private static String getXmlProlog()

in src/main/java/org/apache/commons/io/input/XmlStreamReader.java [284:321]
38 lines of code
9 McCabe index (conditional complexity)

    private static String getXmlProlog(final InputStream inputStream, final String guessedEnc) throws IOException {
        String encoding = null;
        if (guessedEnc != null) {
            final byte[] bytes = IOUtils.byteArray();
            inputStream.mark(IOUtils.DEFAULT_BUFFER_SIZE);
            int offset = 0;
            int max = IOUtils.DEFAULT_BUFFER_SIZE;
            int c = inputStream.read(bytes, offset, max);
            int firstGT = -1;
            String xmlProlog = ""; // avoid possible NPE warning (cannot happen; this just silences the warning)
            while (c != -1 && firstGT == -1 && offset < IOUtils.DEFAULT_BUFFER_SIZE) {
                offset += c;
                max -= c;
                c = inputStream.read(bytes, offset, max);
                xmlProlog = new String(bytes, 0, offset, guessedEnc);
                firstGT = xmlProlog.indexOf('>');
            }
            if (firstGT == -1) {
                if (c == -1) {
                    throw new IOException("Unexpected end of XML stream");
                }
                throw new IOException("XML prolog or ROOT element not found on first " + offset + " bytes");
            }
            final int bytesRead = offset;
            if (bytesRead > 0) {
                inputStream.reset();
                final BufferedReader bReader = new BufferedReader(new StringReader(xmlProlog.substring(0, firstGT + 1)));
                final StringBuilder prolog = new StringBuilder();
                IOConsumer.forEach(bReader.lines(), prolog::append);
                final Matcher m = ENCODING_PATTERN.matcher(prolog);
                if (m.find()) {
                    encoding = m.group(1).toUpperCase(Locale.ROOT);
                    encoding = encoding.substring(1, encoding.length() - 1);
                }
            }
        }
        return encoding;
    }