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Apache Commons Validator provides the building blocks for both client side validation and server side data validation. It may be used standalone or with a framework like Struts.

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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.validator.routines.checkdigit;

/**
 * Modulus 10 ABA Number (or Routing Transit Number (RTN)) Check Digit
 * calculation/validation.
 *
 * 
 * ABA Numbers (or Routing Transit Numbers) are a nine digit numeric code used
 * to identify American financial institutions for things such as checks or deposits
 * (ABA stands for the American Bankers Association).
 * 
 *
 * Check digit calculation is based on modulus 10 with digits being weighted
 * based on their position (from right to left) as follows:
 *
 * 
 *   Digits 1, 4 and & 7 are weighted 1
 *   Digits 2, 5 and & 8 are weighted 7
 *   Digits 3, 6 and & 9 are weighted 3
 * 
 *
 * 
 * For further information see
 *  Wikipedia -
 *  Routing transit number.
 * 
 *
 * @version $Revision: 1739356 $
 * @since Validator 1.4
 */
public final class ABANumberCheckDigit extends ModulusCheckDigit {

    private static final long serialVersionUID = -8255937433810380145L;

    /** Singleton Routing Transit Number Check Digit instance */
    public static final CheckDigit ABAN_CHECK_DIGIT = new ABANumberCheckDigit();

    /** weighting given to digits depending on their right position */
    private static final int[] POSITION_WEIGHT = new int[] {3, 1, 7};

    /**
     * Construct a modulus 10 Check Digit routine for ABA Numbers.
     */
    public ABANumberCheckDigit() {
        super(10); // CHECKSTYLE IGNORE MagicNumber
    }

    /**
     * Calculates the weighted value of a character in the
     * code at a specified position.
     * 
     * ABA Routing numbers are weighted in the following manner:
     * 

     *     left position: 1  2  3  4  5  6  7  8  9
     *            weight: 3  7  1  3  7  1  3  7  1
     * 
     *
     * @param charValue The numeric value of the character.
     * @param leftPos The position of the character in the code, counting from left to right
     * @param rightPos The positionof the character in the code, counting from right to left
     * @return The weighted value of the character.
     */
    @Override
    protected int weightedValue(int charValue, int leftPos, int rightPos) {
        int weight = POSITION_WEIGHT[rightPos % 3]; // CHECKSTYLE IGNORE MagicNumber
        return charValue * weight;
    }

}