/* Copyright (c) 2012 Google Inc.
 * Copyright (c) 2015 The Qt Company Ltd.
 *
 * 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.qtproject.qt.android.purchasing;

// This code was converted from code at http://iharder.sourceforge.net/base64/
// Lots of extraneous features were removed.
/* The original code said:
 * <p>
 * I am placing this code in the Public Domain. Do with it as you will.
 * This software comes with no guarantees or warranties but with
 * plenty of well-wishing instead!
 * Please visit
 * <a href="http://iharder.net/xmlizable">http://iharder.net/xmlizable</a>
 * periodically to check for updates or to contribute improvements.
 * </p>
 *
 * @author Robert Harder
 * @author rharder@usa.net
 * @version 1.3
 */

/**
 * Base64 converter class. This code is not a complete MIME encoder;
 * it simply converts binary data to base64 data and back.
 *
 * <p>Note {@link CharBase64} is a GWT-compatible implementation of this
 * class.
 */
public class Base64 {
    /** Specify encoding (value is {@code true}). */
    public final static boolean ENCODE = true;

    /** Specify decoding (value is {@code false}). */
    public final static boolean DECODE = false;

    /** The equals sign (=) as a byte. */
    private final static byte EQUALS_SIGN = (byte) '=';

    /** The new line character (\n) as a byte. */
    private final static byte NEW_LINE = (byte) '\n';

    /**
     * The 64 valid Base64 values.
     */
    private final static byte[] ALPHABET =
        {(byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F',
        (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K',
        (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O', (byte) 'P',
        (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U',
        (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',
        (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e',
        (byte) 'f', (byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j',
        (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o',
        (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't',
        (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y',
        (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3',
        (byte) '4', (byte) '5', (byte) '6', (byte) '7', (byte) '8',
        (byte) '9', (byte) '+', (byte) '/'};

    /**
     * The 64 valid web safe Base64 values.
     */
    private final static byte[] WEBSAFE_ALPHABET =
        {(byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F',
        (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K',
        (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O', (byte) 'P',
        (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U',
        (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',
        (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e',
        (byte) 'f', (byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j',
        (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o',
        (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't',
        (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y',
        (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3',
        (byte) '4', (byte) '5', (byte) '6', (byte) '7', (byte) '8',
        (byte) '9', (byte) '-', (byte) '_'};

    /**
     * Translates a Base64 value to either its 6-bit reconstruction value
     * or a negative number indicating some other meaning.
     **/
    private final static byte[] DECODABET = {-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
        -5, -5, // Whitespace: Tab and Linefeed
        -9, -9, // Decimal 11 - 12
        -5, // Whitespace: Carriage Return
        -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
        -9, -9, -9, -9, -9, // Decimal 27 - 31
        -5, // Whitespace: Space
        -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 42
        62, // Plus sign at decimal 43
        -9, -9, -9, // Decimal 44 - 46
        63, // Slash at decimal 47
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
        -9, -9, -9, // Decimal 58 - 60
        -1, // Equals sign at decimal 61
        -9, -9, -9, // Decimal 62 - 64
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
        14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
        -9, -9, -9, -9, -9, -9, // Decimal 91 - 96
        26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
        39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
        -9, -9, -9, -9, -9 // Decimal 123 - 127
        /*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
    };

    /** The web safe decodabet */
    private final static byte[] WEBSAFE_DECODABET =
        {-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
        -5, -5, // Whitespace: Tab and Linefeed
        -9, -9, // Decimal 11 - 12
        -5, // Whitespace: Carriage Return
        -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
        -9, -9, -9, -9, -9, // Decimal 27 - 31
        -5, // Whitespace: Space
        -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 44
        62, // Dash '-' sign at decimal 45
        -9, -9, // Decimal 46-47
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
        -9, -9, -9, // Decimal 58 - 60
        -1, // Equals sign at decimal 61
        -9, -9, -9, // Decimal 62 - 64
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
        14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
        -9, -9, -9, -9, // Decimal 91-94
        63, // Underscore '_' at decimal 95
        -9, // Decimal 96
        26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
        39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
        -9, -9, -9, -9, -9 // Decimal 123 - 127
        /*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
        -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
        };

    // Indicates white space in encoding
    private final static byte WHITE_SPACE_ENC = -5;
    // Indicates equals sign in encoding
    private final static byte EQUALS_SIGN_ENC = -1;

