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/*
 * Copyright (C) 2008 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.escape;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

/**
 * An {@link Escaper} that converts literal text into a format safe for
 * inclusion in a particular context (such as an XML document). Typically (but
 * not always), the inverse process of "unescaping" the text is performed
 * automatically by the relevant parser.
 *
 * 

For example, an XML escaper would convert the literal string {@code * "Foo"} into {@code "Foo<Bar>"} to prevent {@code ""} from * being confused with an XML tag. When the resulting XML document is parsed, * the parser API will return this text as the original literal string {@code * "Foo"}. * *

Note: This class is similar to {@link CharEscaper} but with one * very important difference. A CharEscaper can only process Java * UTF16 characters in * isolation and may not cope when it encounters surrogate pairs. This class * facilitates the correct escaping of all Unicode characters. * *

As there are important reasons, including potential security issues, to * handle Unicode correctly if you are considering implementing a new escaper * you should favor using UnicodeEscaper wherever possible. * *

A {@code UnicodeEscaper} instance is required to be stateless, and safe * when used concurrently by multiple threads. * *

Several popular escapers are defined as constants in classes like {@link * com.google.common.html.HtmlEscapers}, {@link * com.google.common.xml.XmlEscapers}, and {@link SourceCodeEscapers}. To create * your own escapers extend this class and implement the {@link #escape(int)} * method. * * @author David Beaumont * @since 15.0 */ @Beta @GwtCompatible public abstract class UnicodeEscaper extends Escaper { /** The amount of padding (chars) to use when growing the escape buffer. */ private static final int DEST_PAD = 32; /** Constructor for use by subclasses. */ protected UnicodeEscaper() {} /** * Returns the escaped form of the given Unicode code point, or {@code null} * if this code point does not need to be escaped. When called as part of an * escaping operation, the given code point is guaranteed to be in the range * {@code 0 <= cp <= Character#MAX_CODE_POINT}. * *

If an empty array is returned, this effectively strips the input * character from the resulting text. * *

If the character does not need to be escaped, this method should return * {@code null}, rather than an array containing the character representation * of the code point. This enables the escaping algorithm to perform more * efficiently. * *

If the implementation of this method cannot correctly handle a * particular code point then it should either throw an appropriate runtime * exception or return a suitable replacement character. It must never * silently discard invalid input as this may constitute a security risk. * * @param cp the Unicode code point to escape if necessary * @return the replacement characters, or {@code null} if no escaping was * needed */ protected abstract char[] escape(int cp); /** * Scans a sub-sequence of characters from a given {@link CharSequence}, * returning the index of the next character that requires escaping. * *

Note: When implementing an escaper, it is a good idea to override * this method for efficiency. The base class implementation determines * successive Unicode code points and invokes {@link #escape(int)} for each of * them. If the semantics of your escaper are such that code points in the * supplementary range are either all escaped or all unescaped, this method * can be implemented more efficiently using {@link CharSequence#charAt(int)}. * *

Note however that if your escaper does not escape characters in the * supplementary range, you should either continue to validate the correctness * of any surrogate characters encountered or provide a clear warning to users * that your escaper does not validate its input. * *

See {@link com.google.common.net.PercentEscaper} for an example. * * @param csq a sequence of characters * @param start the index of the first character to be scanned * @param end the index immediately after the last character to be scanned * @throws IllegalArgumentException if the scanned sub-sequence of {@code csq} * contains invalid surrogate pairs */ protected int nextEscapeIndex(CharSequence csq, int start, int end) { int index = start; while (index < end) { int cp = codePointAt(csq, index, end); if (cp < 0 || escape(cp) != null) { break; } index += Character.isSupplementaryCodePoint(cp) ? 2 : 1; } return index; } /** * Returns the escaped form of a given literal string. * *

If you are escaping input in arbitrary successive chunks, then it is not * generally safe to use this method. If an input string ends with an * unmatched high surrogate character, then this method will throw * {@link IllegalArgumentException}. You should ensure your input is valid UTF-16 before calling this * method. * *

Note: When implementing an escaper it is a good idea to override * this method for efficiency by inlining the implementation of * {@link #nextEscapeIndex(CharSequence, int, int)} directly. Doing this for * {@link com.google.common.net.PercentEscaper} more than doubled the * performance for unescaped strings (as measured by {@link * CharEscapersBenchmark}). * * @param string the literal string to be escaped * @return the escaped form of {@code string} * @throws NullPointerException if {@code string} is null * @throws IllegalArgumentException if invalid surrogate characters are * encountered */ @Override public String escape(String string) { checkNotNull(string); int end = string.length(); int index = nextEscapeIndex(string, 0, end); return index == end ? string : escapeSlow(string, index); } /** * Returns the escaped form of a given literal string, starting at the given * index. This method is called by the {@link #escape(String)} method when it * discovers that escaping is required. It is protected to allow subclasses * to override the fastpath escaping function to inline their escaping test. * See {@link CharEscaperBuilder} for an example usage. * *

