1   /*
2    * Copyright (C) 2009 The Guava Authors
3    *
4    * Licensed under the Apache License, Version 2.0 (the "License");
5    * you may not use this file except in compliance with the License.
6    * You may obtain a copy of the License at
7    *
8    * http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS,
12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13   * See the License for the specific language governing permissions and
14   * limitations under the License.
15   */
16  
17  package com.google.common.escape;
18  
19  import static com.google.common.base.Preconditions.checkNotNull;
20  
21  import com.google.common.annotations.Beta;
22  import com.google.common.annotations.GwtCompatible;
23  
24  import java.util.Map;
25  
26  /**
27   * A {@link CharEscaper} that uses an array to quickly look up replacement
28   * characters for a given {@code char} value. An additional safe range is
29   * provided that determines whether {@code char} values without specific
30   * replacements are to be considered safe and left unescaped or should be
31   * escaped in a general way.
32   *
33   * <p>A good example of usage of this class is for Java source code escaping
34   * where the replacement array contains information about special ASCII
35   * characters such as {@code \\t} and {@code \\n} while {@link #escapeUnsafe}
36   * is overridden to handle general escaping of the form {@code \\uxxxx}.
37   *
38   * <p>The size of the data structure used by {@link ArrayBasedCharEscaper} is
39   * proportional to the highest valued character that requires escaping.
40   * For example a replacement map containing the single character
41   * '{@code \}{@code u1000}' will require approximately 16K of memory. If you
42   * need to create multiple escaper instances that have the same character
43   * replacement mapping consider using {@link ArrayBasedEscaperMap}.
44   *
45   * @author Sven Mawson
46   * @author David Beaumont
47   * @since 15.0
48   */
49  @Beta
50  @GwtCompatible
51  public abstract class ArrayBasedCharEscaper extends CharEscaper {
52    // The replacement array (see ArrayBasedEscaperMap).
53    private final char[][] replacements;
54    // The number of elements in the replacement array.
55    private final int replacementsLength;
56    // The first character in the safe range.
57    private final char safeMin;
58    // The last character in the safe range.
59    private final char safeMax;
60  
61    /**
62     * Creates a new ArrayBasedCharEscaper instance with the given replacement map
63     * and specified safe range. If {@code safeMax < safeMin} then no characters
64     * are considered safe.
65     *
66     * <p>If a character has no mapped replacement then it is checked against the
67     * safe range. If it lies outside that, then {@link #escapeUnsafe} is
68     * called, otherwise no escaping is performed.
69     *
70     * @param replacementMap a map of characters to their escaped representations
71     * @param safeMin the lowest character value in the safe range
72     * @param safeMax the highest character value in the safe range
73     */
74    protected ArrayBasedCharEscaper(Map<Character, String> replacementMap,
75        char safeMin, char safeMax) {
76  
77      this(ArrayBasedEscaperMap.create(replacementMap), safeMin, safeMax);
78    }
79  
80    /**
81     * Creates a new ArrayBasedCharEscaper instance with the given replacement map
82     * and specified safe range. If {@code safeMax < safeMin} then no characters
83     * are considered safe. This initializer is useful when explicit instances of
84     * ArrayBasedEscaperMap are used to allow the sharing of large replacement
85     * mappings.
86     *
87     * <p>If a character has no mapped replacement then it is checked against the
88     * safe range. If it lies outside that, then {@link #escapeUnsafe} is
89     * called, otherwise no escaping is performed.
90     *
91     * @param escaperMap the mapping of characters to be escaped
92     * @param safeMin the lowest character value in the safe range
93     * @param safeMax the highest character value in the safe range
94     */
95    protected ArrayBasedCharEscaper(ArrayBasedEscaperMap escaperMap,
96        char safeMin, char safeMax) {
97  
98      checkNotNull(escaperMap);  // GWT specific check (do not optimize)
99      this.replacements = escaperMap.getReplacementArray();
100     this.replacementsLength = replacements.length;
101     if (safeMax < safeMin) {
102       // If the safe range is empty, set the range limits to opposite extremes
103       // to ensure the first test of either value will (almost certainly) fail.
104       safeMax = Character.MIN_VALUE;
105       safeMin = Character.MAX_VALUE;
106     }
107     this.safeMin = safeMin;
108     this.safeMax = safeMax;
109   }
110 
111   /*
112    * This is overridden to improve performance. Rough benchmarking shows that
113    * this almost doubles the speed when processing strings that do not require
114    * any escaping.
115    */
116   @Override
117   public final String escape(String s) {
118     checkNotNull(s);  // GWT specific check (do not optimize).
119     for (int i = 0; i < s.length(); i++) {
120       char c = s.charAt(i);
121       if ((c < replacementsLength && replacements[c] != null) ||
122           c > safeMax || c < safeMin) {
123         return escapeSlow(s, i);
124       }
125     }
126     return s;
127   }
128 
129   /**
130    * Escapes a single character using the replacement array and safe range
131    * values. If the given character does not have an explicit replacement and
132    * lies outside the safe range then {@link #escapeUnsafe} is called.
133    */
134   @Override protected final char[] escape(char c) {
135     if (c < replacementsLength) {
136       char[] chars = replacements[c];
137       if (chars != null) {
138         return chars;
139       }
140     }
141     if (c >= safeMin && c <= safeMax) {
142       return null;
143     }
144     return escapeUnsafe(c);
145   }
146 
147   /**
148    * Escapes a {@code char} value that has no direct explicit value in the
149    * replacement array and lies outside the stated safe range. Subclasses should
150    * override this method to provide generalized escaping for characters.
151    *
152    * <p>Note that arrays returned by this method must not be modified once they
153    * have been returned. However it is acceptable to return the same array
154    * multiple times (even for different input characters).
155    *
156    * @param c the character to escape
157    * @return the replacement characters, or {@code null} if no escaping was
158    *         required
159    */
160   // TODO(user,cpovirk): Rename this something better once refactoring done
161   protected abstract char[] escapeUnsafe(char c);
162 }