public static void decode()

in src/main/java/org/apache/xml/security/utils/Base64.java [693:762]
60 lines of code
15 McCabe index (conditional complexity)

    public static void decode(InputStream is, OutputStream os)
        throws Base64DecodingException, IOException {
        //byte[] decodedData = null;
        byte b1 = 0, b2 = 0, b3 = 0, b4 = 0;

        int index = 0;
        byte[] data = new byte[4];
        int read;
        //the begin
        while ((read = is.read()) > 0) {
            byte readed = (byte)read;
            if (isWhiteSpace(readed)) {
                continue;
            }
            if (isPad(readed)) {
                data[index++] = readed;
                if (index == 3) {
                    data[index++] = (byte)is.read();
                }
                break;
            }

            if ((data[index++] = readed) == -1) {
                //if found "no data" just return null
                throw new Base64DecodingException("decoding.general");
            }

            if (index != 4) {
                continue;
            }
            index = 0;
            b1 = base64Alphabet[data[0]];
            b2 = base64Alphabet[data[1]];
            b3 = base64Alphabet[data[2]];
            b4 = base64Alphabet[data[3]];

            os.write((byte)(b1 << 2 | b2 >> 4));
            os.write((byte)(((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)));
            os.write((byte)(b3 << 6 | b4));
        }

        byte d1 = data[0], d2 = data[1], d3 = data[2], d4 = data[3];
        b1 = base64Alphabet[d1];
        b2 = base64Alphabet[d2];
        b3 = base64Alphabet[d3];
        b4 = base64Alphabet[d4];
        if (b3 == -1 || b4 == -1) { //Check if they are PAD characters
            if (isPad(d3) && isPad(d4)) {               //Two PAD e.g. 3c[Pad][Pad]
                if ((b2 & 0xf) != 0) { //last 4 bits should be zero
                    throw new Base64DecodingException("decoding.general");
                }
                os.write((byte)(b1 << 2 | b2 >> 4));
            } else if (!isPad(d3) && isPad(d4)) {               //One PAD  e.g. 3cQ[Pad]
                b3 = base64Alphabet[d3];
                if ((b3 & 0x3) != 0) { //last 2 bits should be zero
                    throw new Base64DecodingException("decoding.general");
                }
                os.write((byte)(b1 << 2 | b2 >> 4));
                os.write((byte)(((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)));
            } else {
                //an error  like "3c[Pad]r", "3cdX", "3cXd", "3cXX" where X is non data
                throw new Base64DecodingException("decoding.general");
            }
        } else {
            //No PAD e.g 3cQl
            os.write((byte)(b1 << 2 | b2 >> 4));
            os.write((byte)(((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf)));
            os.write((byte)(b3 << 6 | b4));
        }
    }