Utf8.java
/*
* Copyright (C) 2013 The Guava 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.common.base;
import static com.google.common.base.Preconditions.checkPositionIndexes;
import static java.lang.Character.MAX_SURROGATE;
import static java.lang.Character.MIN_SURROGATE;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
/**
* Low-level, high-performance utility methods related to the {@linkplain Charsets#UTF_8 UTF-8}
* character encoding. UTF-8 is defined in section D92 of <a
* href="http://www.unicode.org/versions/Unicode6.2.0/ch03.pdf">The Unicode Standard Core
* Specification, Chapter 3</a>.
*
* <p>The variant of UTF-8 implemented by this class is the restricted definition of UTF-8
* introduced in Unicode 3.1. One implication of this is that it rejects <a
* href="http://www.unicode.org/versions/corrigendum1.html">"non-shortest form"</a> byte sequences,
* even though the JDK decoder may accept them.
*
* @author Martin Buchholz
* @author Clément Roux
* @since 16.0
*/
@Beta
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public final 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 static int encodedLength(CharSequence sequence) {
// Warning to maintainers: this implementation is highly optimized.
int utf16Length = sequence.length();
int utf8Length = utf16Length;
int i = 0;
// This loop optimizes for pure ASCII.
while (i < utf16Length && sequence.charAt(i) < 0x80) {
i++;
}
// This loop optimizes for chars less than 0x800.
for (; i < utf16Length; i++) {
char c = sequence.charAt(i);
if (c < 0x800) {
utf8Length += ((0x7f - c) >>> 31); // branch free!
} else {
utf8Length += encodedLengthGeneral(sequence, i);
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(CharSequence sequence, int start) {
int utf16Length = sequence.length();
int utf8Length = 0;
for (int i = start; i < utf16Length; i++) {
char c = sequence.charAt(i);
if (c < 0x800) {
utf8Length += (0x7f - c) >>> 31; // branch free!
} else {
utf8Length += 2;
// jdk7+: if (Character.isSurrogate(c)) {
if (MIN_SURROGATE <= c && c <= MAX_SURROGATE) {
// Check that we have a well-formed surrogate pair.
if (Character.codePointAt(sequence, i) == c) {
throw new IllegalArgumentException(unpairedSurrogateMsg(i));
}
i++;
}
}
}
return utf8Length;
}
/**
* Returns {@code true} if {@code bytes} is a <i>well-formed</i> UTF-8 byte sequence according to
* Unicode 6.0. Note that this is a stronger criterion than simply whether the bytes can be
* decoded. For example, some versions of the JDK decoder will accept "non-shortest form" byte
* sequences, but encoding never reproduces these. Such byte sequences are <i>not</i> considered
* well-formed.
*
* <p>This method returns {@code true} if and only if {@code Arrays.equals(bytes, new
* String(bytes, UTF_8).getBytes(UTF_8))} does, but is more efficient in both time and space.
*/
public static boolean isWellFormed(byte[] bytes) {
return isWellFormed(bytes, 0, bytes.length);
}
/**
* Returns whether the given byte array slice is a well-formed UTF-8 byte sequence, as defined by
* {@link #isWellFormed(byte[])}. Note that this can be false even when {@code
* isWellFormed(bytes)} is true.
*
* @param bytes the input buffer
* @param off the offset in the buffer of the first byte to read
* @param len the number of bytes to read from the buffer
*/
public static boolean isWellFormed(byte[] bytes, int off, int len) {
int end = off + len;
checkPositionIndexes(off, end, bytes.length);
// Look for the first non-ASCII character.
for (int i = off; i < end; i++) {
if (bytes[i] < 0) {
return isWellFormedSlowPath(bytes, i, end);
}
}
return true;
}
private static boolean isWellFormedSlowPath(byte[] bytes, int off, int end) {
int index = off;
while (true) {
int byte1;
// Optimize for interior runs of ASCII bytes.
do {
if (index >= end) {
return true;
}
} while ((byte1 = bytes[index++]) >= 0);
if (byte1 < (byte) 0xE0) {
// Two-byte form.
if (index == end) {
return false;
}
// Simultaneously check 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 + 1 >= end) {
return false;
}
int byte2 = bytes[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 && (byte) 0xA0 <= byte2)
// Third byte trailing-byte test.
|| bytes[index++] > (byte) 0xBF) {
return false;
}
} else {
// Four-byte form.
if (index + 2 >= end) {
return false;
}
int byte2 = bytes[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
// Third byte trailing-byte test
|| bytes[index++] > (byte) 0xBF
// Fourth byte trailing-byte test
|| bytes[index++] > (byte) 0xBF) {
return false;
}
}
}
}
private static String unpairedSurrogateMsg(int i) {
return "Unpaired surrogate at index " + i;
}
private Utf8() {}
}