# MimeUtility.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.ByteArrayOutputStream;
    import java.io.UnsupportedEncodingException;
    import java.util.Base64;
    import java.util.HashMap;
    import java.util.Locale;
    import java.util.Map;

    /**
     * Utility class to decode MIME texts.
     *
     * @since 1.3
     */
    public final class MimeUtility {

        /**
         * The {@code US-ASCII} charset identifier constant.
         */
        private static final String US_ASCII_CHARSET = "US-ASCII";

        /**
         * The marker to indicate text is encoded with BASE64 algorithm.
         */
        private static final String BASE64_ENCODING_MARKER = "B";

        /**
         * The marker to indicate text is encoded with QuotedPrintable algorithm.
         */
        private static final String QUOTEDPRINTABLE_ENCODING_MARKER = "Q";

        /**
         * If the text contains any encoded tokens, those tokens will be marked with "=?".
         */
        private static final String ENCODED_TOKEN_MARKER = "=?";

        /**
         * If the text contains any encoded tokens, those tokens will terminate with "=?".
         */
        private static final String ENCODED_TOKEN_FINISHER = "?=";

        /**
         * The linear whitespace chars sequence.
         */
        private static final String LINEAR_WHITESPACE = " \t\r\n";

        /**
         * Mappings between MIME and Java charset.
         */
        private static final Map<String, String> MIME2JAVA = new HashMap<>();

        static {
            MIME2JAVA.put("iso-2022-cn", "ISO2022CN");
            MIME2JAVA.put("iso-2022-kr", "ISO2022KR");
            MIME2JAVA.put("utf-8", "UTF8");
            MIME2JAVA.put("utf8", "UTF8");
            MIME2JAVA.put("ja_jp.iso2022-7", "ISO2022JP");
            MIME2JAVA.put("ja_jp.eucjp", "EUCJIS");
            MIME2JAVA.put("euc-kr", "KSC5601");
            MIME2JAVA.put("euckr", "KSC5601");
            MIME2JAVA.put("us-ascii", "ISO-8859-1");
            MIME2JAVA.put("x-us-ascii", "ISO-8859-1");
        }

        /**
         * Decode a string of text obtained from a mail header into
         * its proper form.  The text generally will consist of a
         * string of tokens, some of which may be encoded using
         * base64 encoding.
         *
         * @param text   The text to decode.
         * @return The decoded text string.
         * @throws UnsupportedEncodingException if the detected encoding in the input text is not supported.
         */
        public static String decodeText(final String text) throws UnsupportedEncodingException {
            // if the text contains any encoded tokens, those tokens will be marked with "=?".  If the
            // source string doesn't contain that sequent, no decoding is required.
            if (!text.contains(ENCODED_TOKEN_MARKER)) {
                return text;
            }

            int offset = 0;
            final int endOffset = text.length();

            int startWhiteSpace = -1;
            int endWhiteSpace = -1;

            final StringBuilder decodedText = new StringBuilder(text.length());

            boolean previousTokenEncoded = false;

            while (offset < endOffset) {
                char ch = text.charAt(offset);

                // is this a whitespace character?
                if (LINEAR_WHITESPACE.indexOf(ch) != -1) { // whitespace found
                    startWhiteSpace = offset;
                    while (offset < endOffset) {
                        // step over the white space characters.
                        ch = text.charAt(offset);
                        if (LINEAR_WHITESPACE.indexOf(ch) == -1) {
                            // record the location of the first non lwsp and drop down to process the
                            // token characters.
                            endWhiteSpace = offset;
                            break;
                        }
                        offset++;
                    }
                } else {
                    // we have a word token.  We need to scan over the word and then try to parse it.
                    final int wordStart = offset;

                    while (offset < endOffset) {
                        // step over the non white space characters.
                        ch = text.charAt(offset);
                        if (LINEAR_WHITESPACE.indexOf(ch) != -1) {
                            break;
                        }
                        offset++;

                        //NB:  Trailing whitespace on these header strings will just be discarded.
                    }
                    // pull out the word token.
                    final String word = text.substring(wordStart, offset);
                    // is the token encoded?  decode the word
                    if (word.startsWith(ENCODED_TOKEN_MARKER)) {
                        try {
                            // if this gives a parsing failure, treat it like a non-encoded word.
                            final String decodedWord = decodeWord(word);