    /** Defeats instantiation. */
    private Base64() {
    }

    /* ********  E N C O D I N G   M E T H O D S  ******** */

    /**
     * Encodes up to three bytes of the array <var>source</var>
     * and writes the resulting four Base64 bytes to <var>destination</var>.
     * The source and destination arrays can be manipulated
     * anywhere along their length by specifying
     * <var>srcOffset</var> and <var>destOffset</var>.
     * This method does not check to make sure your arrays
     * are large enough to accommodate <var>srcOffset</var> + 3 for
     * the <var>source</var> array or <var>destOffset</var> + 4 for
     * the <var>destination</var> array.
     * The actual number of significant bytes in your array is
     * given by <var>numSigBytes</var>.
     *
     * @param source the array to convert
     * @param srcOffset the index where conversion begins
     * @param numSigBytes the number of significant bytes in your array
     * @param destination the array to hold the conversion
     * @param destOffset the index where output will be put
     * @param alphabet is the encoding alphabet
     * @return the <var>destination</var> array
     * @since 1.3
     */
    private static byte[] encode3to4(byte[] source, int srcOffset,
            int numSigBytes, byte[] destination, int destOffset, byte[] alphabet) {
        //           1         2         3
        // 01234567890123456789012345678901 Bit position
        // --------000000001111111122222222 Array position from threeBytes
        // --------|    ||    ||    ||    | Six bit groups to index alphabet
        //          >>18  >>12  >> 6  >> 0  Right shift necessary
        //                0x3f  0x3f  0x3f  Additional AND

        // Create buffer with zero-padding if there are only one or two
        // significant bytes passed in the array.
        // We have to shift left 24 in order to flush out the 1's that appear
        // when Java treats a value as negative that is cast from a byte to an int.
        int inBuff =
                (numSigBytes > 0 ? ((source[srcOffset] << 24) >>> 8) : 0)
                | (numSigBytes > 1 ? ((source[srcOffset + 1] << 24) >>> 16) : 0)
                | (numSigBytes > 2 ? ((source[srcOffset + 2] << 24) >>> 24) : 0);

        switch (numSigBytes) {
            case 3:
                destination[destOffset] = alphabet[(inBuff >>> 18)];
                destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
                destination[destOffset + 3] = alphabet[(inBuff) & 0x3f];
                return destination;
            case 2:
                destination[destOffset] = alphabet[(inBuff >>> 18)];
                destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
                destination[destOffset + 3] = EQUALS_SIGN;
                return destination;
            case 1:
                destination[destOffset] = alphabet[(inBuff >>> 18)];
                destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                destination[destOffset + 2] = EQUALS_SIGN;
                destination[destOffset + 3] = EQUALS_SIGN;
                return destination;
            default:
                return destination;
        } // end switch
    } // end encode3to4

    /**
     * Encodes a byte array into Base64 notation.
     * Equivalent to calling
     * {@code encodeBytes(source, 0, source.length)}
     *
     * @param source The data to convert
     * @since 1.4
     */
    public static String encode(byte[] source) {
        return encode(source, 0, source.length, ALPHABET, true);
    }

    /**
     * Encodes a byte array into web safe Base64 notation.
     *
     * @param source The data to convert
     * @param doPadding is {@code true} to pad result with '=' chars
     *        if it does not fall on 3 byte boundaries
     */
    public static String encodeWebSafe(byte[] source, boolean doPadding) {
        return encode(source, 0, source.length, WEBSAFE_ALPHABET, doPadding);
    }

    /**
     * Encodes a byte array into Base64 notation.
     *
     * @param source the data to convert
     * @param off offset in array where conversion should begin
     * @param len length of data to convert
     * @param alphabet the encoding alphabet
     * @param doPadding is {@code true} to pad result with '=' chars
     * if it does not fall on 3 byte boundaries
     * @since 1.4
     */
    public static String encode(byte[] source, int off, int len, byte[] alphabet,
            boolean doPadding) {
        byte[] outBuff = encode(source, off, len, alphabet, Integer.MAX_VALUE);
        int outLen = outBuff.length;

        // If doPadding is false, set length to truncate '='
        // padding characters
        while (doPadding == false && outLen > 0) {
            if (outBuff[outLen - 1] != '=') {
                break;
            }
            outLen -= 1;
        }

        return new String(outBuff, 0, outLen);
    }

    /**
     * Encodes a byte array into Base64 notation.
     *
     * @param source the data to convert
     * @param off offset in array where conversion should begin
     * @param len length of data to convert
     * @param alphabet is the encoding alphabet
     * @param maxLineLength maximum length of one line.
     * @return the BASE64-encoded byte array
     */
    public static byte[] encode(byte[] source, int off, int len, byte[] alphabet,
            int maxLineLength) {
        int lenDiv3 = (len + 2) / 3; // ceil(len / 3)
        int len43 = lenDiv3 * 4;
        byte[] outBuff = new byte[len43 // Main 4:3
                                  + (len43 / maxLineLength)]; // New lines

        int d = 0;
        int e = 0;
        int len2 = len - 2;
        int lineLength = 0;
        for (; d < len2; d += 3, e += 4) {

