# Base64Decoder.java

    /*
     * Licensed to the Apache Software Foundation (ASF) under one or more
     * contributor license agreements.  See the NOTICE file distributed with
     * this work for additional information regarding copyright ownership.
     * The ASF licenses this file to You 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.apache.commons.fileupload.util.mime;

    import java.io.IOException;
    import java.io.OutputStream;

    /**
     * @since 1.3
     */
    final class Base64Decoder {

        /**
         * Decoding table value for invalid bytes.
         */
        private static final int INVALID_BYTE = -1; // must be outside range 0-63

        /**
         * Decoding table value for padding bytes, so can detect PAD afer conversion.
         */
        private static final int PAD_BYTE = -2; // must be outside range 0-63

        /**
         * Mask to treat byte as unsigned integer.
         */
        private static final int MASK_BYTE_UNSIGNED = 0xFF;

        /**
         * Number of bytes per encoded chunk - 4 6bit bytes produce 3 8bit bytes on output.
         */
        private static final int INPUT_BYTES_PER_CHUNK = 4;

        /**
         * Set up the encoding table.
         */
        private static final byte[] ENCODING_TABLE = {
            (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 padding byte.
         */
        private static final byte PADDING = (byte) '=';

        /**
         * Set up the decoding table; this is indexed by a byte converted to an unsigned int,
         * so must be at least as large as the number of different byte values,
         * positive and negative and zero.
         */
        private static final byte[] DECODING_TABLE = new byte[Byte.MAX_VALUE - Byte.MIN_VALUE + 1];

        static {
            // Initialise as all invalid characters
            for (int i = 0; i < DECODING_TABLE.length; i++) {
                DECODING_TABLE[i] = INVALID_BYTE;
            }
            // set up valid characters
            for (int i = 0; i < ENCODING_TABLE.length; i++) {
                DECODING_TABLE[ENCODING_TABLE[i]] = (byte) i;
            }
            // Allow pad byte to be easily detected after conversion
            DECODING_TABLE[PADDING] = PAD_BYTE;
        }

        /**
         * Hidden constructor, this class must not be instantiated.
         */
        private Base64Decoder() {
            // do nothing
        }

        /**
         * Decode the base 64 encoded byte data writing it to the given output stream,
         * whitespace characters will be ignored.
         *
         * @param data the buffer containing the Base64-encoded data
         * @param out the output stream to hold the decoded bytes
         *
         * @return the number of bytes produced.
         * @throws IOException thrown when the padding is incorrect or the input is truncated.
         */
        public static int decode(byte[] data, OutputStream out) throws IOException {
            int outLen = 0;
            byte[] cache = new byte[INPUT_BYTES_PER_CHUNK];
            int cachedBytes = 0;

            for (byte b : data) {
                final byte d = DECODING_TABLE[MASK_BYTE_UNSIGNED & b];
                if (d == INVALID_BYTE) {
                    continue; // Ignore invalid bytes
                }
                cache[cachedBytes++] = d;
                if (cachedBytes == INPUT_BYTES_PER_CHUNK) {
                    // CHECKSTYLE IGNORE MagicNumber FOR NEXT 4 LINES
                    final byte b1 = cache[0];
                    final byte b2 = cache[1];
                    final byte b3 = cache[2];
                    final byte b4 = cache[3];
                    if (b1 == PAD_BYTE || b2 == PAD_BYTE) {
                        throw new IOException("Invalid Base64 input: incorrect padding, first two bytes cannot be padding");
                    }
                    // Convert 4 6-bit bytes to 3 8-bit bytes
                    // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
                    out.write((b1 << 2) | (b2 >> 4)); // 6 bits of b1 plus 2 bits of b2
                    outLen++;
                    if (b3 != PAD_BYTE) {
                        // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
                        out.write((b2 << 4) | (b3 >> 2)); // 4 bits of b2 plus 4 bits of b3
                        outLen++;
                        if (b4 != PAD_BYTE) {
                            // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
                            out.write((b3 << 6) | b4);        // 2 bits of b3 plus 6 bits of b4
                            outLen++;
                        }
                    } else if (b4 != PAD_BYTE) { // if byte 3 is pad, byte 4 must be pad too
                        throw new // line wrap to avoid 120 char limit
                        IOException("Invalid Base64 input: incorrect padding, 4th byte must be padding if 3rd byte is");
                    }
                    cachedBytes = 0;
                }
            }
            // Check for anything left over
            if (cachedBytes != 0) {
                throw new IOException("Invalid Base64 input: truncated");
            }
            return outLen;
        }
    }