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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;
 
An 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 "Foo<Bar>" into "Foo&lt;Bar&gt;" to prevent "<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 "Foo<Bar>".

Note: This class is similar to 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 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 com.google.common.html.HtmlEscapers, com.google.common.xml.XmlEscapers, and SourceCodeEscapers. To create your own escapers extend this class and implement the escape(int) method.

Author(s):
David Beaumont
Since:
15.0
 
 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;

  
Constructor for use by subclasses.
 
   protected UnicodeEscaper() {}

  
Returns the escaped form of the given Unicode code point, or 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 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 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.

Parameters:
cp the Unicode code point to escape if necessary
Returns:
the replacement characters, or null if no escaping was needed
 
   protected abstract char[] escape(int cp);

  
Scans a sub-sequence of characters from a given java.lang.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 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 java.lang.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 com.google.common.net.PercentEscaper for an example.

Parameters:
csq a sequence of characters
start the index of the first character to be scanned
end the index immediately after the last character to be scanned
Throws:
java.lang.IllegalArgumentException if the scanned sub-sequence of csq contains invalid surrogate pairs
  protected int nextEscapeIndex(CharSequence csqint startint end) {
    int index = start;
    while (index < end) {
      int cp = codePointAt(csqindexend);
      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 java.lang.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 nextEscapeIndex(java.lang.CharSequence,int,int) directly. Doing this for com.google.common.net.PercentEscaper more than doubled the performance for unescaped strings (as measured by CharEscapersBenchmark).

Parameters:
string the literal string to be escaped
Returns:
the escaped form of string
Throws:
java.lang.NullPointerException if string is null
java.lang.IllegalArgumentException if invalid surrogate characters are encountered
  public String escape(String string) {
    checkNotNull(string);
    int end = string.length();
    int index = nextEscapeIndex(string, 0, end);
    return index == end ? string : escapeSlow(stringindex);
  }

  
Returns the escaped form of a given literal string, starting at the given index. This method is called by the escape(java.lang.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 CharEscaperBuilder for an example usage.

This method is not reentrant and may only be invoked by the top level escape(java.lang.String) method.

Parameters:
s the literal string to be escaped
index the index to start escaping from
Returns:
the escaped form of string
Throws:
java.lang.NullPointerException if string is null
java.lang.IllegalArgumentException if invalid surrogate characters are encountered
  protected final String escapeSlow(String sint 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(sindexend);
      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 = growBuffer(destdestIndexdestLength);
        }
        // If we have skipped any characters, we need to copy them now.
        if (charsSkipped > 0) {
          s.getChars(unescapedChunkStartindexdestdestIndex);
          destIndex += charsSkipped;
        }
        if (escaped.length > 0) {
          System.arraycopy(escaped, 0, destdestIndexescaped.length);
          destIndex += escaped.length;
        }
        // If we dealt with an escaped character, reset the unescaped range.
        unescapedChunkStart = nextIndex;
      }
      index = nextEscapeIndex(snextIndexend);
    }
    // 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(destdestIndexendIndex);
      }
      s.getChars(unescapedChunkStartenddestdestIndex);
      destIndex = endIndex;
    }
    return new String(dest, 0, destIndex);
  }

  
Returns the Unicode code point of the character at the given index.

Unlike java.lang.Character.codePointAt(java.lang.CharSequence,int) or java.lang.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 index >= end, java.lang.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 java.lang.IllegalArgumentException is thrown.
  4. If the first character was a low surrogate value, java.lang.IllegalArgumentException is thrown.

Parameters:
seq the sequence of characters from which to decode the code point
index the index of the first character to decode
end the index beyond the last valid character to decode
Returns:
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 seqint indexint end) {
    checkNotNull(seq);
    if (index < end) {
      char c1 = seq.charAt(index++);
      if (c1 < . ||
          c1 > .) {
        // Fast path (first test is probably all we need to do)
        return c1;
      } else if (c1 <= .) {
        // 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(c1c2);
        }
        throw new IllegalArgumentException(
            "Expected low surrogate but got char '" + c2 +
            "' with value " + (intc2 + " at index " + index +
            " in '" + seq + "'");
      } else {
        throw new IllegalArgumentException(
            "Unexpected low surrogate character '" + c1 +
            "' with value " + (intc1 + " 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[] destint indexint size) {
    char[] copy = new char[size];
    if (index > 0) {
      System.arraycopy(dest, 0, copy, 0, index);
    }
    return copy;
  }
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