            // The following block of code is the same as
            // encode3to4( source, d + off, 3, outBuff, e, alphabet );
            // but inlined for faster encoding (~20% improvement)
            int inBuff =
                    ((source[d + off] << 24) >>> 8)
                    | ((source[d + 1 + off] << 24) >>> 16)
                    | ((source[d + 2 + off] << 24) >>> 24);
            outBuff[e] = alphabet[(inBuff >>> 18)];
            outBuff[e + 1] = alphabet[(inBuff >>> 12) & 0x3f];
            outBuff[e + 2] = alphabet[(inBuff >>> 6) & 0x3f];
            outBuff[e + 3] = alphabet[(inBuff) & 0x3f];

            lineLength += 4;
            if (lineLength == maxLineLength) {
                outBuff[e + 4] = NEW_LINE;
                e++;
                lineLength = 0;
            } // end if: end of line
        } // end for: each piece of array

        if (d < len) {
            encode3to4(source, d + off, len - d, outBuff, e, alphabet);

            lineLength += 4;
            if (lineLength == maxLineLength) {
                // Add a last newline
                outBuff[e + 4] = NEW_LINE;
                e++;
            }
            e += 4;
        }

        assert (e == outBuff.length);
        return outBuff;
    }


    /* ********  D E C O D I N G   M E T H O D S  ******** */


    /**
     * Decodes four bytes from array <var>source</var>
     * and writes the resulting bytes (up to three of them)
     * to <var>destination</var>.
     * The source and destination arrays can be manipulated
     * anywhere along their length by specifying
     * <var>srcOffset</var> and <var>destOffset</var>.
     * This method does not check to make sure your arrays
     * are large enough to accommodate <var>srcOffset</var> + 4 for
     * the <var>source</var> array or <var>destOffset</var> + 3 for
     * the <var>destination</var> array.
     * This method returns the actual number of bytes that
     * were converted from the Base64 encoding.
     *
     *
     * @param source the array to convert
     * @param srcOffset the index where conversion begins
     * @param destination the array to hold the conversion
     * @param destOffset the index where output will be put
     * @param decodabet the decodabet for decoding Base64 content
     * @return the number of decoded bytes converted
     * @since 1.3
     */
    private static int decode4to3(byte[] source, int srcOffset,
            byte[] destination, int destOffset, byte[] decodabet) {
        // Example: Dk==
        if (source[srcOffset + 2] == EQUALS_SIGN) {
            int outBuff =
                    ((decodabet[source[srcOffset]] << 24) >>> 6)
                    | ((decodabet[source[srcOffset + 1]] << 24) >>> 12);

            destination[destOffset] = (byte) (outBuff >>> 16);
            return 1;
        } else if (source[srcOffset + 3] == EQUALS_SIGN) {
            // Example: DkL=
            int outBuff =
                    ((decodabet[source[srcOffset]] << 24) >>> 6)
                    | ((decodabet[source[srcOffset + 1]] << 24) >>> 12)
                    | ((decodabet[source[srcOffset + 2]] << 24) >>> 18);

            destination[destOffset] = (byte) (outBuff >>> 16);
            destination[destOffset + 1] = (byte) (outBuff >>> 8);
            return 2;
        } else {
            // Example: DkLE
            int outBuff =
                    ((decodabet[source[srcOffset]] << 24) >>> 6)
                    | ((decodabet[source[srcOffset + 1]] << 24) >>> 12)
                    | ((decodabet[source[srcOffset + 2]] << 24) >>> 18)
                    | ((decodabet[source[srcOffset + 3]] << 24) >>> 24);

            destination[destOffset] = (byte) (outBuff >> 16);
            destination[destOffset + 1] = (byte) (outBuff >> 8);
            destination[destOffset + 2] = (byte) (outBuff);
            return 3;
        }
    } // end decodeToBytes


    /**
     * Decodes data from Base64 notation.
     *
     * @param s the string to decode (decoded in default encoding)
     * @return the decoded data
     * @since 1.4
     */
    public static byte[] decode(String s) throws Base64DecoderException {
        byte[] bytes = s.getBytes();
        return decode(bytes, 0, bytes.length);
    }

    /**
     * Decodes data from web safe Base64 notation.
     * Web safe encoding uses '-' instead of '+', '_' instead of '/'
     *
     * @param s the string to decode (decoded in default encoding)
     * @return the decoded data
     */
    public static byte[] decodeWebSafe(String s) throws Base64DecoderException {
        byte[] bytes = s.getBytes();
        return decodeWebSafe(bytes, 0, bytes.length);
    }

