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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.impl;

import nl.esi.metis.aisparser.AISMessageAcknowledge;
import nl.esi.metis.aisparser.AISMessageAcknowledgeUnit;
import nl.esi.metis.aisparser.Sixbit;
import nl.esi.metis.aisparser.UtilsSpare;
/* 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
 * 
 */
import nl.esi.metis.aisparser.annotations.AISIllegalValueAnnotation;
import nl.esi.metis.aisparser.provenance.VDMMessageProvenance;

/** Implementation of AIS Message Acknowledgements.
 *  * Field Nr     Field Name                          NrOf Bits   (from, to )
 * ------------------------------------------------------------------------
 *  1           messageID                                   6   (   1,   6)
 *  2           repeatIndicator                             2   (   7,   8)
 *  3           userID                                     30   (   9,  38)
 *  4           spare                                       2   (  39,  40)
 *  5           destinationID1                             30   (  41,  70)
 *  6           sequenceNumberForID1                        2   (  71,  72)
 *  7           destinationID2                             30   (  73, 102)
 *  8           sequenceNumberForID2                        2   ( 103, 104)
 *  9           destinationID3                             30   ( 105, 134)
 * 10           sequenceNumberForID3                        2   ( 135, 136)
 * 11           destinationID4                             30   ( 137, 166)
 * 12           sequenceNumberForID4                        2   ( 167, 168)
 *                                                       ---- +
 *                           (maximum) number of bits     168
 * 
 * @author Pierre van de Laar
 * @author Pierre America
 * @author Brian C. Lane
 */
abstract public class AISMessageAcknowledgeImpl extends AISMessageImpl implements AISMessageAcknowledge {
	
	private static final int UNIT_SIZE = 32;		// destination UNIT has UserId (30 bits) and Sequence number (2 bits)

	public static final int MINLENGTH = 72;
	public static final int MAXLENGTH = 168;		
	// AIS Acknowledge Messages have 3 optional destinations
	// hence: MAXLENGTH == MINLENGTH + 3 * (30+2)
												
	public static boolean validLength(int length)
	{
		return (	( length == MINLENGTH ) ||
					( length == MINLENGTH + UNIT_SIZE ) ||
					( length == MINLENGTH + 2* UNIT_SIZE) ||
					( length == MAXLENGTH ) 
				);
	}
	
	/**
	 * Return the difference in available and needed bits to parse this sixbit as an AISMessage
	 * A positive difference indicates that there are more bits available than needed by the standard.
	 * @param sb the sixbit
	 * @return the difference in available and needed bits
	 */
	public static int differenceInBits(Sixbit sb)
	{
		final int available = sb.available();
		if (available >= MAXLENGTH)
		{
			return (available - MAXLENGTH);
		}
		else if (available >= MINLENGTH)
		{
			return (available - MINLENGTH) % UNIT_SIZE;
		}
		else
		{
			return available - MINLENGTH;
		}
	}
		
	private static final int SPARE_FROM = 39;
	private static final int SPARE_TO = 40;

	private int spare;
	/** spare
	 * @return int value of spare (2 bits [39,40])
	 */
	public int getSpare() { return spare; }

	private static final int UNITS_FROM = 41;

	private AISMessageAcknowledgeUnit[] units;
	/** units
	 * @return AISMessageAcknowledgeUnit[] value of units
	 */
	public AISMessageAcknowledgeUnit[] getUnits() { return units; }

	/** AISMessage Acknowledge constructor
	 * @param content AIS content
	 * @param prov the provenance of the message
	 * @precondition validLength(content.length())
	 */
	public AISMessageAcknowledgeImpl(Sixbit content, VDMMessageProvenance prov)
	{
		super(content, prov);
		assert(validLength(content.length()));
		
		spare = content.getIntFromTo(SPARE_FROM,SPARE_TO);
	    if (!UtilsSpare.isSpareSemanticallyCorrect(spare))
	    {
	    	annotations.add(new AISIllegalValueAnnotation("getSpare", spare, UtilsSpare.range));
	    }
		int nrofUnits = (content.length()-40)/UNIT_SIZE;
		units = new AISMessageAcknowledgeUnit[nrofUnits];
		for(int i = 0; i < nrofUnits; i++ )
		{
			int fromIndex = UNITS_FROM + i * UNIT_SIZE;
			units[i] = new AISMessageAcknowledgeUnitImpl(fromIndex, content);
		}
	}
}