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The MongoDB Java Driver uber-artifact, containing mongodb-driver, mongodb-driver-core, and bson
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/*
 * Copyright (c) 2008-2014 MongoDB, Inc.
 * Copyright 1999,2005 The Apache Software Foundation.
 *
 * Licensed 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 com.mongodb.connection;

import com.mongodb.MongoException;

import java.io.UnsupportedEncodingException;

/**
 * Provides Base64 encoding and decoding.
 * This class implements Base64 encoding
 * Thanks to Apache Commons project. This class refactored from org.apache.commons.codec.binary
 * Original Thanks to commons project in
 * ws.apache.org for this code. 
 */
class Base64Codec {
    private static final int BYTES_PER_UNENCODED_BLOCK = 3;
    private static final int BYTES_PER_ENCODED_BLOCK = 4;

    /**
     * Mask used to extract 6 bits, used when encoding
     */
    private static final int SIX_BIT_MASK = 0x3f;

    /**
     * padding char
     */
    private static final byte PAD = '=';

    /**
     * This array is a lookup table that translates 6-bit positive integer index values into their "Base64 Alphabet"
     * equivalents as specified in Table 1 of RFC 2045.
     */
    private static final byte[] ENCODE_TABLE = {'A', 'B', 'C', 'D', 'E', 'F',
            'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S',
            'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
            'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
            't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5',
            '6', '7', '8', '9', '+', '/'};

    private static final int[] DECODE_TABLE = new int[128];

    static {
        for (int i = 0; i < ENCODE_TABLE.length; i++) {
            DECODE_TABLE[ENCODE_TABLE[i]] = i;
        }
    }

    public byte[] decode(final String s) {
        int delta = s.endsWith("==") ? 2 : s.endsWith("=") ? 1 : 0;
        byte[] buffer = new byte[s.length() * BYTES_PER_UNENCODED_BLOCK / BYTES_PER_ENCODED_BLOCK - delta];
        int mask = 0xFF;
        int pos = 0;
        for (int i = 0; i < s.length(); i += BYTES_PER_ENCODED_BLOCK) {
            int c0 = DECODE_TABLE[s.charAt(i)];
            int c1 = DECODE_TABLE[s.charAt(i + 1)];
            buffer[pos++] = (byte) (((c0 << 2) | (c1 >> 4)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c2 = DECODE_TABLE[s.charAt(i + 2)];
            buffer[pos++] = (byte) (((c1 << 4) | (c2 >> 2)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c3 = DECODE_TABLE[s.charAt(i + 3)];
            buffer[pos++] = (byte) (((c2 << 6) | c3) & mask);
        }
        return buffer;
    }

    public String encode(final byte[] in) {

        int modulus = 0;
        int bitWorkArea = 0;
        int numEncodedBytes = (in.length / BYTES_PER_UNENCODED_BLOCK) * BYTES_PER_ENCODED_BLOCK
                + ((in.length % BYTES_PER_UNENCODED_BLOCK == 0) ? 0 : 4);

        byte[] buffer = new byte[numEncodedBytes];
        int pos = 0;

        for (int b : in) {
            modulus = (modulus + 1) % BYTES_PER_UNENCODED_BLOCK;

            if (b < 0) {
                b += 256;
            }

            bitWorkArea = (bitWorkArea << 8) + b; //  BITS_PER_BYTE
            if (0 == modulus) { // 3 bytes = 24 bits = 4 * 6 bits to extract
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 18) & SIX_BIT_MASK];
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 12) & SIX_BIT_MASK];
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 6) & SIX_BIT_MASK];
                buffer[pos++] = ENCODE_TABLE[bitWorkArea & SIX_BIT_MASK];
            }
        }

        switch (modulus) { // 0-2
            case 1: // 8 bits = 6 + 2
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 2) & SIX_BIT_MASK]; // top 6 bits
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea << 4) & SIX_BIT_MASK]; // remaining 2
                buffer[pos++] = PAD;
                buffer[pos] = PAD; // Last entry no need to ++
                break;

            case 2: // 16 bits = 6 + 6 + 4
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 10) & SIX_BIT_MASK];
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea >> 4) & SIX_BIT_MASK];
                buffer[pos++] = ENCODE_TABLE[(bitWorkArea << 2) & SIX_BIT_MASK];
                buffer[pos] = PAD; // Last entry no need to ++
                break;
            default:
                break;
        }

        try {
            return new String(buffer, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new MongoException("UTF-8 Charset is not available");
        }
    }
}