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This package supports the parsing of AIS messages in Java. AIS, the Automatic Identification System, is a system aiming at improving maritime safety by exchanging messages between ships, other vehicles in particular aircraft involved in search-and-rescue (SAR), and (fixed) base stations. To be precise, this package support the ITU-R M.1371-4 AIS standard. See our extensive javadoc and in particular the class AISParser for more information on how to use this package. The parser was used in the Poseidon project, and is improved in the Metis project to better handle uncertain information. Both projects were led by the Embedded Systems Institute. In both projects Thales Nederlands was the carrying industrial partner, and multiple Dutch universities participated.

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package nl.esi.metis.aisparser;
/* ESI AIS Parser
 * 
 * Copyright 2011/2012 by Pierre van de Laar, Pierre America (Embedded Systems Institute)
 * Copyright 2008 by Brian C. Lane 
 * All Rights Reserved
 * 
 */

/** This class represents the SOTDMA communication state as expressed in type 1 or 2 AIS messages.
 * See page 40 of the standard.
 *  * Field Nr     Field Name                         NrOf Bits   (from, to )
 * ------------------------------------------------------------------------
 *  1           syncState                                  2   (   1,   2)
 *  2           slotTimeOut                                3   (   3,   5)
 *  3           subMessage                                14   (   6,  19)
 *                                                      ---- +
 *                       (maximum) number of bits         19
 * 
 * @author Pierre van de Laar
 * @author Brian C. Lane
 */
public class Sotdma extends CommunicationState {
	/** The first bit of the sync state */
	private static final int SYNCSTATE_FROM = 0;

	/** The last bit of the sync state */
	private static final int SYNCSTATE_TO = 1;

	/** The sync state */
	private int syncState;

	/** Returns the sync state.
	 * @return an integer value representing the sync state (0-3)
	 */
	public int getSyncState() { return syncState; }

	/** The first bit of the slot time-out */
	private static final int SLOTTIMEOUT_FROM = 2;

	/** The last bit of the slot time-out */
	private static final int SLOTTIMEOUT_TO = 4;

	/** The  slot time-out */
	private int slotTimeOut;

	/** Returns the slot time-out. This specifies the frames remaining until a new slot is selected.
	 * @return an integer value representing the slot time-out: 

	 * 0 means that this was the last transmission in this slot 

	 * 1-7 means that 1 to 7 frames respectively are left until slot change
	 */
	public int getSlotTimeOut() { return slotTimeOut; }

	/** The first bit of the submessage */
	private static final int SUBMESSAGE_FROM = 5;

	/** The last bit of the submessage */
	private static final int SUBMESSAGE_TO = 18;

	/** The  submessage */
	private int subMessage;

	/** Returns the submessage.
	 * The meaning of submessage depends on the current value in slot time-out
	 * as described in Table 19 on page 40 of the standard.
	 * @return an integer value representing the submessage (14 bits)
	 */
	public int getSubMessage() { return subMessage; }

	/** Constructs an SOTDMA object.
	 * @param offset the position in the message from where to extract information
	 * @param content the content of the message
	 */
	public Sotdma( int offset, Sixbit content )
	{
		assert(offset+SUBMESSAGE_TO <= content.length());

		syncState = content.getIntFromTo(offset+SYNCSTATE_FROM,offset+SYNCSTATE_TO);
		slotTimeOut = content.getIntFromTo(offset+SLOTTIMEOUT_FROM,offset+SLOTTIMEOUT_TO);
		subMessage = content.getIntFromTo(offset+SUBMESSAGE_FROM,offset+SUBMESSAGE_TO);
	}
	

	/** Generates a String representing the Sotdma message. 
	 * Format: all fields are shown in the order as specified by the standard separated by the SEPARATOR string.
	 */
	@Override
	public String toString() {
		return Integer.toString(syncState) + SEPARATOR + Integer.toString(slotTimeOut) + SEPARATOR + Integer.toString(subMessage);
	}
}