/*
 * Copyright (C) 2008 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.escape;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;

/**
 * An object that converts literal text into a format safe for inclusion in a
 * particular context (such as an XML document). Typically (but not always), the
 * inverse process of "unescaping" the text is performed automatically by the
 * relevant parser.
 *
 * <p>
 * For example, an XML escaper would convert the literal string
 * {@code "Foo<Bar>"} into {@code
 * "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from being confused with an XML
 * tag. When the resulting XML document is parsed, the parser API will return
 * this text as the original literal string {@code "Foo<Bar>"}.
 *
 * <p>
 * An {@code Escaper} instance is required to be stateless, and safe when used
 * concurrently by multiple threads.
 *
 * <p>
 * Because, in general, escaping operates on the code points of a string and not
 * on its individual {@code char} values, it is not safe to assume that
 * {@code escape(s)} is equivalent to
 * {@code escape(s.substring(0, n)) + escape(s.substing(n))} for arbitrary
 * {@code n}. This is because of the possibility of splitting a surrogate pair.
 * The only case in which it is safe to escape strings and concatenate the
 * results is if you can rule out this possibility, either by splitting an
 * existing long string into short strings adaptively around
 * {@linkplain Character#isHighSurrogate surrogate}
 * {@linkplain Character#isLowSurrogate pairs}, or by starting with short
 * strings already known to be free of unpaired surrogates.
 *
 * <p>
 * The two primary implementations of this interface are {@link CharEscaper} and
 * {@link UnicodeEscaper}. They are heavily optimized for performance and
 * greatly simplify the task of implementing new escapers. It is strongly
 * recommended that when implementing a new escaper you extend one of these
 * classes. If you find that you are unable to achieve the desired behavior
 * using either of these classes, please contact the Java libraries team for
 * advice.
 *
 * <p>
 * Several popular escapers are defined as constants in classes like
 * {@link com.google.common.html.HtmlEscapers},
 * {@link com.google.common.xml.XmlEscapers}, and {@link SourceCodeEscapers}. To
 * create your own escapers, use {@link CharEscaperBuilder}, or extend
 * {@code CharEscaper} or {@code UnicodeEscaper}.
 *
 * @author David Beaumont
 * @since 15.0
 */
@Beta
@GwtCompatible
public abstract class Escaper {
	// TODO(user): evaluate custom implementations, considering package private
	// constructor.
	/** Constructor for use by subclasses. */
	protected Escaper() {
	}

	/**
	 * Returns the escaped form of a given literal string.
	 *
	 * <p>
	 * Note that this method may treat input characters differently depending on the
	 * specific escaper implementation.
	 *
	 * <ul>
	 * <li>{@link UnicodeEscaper} handles
	 * <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> correctly, including
	 * surrogate character pairs. If the input is badly formed the escaper should
	 * throw {@link IllegalArgumentException}.
	 * <li>{@link CharEscaper} handles Java characters independently and does not
	 * verify the input for well formed characters. A {@code CharEscaper} should not
	 * be used in situations where input is not guaranteed to be restricted to the
	 * Basic Multilingual Plane (BMP).
	 * </ul>
	 *
	 * @param string the literal string to be escaped
	 * @return the escaped form of {@code string}
	 * @throws NullPointerException     if {@code string} is null
	 * @throws IllegalArgumentException if {@code string} contains badly formed
	 *                                  UTF-16 or cannot be escaped for any other
	 *                                  reason
	 */
	public abstract String escape(String string);

	private final Function<String, String> asFunction = new Function<String, String>() {
		@Override
		public String apply(String from) {
			return escape(from);
		}
	};

	/**
	 * Returns a {@link Function} that invokes {@link #escape(String)} on this
	 * escaper.
	 */
	public final Function<String, String> asFunction() {
		return asFunction;
	}
}
