/**
	 * detectEncoding.java - Returns the character encoding of an input stream containin an XML file.<br/> 
	 * Copyright (c) 2009 Alexander Hristov .
	 *
	 * Licensed under the LGPL License - http://www.gnu.org/licenses/lgpl.txt
	 * 
	 * This library is free software; you can redistribute it and/or
	 * modify it under the terms of the GNU Lesser General Public
	 * License as published by the Free Software Foundation; either
	 * version 2.1 of the License, or (at your option) any later version.
	 *
	 * This library is distributed in the hope that it will be useful,
	 * but WITHOUT ANY WARRANTY; without even the implied warranty of
	 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
	 * Lesser General Public License for more details.
	 *
	 * You should have received a copy of the GNU Lesser General Public
	 * License along with this library; if not, write to the Free Software
	 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA  
	 */
	 
	 
	 /**
	 * Returns the character encoding of an input stream containin an XML file.<br/> 
	 * 
	 * The encoding is detected using the guidelines specified in the
	 * <a href='http://www.w3.org/TR/xml/#sec-guessing'>XML W3C Specification</a>,
	 * and the method was designed to be as fast as possible, without extensive
	 * string operations or regular expressions<br/> <br/> 
	 * 
	 * <code>
	 * A sample use would be<br/><br/>
	 * InputStream in = ...; <br/>
	 * String encoding = detectEncoding(in);<br/>
	 * BufferedReader reader = new BufferedReader(new InputStreamReader(in,encoding)); <br/>
	 * </code><br/> 
	 * 
	 * and from that point you can happily read text from the input stream.
	 * 
	 * @param in
	 *          Stream containing the data to be read. The stream must support
	 *          mark()/reset(), otherwise the caller should wrap that stream in a
	 *          {@link BufferedInputStream} before invokin the method. After the
	 *          call, the stream is positioned at the &lt; character (this means
	 *          that if there were any byte-order-marks, they are skipped).
	 * 
	 * @return Detected encoding, using the canonical name in java.io (see <a
	 *         href=
	 *         'http://java.sun.com/j2se/1.4.2/docs/guide/intl/encoding.doc.html'>Supported
	 *         Encodings</a> ).
	 * 
	 * @author Alexander Hristov
	 */
	public static String detectEncoding(InputStream in) throws IOException {
		String encoding = null;
		in.mark(400);
		int ignoreBytes = 0;
		boolean readEncoding = false;
		byte[] buffer = new byte[400];
		int read = in.read(buffer, 0, 4);
		switch (buffer[0]) {
		case (byte) 0x00:
			if (buffer[1] == (byte) 0x00 && buffer[2] == (byte) 0xFE
					&& buffer[3] == (byte) 0xFF) {
				ignoreBytes = 4;
				encoding = "UTF_32BE";
			} else if (buffer[1] == (byte) 0x00 && buffer[2] == (byte) 0x00
					&& buffer[3] == (byte) 0x3C) {
				encoding = "UTF_32BE";
				readEncoding = true;
			} else if (buffer[1] == (byte) 0x3C && buffer[2] == (byte) 0x00
					&& buffer[3] == (byte) 0x3F) {
				encoding = "UnicodeBigUnmarked";
				readEncoding = true;
			}
			break;
		case (byte) 0xFF:
			if (buffer[1] == (byte) 0xFE && buffer[2] == (byte) 0x00
					&& buffer[3] == (byte) 0x00) {
				ignoreBytes = 4;
				encoding = "UTF_32LE";
			} else if (buffer[1] == (byte) 0xFE) {
				ignoreBytes = 2;
				encoding = "UnicodeLittleUnmarked";
			}
			break;

		case (byte) 0x3C:
			readEncoding = true;
			if (buffer[1] == (byte) 0x00 && buffer[2] == (byte) 0x00
					&& buffer[3] == (byte) 0x00) {
				encoding = "UTF_32LE";
			} else if (buffer[1] == (byte) 0x00 && buffer[2] == (byte) 0x3F
					&& buffer[3] == (byte) 0x00) {
				encoding = "UnicodeLittleUnmarked";
			} else if (buffer[1] == (byte) 0x3F && buffer[2] == (byte) 0x78
					&& buffer[3] == (byte) 0x6D) {
				encoding = "ASCII";
			}
			break;
		case (byte) 0xFE:
			if (buffer[1] == (byte) 0xFF) {
				encoding = "UnicodeBigUnmarked";
				ignoreBytes = 2;
			}
			break;
		case (byte) 0xEF:
			if (buffer[1] == (byte) 0xBB && buffer[2] == (byte) 0xBF) {
				encoding = "UTF8";
				ignoreBytes = 3;
			}
			break;
		case (byte) 0x4C:
			if (buffer[1] == (byte) 0x6F && buffer[2] == (byte) 0xA7
					&& buffer[3] == (byte) 0x94) {
				encoding = "CP037";
			}
			break;
		}
		if (encoding == null) {
			encoding = System.getProperty("file.encoding");
		}
		if (readEncoding) {
			read = in.read(buffer, 4, buffer.length - 4);
			Charset cs = Charset.forName(encoding);
			String s = new String(buffer, 4, read, cs);
			int pos = s.indexOf("encoding");
			if (pos == -1) {
				encoding = System.getProperty("file.encoding");
			} else {
				char delim;
				int start = s.indexOf(delim = '\'', pos);
				if (start == -1)
					start = s.indexOf(delim = '"', pos);
				if (start == -1)
					notifyEncodingError(buffer);
				int end = s.indexOf(delim, start + 1);
				if (end == -1)
					notifyEncodingError(buffer);
				encoding = s.substring(start + 1, end);
			}
		}

		in.reset();
		while (ignoreBytes-- > 0)
			in.read();
		return encoding;
	}