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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. * *

Popular escapers are defined as constants in classes like {@link * com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. 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); /** * 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); } /** * 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, 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) { if (size < 0) { // overflow - should be OutOfMemoryError but GWT/j2cl don't support it throw new AssertionError("Cannot increase internal buffer any further"); } char[] copy = new char[size]; if (index > 0) { System.arraycopy(dest, 0, copy, 0, index); } return copy; } }




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