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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.lang3.text.translate;

import java.io.IOException;
import java.io.Writer;

/**
 * Translates escaped Unicode values of the form \\u+\d\d\d\d back to
 * Unicode. It supports multiple 'u' characters and will work with or
 * without the +.
 *
 * @since 3.0
 * @deprecated As of 3.6, use Apache Commons Text
 * 
 * UnicodeUnescaper instead
 */
@Deprecated
public class UnicodeUnescaper extends CharSequenceTranslator {

    /**
     * Constructs a new instance.
     */
    public UnicodeUnescaper() {
        // empty
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int translate(final CharSequence input, final int index, final Writer out) throws IOException {
        if (input.charAt(index) == '\\' && index + 1 < input.length() && input.charAt(index + 1) == 'u') {
            // consume optional additional 'u' chars
            int i = 2;
            while (index + i < input.length() && input.charAt(index + i) == 'u') {
                i++;
            }

            if (index + i < input.length() && input.charAt(index + i) == '+') {
                i++;
            }

            if (index + i + 4 <= input.length()) {
                // Get 4 hex digits
                final CharSequence unicode = input.subSequence(index + i, index + i + 4);

                try {
                    final int value = Integer.parseInt(unicode.toString(), 16);
                    out.write((char) value);
                } catch (final NumberFormatException nfe) {
                    throw new IllegalArgumentException("Unable to parse unicode value: " + unicode, nfe);
                }
                return i + 4;
            }
            throw new IllegalArgumentException("Less than 4 hex digits in unicode value: '" + input.subSequence(index, input.length())
                    + "' due to end of CharSequence");
        }
        return 0;
    }
}