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Apache Commons Text is a library focused on algorithms working on strings.
There is a newer version: 1.12.0
/*
 * 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.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 1.0
 */
public class OctalUnescaper extends CharSequenceTranslator {

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