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210 ira 1 
/*******************************************************************************
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 * File: P2_EncDec.java
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 * Author: Ira W. Snyder (devel@irasnyder.com)
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 * License: GNU General Public License v2
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 * Class: CS380 - Computer Networking
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 *
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 * Assignment: Project #2
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 * Date Last Modified: 2006-02-04
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 *
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 * Purpose: Encode / Decode a number to / from 4B5B Encoding.
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 *
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 ******************************************************************************/
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import java.io.*;
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import java.util.*;
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public class P2_EncDec
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{
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    public static final String[][] encodingTable = {
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        { "0000", "11110" },
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        { "0001", "01001" },
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        { "0010", "10100" },
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        { "0011", "10101" },
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        { "0100", "01010" },
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        { "0101", "01011" },
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        { "0110", "01110" },
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        { "0111", "01111" },
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        { "1000", "10010" },
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        { "1001", "10011" },
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        { "1010", "10110" },
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        { "1011", "10111" },
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        { "1100", "11010" },
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        { "1101", "11011" },
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        { "1110", "11100" },
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        { "1111", "11101" }};
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    /**
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     * Method: convert4B()
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     * Purpose: Convert a number from it's 4B representation to the
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     * corresponding 5B representation.
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     */
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    public static byte[] convert4B (byte[] _4BNum)
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    {
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        String s = convertToString (_4BNum);
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        int i = encodingTable.length - 1;
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        for (; i >= 0; i--)
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            if (encodingTable[i][0].equals(s))
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                return convertToByteArray(encodingTable[i][1]);
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        throw new IllegalArgumentException();
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    }
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    /**
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     * Method: convert5B()
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     * Purpose: Convert a number from it's 5B representation to the
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     * corresponding 4B representation.
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     */
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    public static byte[] convert5B (byte[] _5BNum)
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    {
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        int i = encodingTable.length - 1;
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        String s = convertToString (_5BNum);
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        for (; i>=0; i--)
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            if (encodingTable[i][1].equals(s))
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                return convertToByteArray(encodingTable[i][0]);
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        throw new IllegalArgumentException();
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    }
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    /**
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     * Method: convertToByteArray()
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     * Purpose: Convert a String to a byte[], useful for sending
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     * over the network in a Datagram.
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     */
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    public static byte[] convertToByteArray (String num)
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    {
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        int i;
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        byte[] ret = new byte[num.length()];
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        byte temp;
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        for (i=0; i<ret.length; i++)
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        {
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            if (num.charAt(i) == '0')
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                temp = 0;
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            else if (num.charAt(i) == '1')
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                temp = 1;
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            else
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                throw new IllegalArgumentException();
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            ret[i] = temp;
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        }
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        return ret;
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    }
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    /**
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     * Method: convertToString()
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     * Purpose: Convert a byte[] to a String. This is really useful for
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     * performing table lookups in the Encoder / Decoder.
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     */
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    public static String convertToString (byte[] num)
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    {
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        int i;
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        String s = new String();
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        for (i=0; i<num.length; i++)
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            s = new String (s + num[i]);
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        return s;
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    }
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}
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