    /**
     * Decodes Base64 content in byte array format and returns
     * the decoded byte array.
     *
     * @param source The Base64 encoded data
     * @return decoded data
     * @since 1.3
     * @throws Base64DecoderException
     */
    public static byte[] decode(byte[] source) throws Base64DecoderException {
        return decode(source, 0, source.length);
    }

    /**
     * Decodes web safe Base64 content in byte array format and returns
     * the decoded data.
     * Web safe encoding uses '-' instead of '+', '_' instead of '/'
     *
     * @param source the string to decode (decoded in default encoding)
     * @return the decoded data
     */
    public static byte[] decodeWebSafe(byte[] source)
            throws Base64DecoderException {
        return decodeWebSafe(source, 0, source.length);
    }

    /**
     * Decodes Base64 content in byte array format and returns
     * the decoded byte array.
     *
     * @param source the Base64 encoded data
     * @param off    the offset of where to begin decoding
     * @param len    the length of characters to decode
     * @return decoded data
     * @since 1.3
     * @throws Base64DecoderException
     */
    public static byte[] decode(byte[] source, int off, int len)
            throws Base64DecoderException {
        return decode(source, off, len, DECODABET);
    }

    /**
     * Decodes web safe Base64 content in byte array format and returns
     * the decoded byte array.
     * Web safe encoding uses '-' instead of '+', '_' instead of '/'
     *
     * @param source the Base64 encoded data
     * @param off    the offset of where to begin decoding
     * @param len    the length of characters to decode
     * @return decoded data
     */
    public static byte[] decodeWebSafe(byte[] source, int off, int len)
            throws Base64DecoderException {
        return decode(source, off, len, WEBSAFE_DECODABET);
    }

    /**
     * Decodes Base64 content using the supplied decodabet and returns
     * the decoded byte array.
     *
     * @param source the Base64 encoded data
     * @param off the offset of where to begin decoding
     * @param len the length of characters to decode
     * @param decodabet the decodabet for decoding Base64 content
     * @return decoded data
     */
    public static byte[] decode(byte[] source, int off, int len, byte[] decodabet)
            throws Base64DecoderException {
        int len34 = len * 3 / 4;
        byte[] outBuff = new byte[2 + len34]; // Upper limit on size of output
        int outBuffPosn = 0;

        byte[] b4 = new byte[4];
        int b4Posn = 0;
        int i = 0;
        byte sbiCrop = 0;
        byte sbiDecode = 0;
        for (i = 0; i < len; i++) {
            sbiCrop = (byte) (source[i + off] & 0x7f); // Only the low seven bits
            sbiDecode = decodabet[sbiCrop];

            if (sbiDecode >= WHITE_SPACE_ENC) { // White space Equals sign or better
                if (sbiDecode >= EQUALS_SIGN_ENC) {
                    // An equals sign (for padding) must not occur at position 0 or 1
                    // and must be the last byte[s] in the encoded value
                    if (sbiCrop == EQUALS_SIGN) {
                        int bytesLeft = len - i;
                        byte lastByte = (byte) (source[len - 1 + off] & 0x7f);
                        if (b4Posn == 0 || b4Posn == 1) {
                            throw new Base64DecoderException(
                                    "invalid padding byte '=' at byte offset " + i);
                        } else if ((b4Posn == 3 && bytesLeft > 2)
                                || (b4Posn == 4 && bytesLeft > 1)) {
                            throw new Base64DecoderException(
                                    "padding byte '=' falsely signals end of encoded value "
                                            + "at offset " + i);
                        } else if (lastByte != EQUALS_SIGN && lastByte != NEW_LINE) {
                            throw new Base64DecoderException(
                                    "encoded value has invalid trailing byte");
                        }
                        break;
                    }

                    b4[b4Posn++] = sbiCrop;
                    if (b4Posn == 4) {
                        outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
                        b4Posn = 0;
                    }
                }
            } else {
                throw new Base64DecoderException("Bad Base64 input character at " + i
                        + ": " + source[i + off] + "(decimal)");
            }
        }

        // Because web safe encoding allows non padding base64 encodes, we
        // need to pad the rest of the b4 buffer with equal signs when
        // b4Posn != 0.  There can be at most 2 equal signs at the end of
        // four characters, so the b4 buffer must have two or three
        // characters.  This also catches the case where the input is
        // padded with EQUALS_SIGN
        if (b4Posn != 0) {
            if (b4Posn == 1) {
                throw new Base64DecoderException("single trailing character at offset "
                        + (len - 1));
            }
            b4[b4Posn++] = EQUALS_SIGN;
            outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
        }

        byte[] out = new byte[outBuffPosn];
        System.arraycopy(outBuff, 0, out, 0, outBuffPosn);
        return out;
    }
}
