UnicodeEscaper.java
/*
* Copyright (c) 2010 Google Inc.
*
* 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.api.client.util.escape;
/**
* 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.
*
* <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
* "Foo<Bar>"} to prevent {@code "<Bar>"} 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<Bar>"}.
*
* <p>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.
*
* <p>A {@code UnicodeEscaper} instance is required to be stateless, and safe when used concurrently
* by multiple threads.
*
* <p>Several popular escapers are defined as constants in the class {@link CharEscapers}. To create
* your own escapers extend this class and implement the {@link #escape(int)} method.
*
* @since 1.0
*/
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;
/**
* 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}.
*
* <p>If an empty array is returned, this effectively strips the input character from the
* resulting text.
*
* <p>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.
*
* <p>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.
*
* <p><b>Note:</b> 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)}.
*
* <p>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.
*
* <p>See {@link 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 abstract int nextEscapeIndex(CharSequence csq, int start, int end);
/**
* Returns the escaped form of a given literal string.
*
* <p>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 <a
* href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> before calling this method.
*
* @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 abstract String escape(String string);
/**
* 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.
*
* <p>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.
*
* <p>Unlike {@link Character#codePointAt(CharSequence, int)} or {@link String#codePointAt(int)}
* this method will never fail silently when encountering an invalid surrogate pair.
*
* <p>The behaviour of this method is as follows:
*
* <ol>
* <li>If {@code index >= end}, {@link IndexOutOfBoundsException} is thrown.
* <li><b>If the character at the specified index is not a surrogate, it is returned.</b>
* <li>If the first character was a high surrogate value, then an attempt is made to read the
* next character.
* <ol>
* <li><b>If the end of the sequence was reached, the negated value of the trailing high
* surrogate is returned.</b>
* <li><b>If the next character was a valid low surrogate, the code point value of the
* high/low surrogate pair is returned.</b>
* <li>If the next character was not a low surrogate value, then {@link
* IllegalArgumentException} is thrown.
* </ol>
* <li>If the first character was a low surrogate value, {@link IllegalArgumentException} is
* thrown.
* </ol>
*
* @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) {
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);
} else {
throw new IllegalArgumentException(
"Unexpected low surrogate character '"
+ c1
+ "' with value "
+ (int) c1
+ " at index "
+ (index - 1));
}
}
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;
}
}