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gwt-jackson is a GWT JSON serializer/deserializer mechanism based on Jackson annotations
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package com.github.nmorel.gwtjackson.client.utils;
/*
Copyright (c) 2009, Mark Renouf
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
    * Neither the name of the  nor the
      names of its contributors may be used to endorse or promote products
      derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY Mark Renouf ''AS IS'' AND ANY
EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL  BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/**
 * Custom Base64 encode/decode implementation suitable for use in
 * GWT applications (uses only translatable classes).
 */
public class Base64 {

    private static final String etab =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

    private static byte[] dtab = {
        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
        -1,-1,-1,-1,-1,-1,-1,-1,-1,62,-1,-1,-1,63,52,53,54,
        55,56,57,58,59,60,61,-1,-1,-1,64,-1,-1,-1, 0, 1, 2,
         3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19,
        20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,
        31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,
        48,49,50,51,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
    };

    public static String decode(String data) {
        StringBuilder out = new StringBuilder();

        // length must be multiple of 4 (with padding)
        if (data.length() % 4 != 0)
            return "";

        for (int i = 0; i < data.length();) {
            byte e0 = dtab[data.charAt(i++) & 0x7f];
            byte e1 = dtab[data.charAt(i++) & 0x7f];
            byte e2 = dtab[data.charAt(i++) & 0x7f];
            byte e3 = dtab[data.charAt(i++) & 0x7f];

            // Invalid characters in input
            if (e0 == -1 || e1 == -1 || e2 == -1 || e3 == -1)
                return "";

            byte d0 = (byte) ((e0 << 2) + ((e1 >>> 4) & 0x03));
            byte d1 = (byte) ((e1 << 4) + ((e2 >>> 2) & 0x0f));
            byte d2 = (byte) ((e2 << 6) + (e3 & 0x3f));

            out.append(Character.toString((char) d0));
            if (e2 != 64)
                out.append(Character.toString((char) d1));
            if (e3 != 64)
                out.append(Character.toString((char) d2));
        }
        return out.toString();
    }

    public static String encode(String data) {
        StringBuilder out = new StringBuilder();

        int i = 0;
        int r = data.length();
        while (r > 0) {
            byte d0, d1, d2;
            byte e0, e1, e2, e3;

            d0 = (byte) data.charAt(i++); --r;
            e0 = (byte) (d0 >>> 2);
            e1 = (byte) ((d0 & 0x03) << 4);

            if (r > 0) {
                d1 = (byte) data.charAt(i++); --r;
                e1 += (byte) (d1 >>> 4);
                e2 = (byte) ((d1 & 0x0f) << 2);
            }
            else {
                e2 = 64;
            }

            if (r > 0) {
                d2 = (byte) data.charAt(i++); --r;
                e2 += (byte) (d2 >>> 6);
                e3 = (byte) (d2 & 0x3f);
            }
            else {
                e3 = 64;
            }
            out.append(etab.charAt(e0));
            out.append(etab.charAt(e1));
            out.append(etab.charAt(e2));
            out.append(etab.charAt(e3));
        }

        return out.toString();
    }
}