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'databene commons' is an open source Java library by Volker Bergmann. It provides extensions to the Java core library by utility classes, abstract concepts and concrete implementations.
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/*
 * Copyright (C) 2004-2015 Volker Bergmann ([email protected]).
 * All rights reserved.
 *
 * 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 org.databene.commons;

import java.util.Arrays;

/**
 * Codes/Decodes Base64 strings.
 * @author Volker Bergmann
 * @since 0.2.04
 */
public class Base64Codec {

    private static final char[] DIGITS  = createDigits();
    private static final byte[] INDICES = createIndices();

    private Base64Codec() {}
    
    // interface ----------------------------------------------------------------------------------------

    public static String encode(byte[] source) {
        int srcLength = source.length;
        int srcCursor = 0;
        int unpaddedLength = (srcLength * 4 + 2) / 3;
        int paddedLength = ((srcLength + 2) / 3) * 4;
        char[] buffer = new char[paddedLength];
        int bufferCursor = 0;
        while (srcCursor < srcLength) {
            int d0 = source[srcCursor++] & 0xff;
            int d1 = srcCursor < srcLength ? source[srcCursor++] & 0xff : 0;
            int d2 = srcCursor < srcLength ? source[srcCursor++] & 0xff : 0;
            buffer[bufferCursor++] = DIGITS[d0 >>> 2];
            buffer[bufferCursor++] = DIGITS[((d0 & 0x3) << 4) | (d1 >>> 4)];
            buffer[bufferCursor] = bufferCursor++ < unpaddedLength ? DIGITS[((d1 & 0xf) << 2) | (d2 >>> 6)] : '=';
            buffer[bufferCursor] = bufferCursor++ < unpaddedLength ? DIGITS[d2 & 0x3F] : '=';
        }
        return new String(buffer);
    }

    public static byte[] decode(String code) {
        if (code == null)
            throw new IllegalArgumentException("code is null");
        char[] codeBuffer = code.toCharArray();
        int codeLength = codeBuffer.length;
        if (codeLength % 4 != 0)
            throw new IllegalArgumentException("Length of Base64 encoded input string is not a multiple of 4.");
        while (codeLength > 0 && codeBuffer[codeLength - 1] == '=')
            codeLength--;
        for (int i = 0; i < codeLength; i++)
            if (!isBase64Char(codeBuffer[i]))
                throw new IllegalArgumentException("Not a base64 code: " + code);
        int resultLength = (codeLength * 3) / 4;
        byte[] result = new byte[resultLength];
        int codeCursor = 0;
        int resultCursor = 0;
        while (codeCursor < codeLength) {
            int b0 = INDICES[codeBuffer[codeCursor++]];
            int b1 = INDICES[codeBuffer[codeCursor++]];
            int b2 = (codeCursor < codeLength ? INDICES[codeBuffer[codeCursor++]] : 0);
            int b3 = (codeCursor < codeLength ? INDICES[codeBuffer[codeCursor++]] : 0);
            result[resultCursor++] = (byte) ((b0 << 2) | (b1 >>> 4));
            if (resultCursor < resultLength)
                result[resultCursor++] = (byte) (((b1 & 0xf) << 4) | (b2 >>> 2));
            if (resultCursor < resultLength)
                result[resultCursor++] = (byte) (((b2 & 3) << 6) | b3);
        }
        return result;
    }
    
    // private helpers -------------------------------------------------------------------------------------
    
    public static boolean isBase64Char(char c) {
        return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z') || ('0' <= c && c <= '9') || c == '+' || c =='/';
    }

    private static char[] createDigits() {
        char[] digits = new char[64];
        int i = 0;
        for (char c = 'A'; c <= 'Z'; c++)
            digits[i++] = c;
        for (char c = 'a'; c <= 'z'; c++)
            digits[i++] = c;
        for (char c = '0'; c <= '9'; c++)
            digits[i++] = c;
        digits[i++] = '+';
        digits[i++] = '/';
        return digits;
    }

    private static byte[] createIndices() {
        byte[] indices = new byte[128];
        Arrays.fill(indices, 0, indices.length, (byte) -1);
        for (int i = 0; i < 64; i++)
            indices[DIGITS[i]] = (byte) i;
        return indices;
    }
}