Incomplete sequence
            return false;
          }
          remaining -= 3;

          final int byte2;
          if ((byte2 = UnsafeUtil.getByte(bytes, offset++)) > (byte) 0xBF
              // Check that 1 <= plane <= 16.  Tricky optimized form of:
              // if (byte1 > (byte) 0xF4 ||
              //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
              //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
              || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
              // byte3 trailing-byte test
              || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF
              // byte4 trailing-byte test
              || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
            return false;
          }
        }
      }
    }

    @Override
    protected boolean isValidUtf8BufferDirect(ByteBuffer buffer, final int index, final int limit) {
      if (!buffer.isDirect()) {
        throw new IllegalArgumentException("ByteBuffer must be direct");
      }

      // Bitwise OR combines the sign bits so any negative value fails the check.
      if ((index | limit | buffer.limit() - limit) < 0) {
        throw new ArrayIndexOutOfBoundsException(
            String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, limit));
      }

      long address = addressOffset(buffer) + index;
      int remaining = limit - index;

      // Skip past ASCII characters as quickly as possible.
      final int skipped = unsafeEstimateConsecutiveAscii(address, remaining);
      address += skipped;
      remaining -= skipped;

      for (; ; ) {
        // Optimize for interior runs of ASCII bytes.
        // TODO: Consider checking 8 bytes at a time after some threshold?
        // Maybe after seeing a few in a row that are ASCII, go back to fast mode?
        int byte1 = 0;
        for (; remaining > 0 && (byte1 = UnsafeUtil.getByte(address++)) >= 0; --remaining) {}
        if (remaining == 0) {
          return true;
        }
        remaining--;

        if (byte1 < (byte) 0xE0) {
          // Two-byte form

          if (remaining == 0) {
            // Incomplete sequence
            return false;
          }
          remaining--;

          // Simultaneously checks for illegal trailing-byte in
          // leading position and overlong 2-byte form.
          if (byte1 < (byte) 0xC2 || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
            return false;
          }
        } else if (byte1 < (byte) 0xF0) {
          // Three-byte form

          if (remaining < 2) {
            // Incomplete sequence
            return false;
          }
          remaining -= 2;

          final byte byte2 = UnsafeUtil.getByte(address++);
          if (byte2 > (byte) 0xBF
              // 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)
              // byte3 trailing-byte test
              || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
            return false;
          }
        } else {
          // Four-byte form

          if (remaining < 3) {
            // Incomplete sequence
            return false;
          }
          remaining -= 3;

          final byte byte2 = UnsafeUtil.getByte(address++);
          if (byte2 > (byte) 0xBF
              // Check that 1 <= plane <= 16.  Tricky optimized form of:
              // if (byte1 > (byte) 0xF4 ||
              //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
              //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
              || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
              // byte3 trailing-byte test
              || UnsafeUtil.getByte(address++) > (byte) 0xBF
              // byte4 trailing-byte test
              || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
            return false;
          }
        }
      }
    }

    @Override
    String decodeUtf8(byte[] bytes, int index, int size) throws InvalidProtocolBufferException {
      String s = new String(bytes, index, size, Internal.UTF_8);

      // '\uFFFD' is the UTF-8 default replacement char, which illegal byte sequences get replaced
      // with.
      if (s.indexOf('\uFFFD') < 0) {
        return s;
      }

      // Since s contains '\uFFFD' there are 2 options:
      // 1) The byte array slice is invalid UTF-8.
      // 2) The byte array slice is valid UTF-8 and contains encodings for "\uFFFD".
      // To rule out (1), we encode s and compare it to the byte array slice.
      // If the byte array slice was invalid UTF-8, then we would get a different sequence of bytes.
      if (Arrays.equals(
          s.getBytes(Internal.UTF_8), Arrays.copyOfRange(bytes, index, index + size))) {
        return s;
      }

      throw InvalidProtocolBufferException.invalidUtf8();
    }

    @Override
    String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException {
      // Bitwise OR combines the sign bits so any negative value fails the check.
      if ((index | size | buffer.limit() - index - size) < 0) {
        throw new ArrayIndexOutOfBoundsException(
            String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, size));
      }
      long address = UnsafeUtil.addressOffset(buffer) + index;
      final long addressLimit = address + size;

