/*
 * 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.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.
 * One Google-authored templating system available for external use is <a
 * href="https://developers.google.com/closure/templates/">Closure Templates</a>.
 *
 * <p>HTML escaping is particularly tricky: For example, <a
 * href="https://www.w3.org/TR/html4/types.html#h-6.2">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
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
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() {}
}
