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
17package com.google.common.escape;
18
19import static com.google.common.base.Preconditions.checkNotNull;
20
21import com.google.common.annotations.Beta;
22import com.google.common.annotations.GwtCompatible;
23
24import java.util.Map;
25
26import javax.annotation.Nullable;
27
28/**
29 * A {@link UnicodeEscaper} that uses an array to quickly look up replacement
30 * characters for a given code point. An additional safe range is provided that
31 * determines whether code points without specific replacements are to be
32 * considered safe and left unescaped or should be escaped in a general way.
33 *
34 * <p>A good example of usage of this class is for HTML escaping where the
35 * replacement array contains information about the named HTML entities
36 * such as {@code &amp;} and {@code &quot;} while {@link #escapeUnsafe} is
37 * overridden to handle general escaping of the form {@code &#NNNNN;}.
38 *
39 * <p>The size of the data structure used by {@link ArrayBasedUnicodeEscaper} is
40 * proportional to the highest valued code point that requires escaping.
41 * For example a replacement map containing the single character
42 * '{@code \}{@code u1000}' will require approximately 16K of memory. If you
43 * need to create multiple escaper instances that have the same character
44 * replacement mapping consider using {@link ArrayBasedEscaperMap}.
45 *
46 * @author David Beaumont
47 * @since 15.0
48 */
49@Beta
50@GwtCompatible
51public abstract class ArrayBasedUnicodeEscaper extends UnicodeEscaper {
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 code point in the safe range.
57  private final int safeMin;
58  // The last code point in the safe range.
59  private final int safeMax;
60
61  // Cropped values used in the fast path range checks.
62  private final char safeMinChar;
63  private final char safeMaxChar;
64
65  /**
66   * Creates a new ArrayBasedUnicodeEscaper instance with the given replacement
67   * map and specified safe range. If {@code safeMax < safeMin} then no code
68   * points are considered safe.
69   *
70   * <p>If a code point has no mapped replacement then it is checked against the
71   * safe range. If it lies outside that, then {@link #escapeUnsafe} is
72   * called, otherwise no escaping is performed.
73   *
74   * @param replacementMap a map of characters to their escaped representations
75   * @param safeMin the lowest character value in the safe range
76   * @param safeMax the highest character value in the safe range
77   * @param unsafeReplacement the default replacement for unsafe characters or
78   *     null if no default replacement is required
79   */
80  protected ArrayBasedUnicodeEscaper(Map<Character, String> replacementMap,
81      int safeMin, int safeMax, @Nullable String unsafeReplacement) {
82
83    this(ArrayBasedEscaperMap.create(replacementMap), safeMin, safeMax,
84        unsafeReplacement);
85  }
86
87  /**
88   * Creates a new ArrayBasedUnicodeEscaper instance with the given replacement
89   * map and specified safe range. If {@code safeMax < safeMin} then no code
90   * points are considered safe. This initializer is useful when explicit
91   * instances of ArrayBasedEscaperMap are used to allow the sharing of large
92   * replacement mappings.
93   *
94   * <p>If a code point has no mapped replacement then it is checked against the
95   * safe range. If it lies outside that, then {@link #escapeUnsafe} is
96   * called, otherwise no escaping is performed.
97   *
98   * @param escaperMap the map of replacements
99   * @param safeMin the lowest character value in the safe range
100   * @param safeMax the highest character value in the safe range
101   * @param unsafeReplacement the default replacement for unsafe characters or
102   *     null if no default replacement is required
103   */
104  protected ArrayBasedUnicodeEscaper(ArrayBasedEscaperMap escaperMap,
105      int safeMin, int safeMax, @Nullable String unsafeReplacement) {
106
107    checkNotNull(escaperMap);  // GWT specific check (do not optimize)
108    this.replacements = escaperMap.getReplacementArray();
109    this.replacementsLength = replacements.length;
110    if (safeMax < safeMin) {
111      // If the safe range is empty, set the range limits to opposite extremes
112      // to ensure the first test of either value will fail.
113      safeMax = -1;
114      safeMin = Integer.MAX_VALUE;
115    }
116    this.safeMin = safeMin;
117    this.safeMax = safeMax;
118
119    // This is a bit of a hack but lets us do quicker per-character checks in
120    // the fast path code. The safe min/max values are very unlikely to extend
121    // into the range of surrogate characters, but if they do we must not test
122    // any values in that range. To see why, consider the case where:
123    //   safeMin <= {hi,lo} <= safeMax
124    // where {hi,lo} are characters forming a surrogate pair such that:
125    //   codePointOf(hi, lo) > safeMax
126    // which would result in the surrogate pair being (wrongly) considered safe.
127    // If we clip the safe range used during the per-character tests so it is
128    // below the values of characters in surrogate pairs, this cannot occur.
129    // This approach does mean that we break out of the fast path code in cases
130    // where we don't strictly need to, but this situation will almost never
131    // occur in practice.
132    if (safeMin >= Character.MIN_HIGH_SURROGATE) {
133      // The safe range is empty or the all safe code points lie in or above the
134      // surrogate range. Either way the character range is empty.
135      this.safeMinChar = Character.MAX_VALUE;
136      this.safeMaxChar = 0;
137    } else {
138      // The safe range is non empty and contains values below the surrogate
139      // range but may extend above it. We may need to clip the maximum value.
140      this.safeMinChar = (char) safeMin;
141      this.safeMaxChar = (char) Math.min(safeMax,
142                                         Character.MIN_HIGH_SURROGATE - 1);
143    }
144  }
145
146  /*
147   * This is overridden to improve performance. Rough benchmarking shows that
148   * this almost doubles the speed when processing strings that do not require
149   * any escaping.
150   */
151  @Override
152  public final String escape(String s) {
153    checkNotNull(s);  // GWT specific check (do not optimize)
154    for (int i = 0; i < s.length(); i++) {
155      char c = s.charAt(i);
156      if ((c < replacementsLength && replacements[c] != null) ||
157          c > safeMaxChar || c < safeMinChar) {
158        return escapeSlow(s, i);
159      }
160    }
161    return s;
162  }
163
164  /* Overridden for performance. */
165  @Override
166  protected final int nextEscapeIndex(CharSequence csq, int index, int end) {
167    while (index < end) {
168      char c = csq.charAt(index);
169      if ((c < replacementsLength && replacements[c] != null) ||
170          c > safeMaxChar || c < safeMinChar) {
171        break;
172      }
173      index++;
174    }
175    return index;
176  }
177
178  /**
179   * Escapes a single Unicode code point using the replacement array and safe
180   * range values. If the given character does not have an explicit replacement
181   * and lies outside the safe range then {@link #escapeUnsafe} is called.
182   */
183  @Override
184  protected final char[] escape(int cp) {
185    if (cp < replacementsLength) {
186      char[] chars = replacements[cp];
187      if (chars != null) {
188        return chars;
189      }
190    }
191    if (cp >= safeMin && cp <= safeMax) {
192      return null;
193    }
194    return escapeUnsafe(cp);
195  }
196
197  /**
198   * Escapes a code point that has no direct explicit value in the replacement
199   * array and lies outside the stated safe range. Subclasses should override
200   * this method to provide generalized escaping for code points if required.
201   *
202   * <p>Note that arrays returned by this method must not be modified once they
203   * have been returned. However it is acceptable to return the same array
204   * multiple times (even for different input characters).
205   *
206   * @param cp the Unicode code point to escape
207   * @return the replacement characters, or {@code null} if no escaping was
208   *         required
209   */
210  protected abstract char[] escapeUnsafe(int cp);
211}
212