/*
 * 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.bval.extras.constraints.checkdigit;

/**
 * Modulus 10 <b>Luhn</b> Check Digit calculation/validation.
 * <p>
 * Luhn check digits are used, for example, by:
 * <ul>
 *    <li><a href="http://en.wikipedia.org/wiki/Credit_card">Credit Card Numbers</a></li>
 *    <li><a href="http://en.wikipedia.org/wiki/IMEI">IMEI Numbers</a> - International
 *        Mobile Equipment Identity Numbers</li>
 * </ul>
 * Check digit calculation is based on <i>modulus 10</i> with digits in
 * an <i>odd</i> position (from right to left) being weighted 1 and <i>even</i>
 * position digits being weighted 2 (weighted values greater than 9 have 9 subtracted).
 * <p>
 * See <a href="http://en.wikipedia.org/wiki/Luhn_algorithm">Wikipedia</a>
 * for more details.
 */
public final class LuhnValidator
    extends ModulusValidator<Luhn> {

    /** weighting given to digits depending on their right position */
    private static final int[] POSITION_WEIGHT = new int[] {2, 1};

    public LuhnValidator() {
        super(10);
    }

    /**
     * <p>Calculates the <i>weighted</i> value of a charcter in the
     * code at a specified position.</p>
     *
     * <p>For Luhn (from right to left) <b>odd</b> digits are weighted
     * with a factor of <b>one</b> and <b>even</b> digits with a factor
     * of <b>two</b>. Weighted values > 9, have 9 subtracted</p>
     *
     * {@inheritDoc}
     */
    @Override
    protected int weightedValue( int charValue, int leftPos, int rightPos )
            throws Exception {
        int weight = POSITION_WEIGHT[rightPos % 2];
        int weightedValue = (charValue * weight);
        return (weightedValue > 9 ? (weightedValue - 9) : weightedValue);
    }

}