      // The longest possible resulting String is the same as the number of input bytes, when it is
      // all ASCII. For other cases, this over-allocates and we will truncate in the end.
      char[] resultArr = new char[size];
      int resultPos = 0;

      // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      while (address < addressLimit) {
        byte b = UnsafeUtil.getByte(address);
        if (!DecodeUtil.isOneByte(b)) {
          break;
        }
        address++;
        DecodeUtil.handleOneByte(b, resultArr, resultPos++);
      }

      while (address < addressLimit) {
        byte byte1 = UnsafeUtil.getByte(address++);
        if (DecodeUtil.isOneByte(byte1)) {
          DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
          // It's common for there to be multiple ASCII characters in a run mixed in, so add an
          // extra optimized loop to take care of these runs.
          while (address < addressLimit) {
            byte b = UnsafeUtil.getByte(address);
            if (!DecodeUtil.isOneByte(b)) {
              break;
            }
            address++;
            DecodeUtil.handleOneByte(b, resultArr, resultPos++);
          }
        } else if (DecodeUtil.isTwoBytes(byte1)) {
          if (address >= addressLimit) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleTwoBytes(
              byte1, /* byte2 */ UnsafeUtil.getByte(address++), resultArr, resultPos++);
        } else if (DecodeUtil.isThreeBytes(byte1)) {
          if (address >= addressLimit - 1) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleThreeBytes(
              byte1,
              /* byte2 */ UnsafeUtil.getByte(address++),
              /* byte3 */ UnsafeUtil.getByte(address++),
              resultArr,
              resultPos++);
        } else {
          if (address >= addressLimit - 2) {
            throw InvalidProtocolBufferException.invalidUtf8();
          }
          DecodeUtil.handleFourBytes(
              byte1,
              /* byte2 */ UnsafeUtil.getByte(address++),
              /* byte3 */ UnsafeUtil.getByte(address++),
              /* byte4 */ UnsafeUtil.getByte(address++),
              resultArr,
              resultPos++);
          // 4-byte case requires two chars.
          resultPos++;
        }
      }

      return new String(resultArr, 0, resultPos);
    }

    @Override
    int encodeUtf8(String in, byte[] out, int offset, int length) {
      // On our servers with strings that have optimized internals, empirically doing the naive
      // thing is faster than our best fancy loops.
      return encodeUtf8Naive(in, out, offset, length);
    }

    @Override
    protected void encodeUtf8Internal(String in, ByteBuffer out) {
      // On our servers with strings that have optimized internals, empirically doing the naive
      // thing is faster than our best fancy loops.
      encodeUtf8Naive(in, out);
    }

    /**
     * Counts (approximately) the number of consecutive ASCII characters starting from the given
     * position, using the most efficient method available to the platform.
     *
     * @param bytes the array containing the character sequence
     * @param offset the offset position of the index (same as index + arrayBaseOffset)
     * @param maxChars the maximum number of characters to count
     * @return the number of ASCII characters found. The stopping position will be at or before the
     *     first non-ASCII byte.
     */
    private static int unsafeEstimateConsecutiveAscii(
        byte[] bytes, long offset, final int maxChars) {
      if (maxChars < UNSAFE_COUNT_ASCII_THRESHOLD) {
        // Don't bother with small strings.
        return 0;
      }

      // Read bytes until 8-byte aligned so that we can read longs in the loop below.
      // Byte arrays are already either 8 or 16-byte aligned, so we just need to make sure that
      // the index (relative to the start of the array) is also 8-byte aligned. We do this by
      // ANDing the index with 7 to determine the number of bytes that need to be read before
      // we're 8-byte aligned.
      final int unaligned = 8 - ((int) offset & 7);
      int i;
      for (i = 0; i < unaligned; i++) {
        if (UnsafeUtil.getByte(bytes, offset++) < 0) {
          return i;
        }
      }

      for (; i + 8 <= maxChars; i += 8) {
        if ((UnsafeUtil.getLong(bytes, UnsafeUtil.BYTE_ARRAY_BASE_OFFSET + offset)
                & ASCII_MASK_LONG)
            != 0L) {
          break;
        }
        offset += 8;
      }

      for (; i < maxChars; i++) {
        if (UnsafeUtil.getByte(bytes, offset++) < 0) {
          return i;
        }
      }
      return maxChars;
    }

