# QuotedPrintableDecoder.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 QuotedPrintableDecoder {

        /**
         * The shift value required to create the upper nibble
         * from the first of 2 byte values converted from ascii hex.
         */
        private static final int UPPER_NIBBLE_SHIFT = Byte.SIZE / 2;

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

        /**
         * Decode the encoded byte data writing it to the given output stream.
         *
         * @param data   The array of byte data to decode.
         * @param out    The output stream used to return the decoded data.
         *
         * @return the number of bytes produced.
         * @throws IOException
         */
        public static int decode(byte[] data, OutputStream out) throws IOException {
            int off = 0;
            int length = data.length;
            int endOffset = off + length;
            int bytesWritten = 0;

            while (off < endOffset) {
                byte ch = data[off++];

                // space characters were translated to '_' on encode, so we need to translate them back.
                if (ch == '_') {
                    out.write(' ');
                } else if (ch == '=') {
                    // we found an encoded character.  Reduce the 3 char sequence to one.
                    // but first, make sure we have two characters to work with.
                    if (off + 1 >= endOffset) {
                        throw new IOException("Invalid quoted printable encoding; truncated escape sequence");
                    }

                    byte b1 = data[off++];
                    byte b2 = data[off++];

                    // we've found an encoded carriage return.  The next char needs to be a newline
                    if (b1 == '\r') {
                        if (b2 != '\n') {
                            throw new IOException("Invalid quoted printable encoding; CR must be followed by LF");
                        }
                        // this was a soft linebreak inserted by the encoding.  We just toss this away
                        // on decode.
                    } else {
                        // this is a hex pair we need to convert back to a single byte.
                        int c1 = hexToBinary(b1);
                        int c2 = hexToBinary(b2);
                        out.write((c1 << UPPER_NIBBLE_SHIFT) | c2);
                        // 3 bytes in, one byte out
                        bytesWritten++;
                    }
                } else {
                    // simple character, just write it out.
                    out.write(ch);
                    bytesWritten++;
                }
            }

            return bytesWritten;
        }

        /**
         * Convert a hex digit to the binary value it represents.
         *
         * @param b the ascii hex byte to convert (0-0, A-F, a-f)
         * @return the int value of the hex byte, 0-15
         * @throws IOException if the byte is not a valid hex digit.
         */
        private static int hexToBinary(final byte b) throws IOException {
            // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
            final int i = Character.digit((char) b, 16);
            if (i == -1) {
                throw new IOException("Invalid quoted printable encoding: not a valid hex digit: " + b);
            }
            return i;
        }

    }