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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. The code is tested using the latest revision of the JDK for supported LTS releases: 8, 11, 17 and 21 currently. See https://github.com/apache/commons-lang/blob/master/.github/workflows/maven.yml Please ensure your build environment is up-to-date and kindly report any build issues.

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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;

/**
 * Translate escaped octal Strings back to their octal values.
 *
 * For example, "\45" should go back to being the specific value (a %).
 *
 * Note that this currently only supports the viable range of octal for Java; namely 
 * 1 to 377. This is because parsing Java is the main use case.
 * 
 * @since 3.0
 * @version $Id: OctalUnescaper.java 967237 2010-07-23 20:08:57Z mbenson $
 */
public class OctalUnescaper extends CharSequenceTranslator {

    /**
     * {@inheritDoc}
     */
    @Override
    public int translate(final CharSequence input, final int index, final Writer out) throws IOException {
        int remaining = input.length() - index - 1; // how many characters left, ignoring the first \
        StringBuilder builder = new StringBuilder();
        if(input.charAt(index) == '\\' && remaining > 0 && isOctalDigit(input.charAt(index + 1)) ) {
            int next = index + 1;
            int next2 = index + 2;
            int next3 = index + 3;

            // we know this is good as we checked it in the if block above
            builder.append(input.charAt(next));

            if(remaining > 1 && isOctalDigit(input.charAt(next2))) {
                builder.append(input.charAt(next2));
                if(remaining > 2 && isZeroToThree(input.charAt(next)) && isOctalDigit(input.charAt(next3))) {
                    builder.append(input.charAt(next3));
                }
            }

            out.write( Integer.parseInt(builder.toString(), 8) );
            return 1 + builder.length();
        }
        return 0;
    }

    /**
     * Checks if the given char is an octal digit. Octal digits are the character representations of the digits 0 to 7.
     * @param ch the char to check
     * @return true if the given char is the character representation of one of the digits from 0 to 7
     */
    private boolean isOctalDigit(char ch) {
        return ch >= '0' && ch <= '7';
    }

    /**
     * Checks if the given char is the character representation of one of the digit from 0 to 3.
     * @param ch the char to check
     * @return true if the given char is the character representation of one of the digits from 0 to 3
     */
    private boolean isZeroToThree(char ch) {
        return ch >= '0' && ch <= '3';
    }
}