    /**
     * Same as {@link Utf8#estimateConsecutiveAscii(ByteBuffer, int, int)} except that it uses the
     * most efficient method available to the platform.
     */
    private static int unsafeEstimateConsecutiveAscii(long address, final int maxChars) {
      int remaining = maxChars;
      if (remaining < UNSAFE_COUNT_ASCII_THRESHOLD) {
        // Don't bother with small strings.
        return 0;
      }

      // Read bytes until 8-byte aligned so that we can read longs in the loop below.
      // This is equivalent to (8-address) mod 8, the number of bytes we need to read before we're
      // 8-byte aligned.
      final int unaligned = (int) (-address & 7);
      for (int j = unaligned; j > 0; j--) {
        if (UnsafeUtil.getByte(address++) < 0) {
          return unaligned - j;
        }
      }

      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      // To speed things up further, we're reading longs instead of bytes so we use a mask to
      // determine if any byte in the current long is non-ASCII.
      remaining -= unaligned;
      for (;
          remaining >= 8 && (UnsafeUtil.getLong(address) & ASCII_MASK_LONG) == 0;
          address += 8, remaining -= 8) {}
      return maxChars - remaining;
    }
  }

  /**
   * 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.
   */
  private static class DecodeUtil {

    /** Returns whether this is a single-byte codepoint (i.e., ASCII) with the form '0XXXXXXX'. */
    private static boolean isOneByte(byte b) {
      return b >= 0;
    }

    /**
     * Returns whether this is a two-byte codepoint with the form '10XXXXXX' iff
     * {@link #isOneByte(byte)} is false. This private method works in the limited use in
     * this class where this method is only called when {@link #isOneByte(byte)} has already
     * returned false. It is not suitable for general or public use.
     */
    private static boolean isTwoBytes(byte b) {
      return b < (byte) 0xE0;
    }

    /**
     * Returns whether this is a three-byte codepoint with the form '110XXXXX' iff
     * {@link #isOneByte(byte)} and {@link #isTwoBytes(byte)} are false.
     * This private method works in the limited use in
     * this class where this method is only called when {@link #isOneByte(byte)} an
     * {@link #isTwoBytes(byte)} have already returned false. It is not suitable for general
     * or public use.
     */
    private static boolean isThreeBytes(byte b) {
      return b < (byte) 0xF0;
    }

    private static void handleOneByte(byte byte1, char[] resultArr, int resultPos) {
      resultArr[resultPos] = (char) byte1;
    }

    private static void handleTwoBytes(byte byte1, byte byte2, char[] resultArr, int resultPos)
        throws InvalidProtocolBufferException {
      // Simultaneously checks for illegal trailing-byte in leading position (<= '11000000') and
      // overlong 2-byte, '11000001'.
      if (byte1 < (byte) 0xC2 || isNotTrailingByte(byte2)) {
        throw InvalidProtocolBufferException.invalidUtf8();
      }
      resultArr[resultPos] = (char) (((byte1 & 0x1F) << 6) | trailingByteValue(byte2));
    }

    private static void handleThreeBytes(
        byte byte1, byte byte2, byte byte3, char[] resultArr, int resultPos)
        throws InvalidProtocolBufferException {
      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 InvalidProtocolBufferException.invalidUtf8();
      }
      resultArr[resultPos] =
          (char)
              (((byte1 & 0x0F) << 12) | (trailingByteValue(byte2) << 6) | trailingByteValue(byte3));
    }

    private static void handleFourBytes(
        byte byte1, byte byte2, byte byte3, byte byte4, char[] resultArr, int resultPos)
        throws InvalidProtocolBufferException {
      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 InvalidProtocolBufferException.invalidUtf8();
      }
      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));
    }
  }

  private Utf8() {}
}
