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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 ITDMA communication state as expressed in type 3 AIS messages.
 * See page 41 of the standard.
 *  * Field Nr     Field Name                          NrOf Bits   (from, to )
 * ------------------------------------------------------------------------
 *  1           syncState                                  2    (   1,   2)
 *  2           slotIncrement                             13    (   3,  15)
 *  3           numberOfSlots                              3    (  16,  18)
 *  4           keepFlag                                   1    (  19,  19)
 *                                                      ---- +
 *                           (maximum) number of bits     19
 * 
 * @author Pierre van de Laar
 * @author Pierre America
 * @author Brian C. Lane
 */
public class Itdma 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 increment */
	private static final int SLOTINCREMENT_FROM = 2;

	/** The last bit of the slot increment */
	private static final int SLOTINCREMENT_TO = 14;

	/** The slot increment */
	private int slotIncrement;

	/** Returns the slot increment.
	 * @return an integer value representing the slot increment (13 bits)
	 */
	public int getSlotIncrement() { return slotIncrement; }

	/** The first bit of the number of slots */
	private static final int NUMBEROFSLOTS_FROM = 15;

	/** The last bit of the number of slots */
	private static final int NUMBEROFSLOTS_TO = 17;

	/** The number of slots */
	private int numberOfSlots;

	/** Returns the number of consecutive slots to allocate.
	 * @return an integer value representing the number of slots: 

	 * 0 = 1 slot 

	 * 1 = 2 slots 

	 * 2 = 3 slots 

	 * 3 = 4 slots 

	 * 4 = 5 slots 

	 * 5 = 1 slot; offset = slot increment + 8192 

	 * 6 = 2 slots; offset = slot increment + 8192 

	 * 7 = 3 slots; offset = slot increment + 8192
	 */
	public int getNumberOfSlots() { return numberOfSlots; }

	/** The position of the keep flag bit. */
	private static final int KEEPFLAG_BITINDEX = 18;

	/** The keep flag */
	private boolean keepFlag;

	/** Returns the keep flag.
	 * @return true if the slot remains allocated for one additional frame
	 */
	public boolean getKeepFlag() { return keepFlag; }

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

		syncState = content.getIntFromTo(offset+SYNCSTATE_FROM,offset+SYNCSTATE_TO);
		slotIncrement = content.getIntFromTo(offset+SLOTINCREMENT_FROM,offset+SLOTINCREMENT_TO);
		numberOfSlots = content.getIntFromTo(offset+NUMBEROFSLOTS_FROM,offset+NUMBEROFSLOTS_TO);
		keepFlag = content.getBoolean(offset+KEEPFLAG_BITINDEX);
	}
	
	/** 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(slotIncrement) + SEPARATOR + Integer.toString(numberOfSlots) +  SEPARATOR + keepFlag;
	}
}