This method is not reentrant and may only be invoked by the top level * {@link #escape(String)} method. * * @param s the literal string to be escaped * @param index the index to start escaping from * @return the escaped form of {@code string} * @throws NullPointerException if {@code string} is null * @throws IllegalArgumentException if invalid surrogate characters are * encountered */ protected final String escapeSlow(String s, int index) { int end = s.length(); // Get a destination buffer and setup some loop variables. char[] dest = Platform.charBufferFromThreadLocal(); int destIndex = 0; int unescapedChunkStart = 0; while (index < end) { int cp = codePointAt(s, index, end); if (cp < 0) { throw new IllegalArgumentException( "Trailing high surrogate at end of input"); } // It is possible for this to return null because nextEscapeIndex() may // (for performance reasons) yield some false positives but it must never // give false negatives. char[] escaped = escape(cp); int nextIndex = index + (Character.isSupplementaryCodePoint(cp) ? 2 : 1); if (escaped != null) { int charsSkipped = index - unescapedChunkStart; // This is the size needed to add the replacement, not the full // size needed by the string. We only regrow when we absolutely must. int sizeNeeded = destIndex + charsSkipped + escaped.length; if (dest.length < sizeNeeded) { int destLength = sizeNeeded + (end - index) + DEST_PAD; dest = growBuffer(dest, destIndex, destLength); } // If we have skipped any characters, we need to copy them now. if (charsSkipped > 0) { s.getChars(unescapedChunkStart, index, dest, destIndex); destIndex += charsSkipped; } if (escaped.length > 0) { System.arraycopy(escaped, 0, dest, destIndex, escaped.length); destIndex += escaped.length; } // If we dealt with an escaped character, reset the unescaped range. unescapedChunkStart = nextIndex; } index = nextEscapeIndex(s, nextIndex, end); } // Process trailing unescaped characters - no need to account for escaped // length or padding the allocation. int charsSkipped = end - unescapedChunkStart; if (charsSkipped > 0) { int endIndex = destIndex + charsSkipped; if (dest.length < endIndex) { dest = growBuffer(dest, destIndex, endIndex); } s.getChars(unescapedChunkStart, end, dest, destIndex); destIndex = endIndex; } return new String(dest, 0, destIndex); } /** * Returns the Unicode code point of the character at the given index. * *

Unlike {@link Character#codePointAt(CharSequence, int)} or * {@link String#codePointAt(int)} this method will never fail silently when * encountering an invalid surrogate pair. * *

The behaviour of this method is as follows: *

    *
  1. If {@code index >= end}, {@link IndexOutOfBoundsException} is thrown. *
  2. If the character at the specified index is not a surrogate, it is * returned. *
  3. If the first character was a high surrogate value, then an attempt is * made to read the next character. *
      *
    1. If the end of the sequence was reached, the negated value of * the trailing high surrogate is returned. *
    2. If the next character was a valid low surrogate, the code point * value of the high/low surrogate pair is returned. *
    3. If the next character was not a low surrogate value, then * {@link IllegalArgumentException} is thrown. *
    *
  4. If the first character was a low surrogate value, * {@link IllegalArgumentException} is thrown. *
* * @param seq the sequence of characters from which to decode the code point * @param index the index of the first character to decode * @param end the index beyond the last valid character to decode * @return the Unicode code point for the given index or the negated value of * the trailing high surrogate character at the end of the sequence */ protected static int codePointAt(CharSequence seq, int index, int end) { checkNotNull(seq); if (index < end) { char c1 = seq.charAt(index++); if (c1 < Character.MIN_HIGH_SURROGATE || c1 > Character.MAX_LOW_SURROGATE) { // Fast path (first test is probably all we need to do) return c1; } else if (c1 <= Character.MAX_HIGH_SURROGATE) { // If the high surrogate was the last character, return its inverse if (index == end) { return -c1; } // Otherwise look for the low surrogate following it char c2 = seq.charAt(index); if (Character.isLowSurrogate(c2)) { return Character.toCodePoint(c1, c2); } throw new IllegalArgumentException( "Expected low surrogate but got char '" + c2 + "' with value " + (int) c2 + " at index " + index + " in '" + seq + "'"); } else { throw new IllegalArgumentException( "Unexpected low surrogate character '" + c1 + "' with value " + (int) c1 + " at index " + (index - 1) + " in '" + seq + "'"); } } throw new IndexOutOfBoundsException("Index exceeds specified range"); } /** * Helper method to grow the character buffer as needed, this only happens * once in a while so it's ok if it's in a method call. If the index passed * in is 0 then no copying will be done. */ private static char[] growBuffer(char[] dest, int index, int size) { char[] copy = new char[size]; if (index > 0) { System.arraycopy(dest, 0, copy, 0, index); } return copy; } }




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