Java tutorial
/* * 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; import javax.annotation.CheckReturnValue; /** * 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 Clment Roux * @since 16.0 */ @Beta @GwtCompatible 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) */ @CheckReturnValue 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. */ @CheckReturnValue 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 */ @CheckReturnValue 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() { } }