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/*
 * JUnique - Helps in preventing multiple instances of the same application
 * 
 * Copyright (C) 2008-2010 Carlo Pelliccia (www.sauronsoftware.it)
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License version
 * 2.1, as published by the Free Software Foundation.
 *
 * This program 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 2.1 for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License version 2.1 along with this program.
 * If not, see .
 */
package it.sauronsoftware.junique;

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

/**
 * Static methods for encoding/decoding messages.
 * 
 * @author Carlo Pelliccia
 */
class Message {

	/**
	 * It reads a JUnique formatted message from an InputStream.
	 * 
	 * @param inputStream
	 *            The source stream.
	 * @return The message decoded from the stream.
	 * @throws IOException
	 *             In an I/O error occurs.
	 */
	public static String read(InputStream inputStream) throws IOException {
		// Message length (4 bytes)
		byte[] b = new byte[4];
		if (inputStream.read(b) != 4) {
			throw new IOException("Unexpected end of stream");
		}
		int length = byteArrayToInt(b);
		// Length validation.
		if (length < 0) {
			throw new IOException("Invalid length block");
		} else if (length == 0) {
			return "";
		} else {
			// The message in bytes.
			byte[] message = new byte[length];
			if (inputStream.read(message) != length) {
				throw new IOException("Unexpected end of stream");
			}
			// From bytes to string (utf-8).
			return new String(message, "UTF-8");
		}
	}

	/**
	 * It writes a JUnique formatted message in an OutputStream.
	 * 
	 * @param message
	 *            The message.
	 * @param outputStream
	 *            The OutputStream.
	 * @throws IOException
	 *             In an I/O error occurs.
	 */
	public static void write(String message, OutputStream outputStream)
			throws IOException {
		// Is this message null?
		if (message == null) {
			// Writes a 0 length block.
			outputStream.write(0);
			outputStream.write(0);
			outputStream.write(0);
			outputStream.write(0);
			outputStream.flush();
		} else {
			// Message length.
			int length = message.length();
			// The length block.
			byte[] lenght = intToByteArray(length);
			outputStream.write(lenght);
			outputStream.flush();
			// Message block.
			byte[] b = message.getBytes("UTF-8");
			outputStream.write(b);
			outputStream.flush();
		}
	}

	/**
	 * Encode an integer value in a four elements byte array.
	 * 
	 * @param value
	 *            The value to be encoded.
	 * @return The encoded value, as a four bytes array.
	 */
	private static byte[] intToByteArray(int value) {
		byte[] b = new byte[4];
		for (int i = 0; i < 4; i++) {
			int offset = (3 - i) * 8;
			b[i] = (byte) ((value >> offset) & 0xFF);
		}
		return b;
	}

	/**
	 * Decoded a four bytes lengh array into an integer value.
	 * 
	 * @param b
	 *            The source array (four elements, nothing less, nothing more)
	 * @return The decoded integer value.
	 */
	private static int byteArrayToInt(byte[] b) {
		int value = 0;
		for (int i = 0; i < 4; i++) {
			int shift = (3 - i) * 8;
			value += (b[i] & 0x000000FF) << shift;
		}
		return value;
	}
}