/*
 * Copyright (C) 2009 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.html;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.escape.Escaper;
import com.google.common.escape.Escapers;

/**
 * {@code Escaper} instances suitable for strings to be included in HTML
 * attribute values and <em>most</em> elements' text contents. When possible,
 * avoid manual escaping by using templating systems and high-level APIs that
 * provide autoescaping.
 *
 * <p>HTML escaping is particularly tricky: For example, <a
 * href="http://goo.gl/5TgZb">some elements' text contents must not be HTML
 * escaped</a>. As a result, it is impossible to escape an HTML document
 * correctly without domain-specific knowledge beyond what {@code HtmlEscapers}
 * provides. We strongly encourage the use of HTML templating systems.
 *
 * @author Sven Mawson
 * @author David Beaumont
 * @since 15.0
 */
@Beta
@GwtCompatible
public final class HtmlEscapers {
  /**
   * Returns an {@link Escaper} instance that escapes HTML metacharacters as
   * specified by <a href="http://www.w3.org/TR/html4/">HTML 4.01</a>. The
   * resulting strings can be used both in attribute values and in <em>most</em>
   * elements' text contents, provided that the HTML document's character
   * encoding can encode any non-ASCII code points in the input (as UTF-8 and
   * other Unicode encodings can).
   *
   *
   * <p><b>Note</b>: This escaper only performs minimal escaping to make content
   * structurally compatible with HTML. Specifically, it does not perform entity
   * replacement (symbolic or numeric), so it does not replace non-ASCII code
   * points with character references. This escaper escapes only the following
   * five ASCII characters: {@code '"&<>}.
   */
  public static Escaper htmlEscaper() {
    return HTML_ESCAPER;
  }

  // For each xxxEscaper() method, please add links to external reference pages
  // that are considered authoritative for the behavior of that escaper.

  private static final Escaper HTML_ESCAPER =
      Escapers.builder()
          .addEscape('"', "&quot;")
          // Note: "&apos;" is not defined in HTML 4.01.
          .addEscape('\'', "&#39;")
          .addEscape('&', "&amp;")
          .addEscape('<', "&lt;")
          .addEscape('>', "&gt;")
          .build();

  private HtmlEscapers() {}
}