                            // are any whitespace characters significant?  Append 'em if we've got 'em.
                            if (!previousTokenEncoded && startWhiteSpace != -1) {
                                decodedText.append(text, startWhiteSpace, endWhiteSpace);
                                startWhiteSpace = -1;
                            }
                            // this is definitely a decoded token.
                            previousTokenEncoded = true;
                            // and add this to the text.
                            decodedText.append(decodedWord);
                            // we continue parsing from here...we allow parsing errors to fall through
                            // and get handled as normal text.
                            continue;

                        } catch (final ParseException e) {
                            // just ignore it, skip to next word
                        }
                    }
                    // this is a normal token, so it doesn't matter what the previous token was.  Add the white space
                    // if we have it.
                    if (startWhiteSpace != -1) {
                        decodedText.append(text, startWhiteSpace, endWhiteSpace);
                        startWhiteSpace = -1;
                    }
                    // this is not a decoded token.
                    previousTokenEncoded = false;
                    decodedText.append(word);
                }
            }

            return decodedText.toString();
        }

        /**
         * Parse a string using the RFC 2047 rules for an "encoded-word"
         * type.  This encoding has the syntax:
         *
         * encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
         *
         * @param word   The possibly encoded word value.
         * @return The decoded word.
         * @throws ParseException in case of a parse error of the RFC 2047
         * @throws UnsupportedEncodingException Thrown when Invalid RFC 2047 encoding was found
         */
        private static String decodeWord(final String word) throws ParseException, UnsupportedEncodingException {
            // encoded words start with the characters "=?".  If this not an encoded word, we throw a
            // ParseException for the caller.

            if (!word.startsWith(ENCODED_TOKEN_MARKER)) {
                throw new ParseException("Invalid RFC 2047 encoded-word: " + word);
            }

            final int charsetPos = word.indexOf('?', 2);
            if (charsetPos == -1) {
                throw new ParseException("Missing charset in RFC 2047 encoded-word: " + word);
            }

            // pull out the character set information (this is the MIME name at this point).
            final String charset = word.substring(2, charsetPos).toLowerCase(Locale.ROOT);

            // now pull out the encoding token the same way.
            final int encodingPos = word.indexOf('?', charsetPos + 1);
            if (encodingPos == -1) {
                throw new ParseException("Missing encoding in RFC 2047 encoded-word: " + word);
            }

            final String encoding = word.substring(charsetPos + 1, encodingPos);

            // and finally the encoded text.
            final int encodedTextPos = word.indexOf(ENCODED_TOKEN_FINISHER, encodingPos + 1);
            if (encodedTextPos == -1) {
                throw new ParseException("Missing encoded text in RFC 2047 encoded-word: " + word);
            }

            final String encodedText = word.substring(encodingPos + 1, encodedTextPos);

            // seems a bit silly to encode a null string, but easy to deal with.
            if (encodedText.isEmpty()) {
                return "";
            }

            try {
                // the decoder writes directly to an output stream.
                final ByteArrayOutputStream out = new ByteArrayOutputStream(encodedText.length());

                final byte[] encodedData = encodedText.getBytes(US_ASCII_CHARSET);

                // Base64 encoded?
                if (encoding.equals(BASE64_ENCODING_MARKER)) {
                    out.write(Base64.getDecoder().decode(encodedData));
                } else if (encoding.equals(QUOTEDPRINTABLE_ENCODING_MARKER)) { // maybe quoted printable.
                    QuotedPrintableDecoder.decode(encodedData, out);
                } else {
                    throw new UnsupportedEncodingException("Unknown RFC 2047 encoding: " + encoding);
                }
                // get the decoded byte data and convert into a string.
                final byte[] decodedData = out.toByteArray();
                return new String(decodedData, javaCharset(charset));
            } catch (final Exception e) {
                throw new UnsupportedEncodingException("Invalid RFC 2047 encoding");
            }
        }

        /**
         * Translate a MIME standard character set name into the Java
         * equivalent.
         *
         * @param charset The MIME standard name.
         * @return The Java equivalent for this name.
         */
        private static String javaCharset(final String charset) {
            // nothing in, nothing out.
            if (charset == null) {
                return null;
            }

            final String mappedCharset = MIME2JAVA.get(charset.toLowerCase(Locale.ROOT));
            // if there is no mapping, then the original name is used.  Many of the MIME character set
            // names map directly back into Java.  The reverse isn't necessarily true.
            if (mappedCharset == null) {
                return charset;
            }
            return mappedCharset;
        }

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

    }