/*
 * 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() {
	}
}
