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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.AISMessage16;
import nl.esi.metis.aisparser.AISMessage16Unit;
import nl.esi.metis.aisparser.Sixbit;
import nl.esi.metis.aisparser.UtilsSpare;
/** ESI AIS Parser
 * 
 * Copyright 2011/2012 by Pierre van de Laar (Embedded Systems Institute)
 * Copyright 2008 by Brian C. Lane 
 * All Rights Reserved
 * 
 * @author Pierre van de Laar
 * @author Brian C. Lane
 */
import nl.esi.metis.aisparser.annotations.AISIllegalValueAnnotation;
import nl.esi.metis.aisparser.provenance.VDMMessageProvenance;


/** AIS Message 16
 * Assigned mode command
 * 
 * Field Nr     Field Name                          NrOf Bits   (from, to )
 * ------------------------------------------------------------------------
 *  1	messageID                               	   6	(   1,   6)
 *  2	repeatIndicator                         	   2	(   7,   8)
 *  3	userID                                  	  30	(   9,  38)
 *  4	spare1                                  	   2	(  39,  40)
 *  5	destinationIDA                          	  30	(  41,  70)
 *  6	offsetA                                 	  12	(  71,  82)
 *  7	incrementA                              	  10	(  83,  92)
 *  8	destinationIDB                          	  30	(  93, 122)
 *  9	offsetB                                 	  12	( 123, 134)
 * 10	incrementB                              	  10	( 135, 144)
 * 11	spare2                                  	   0
 *                                                      ---- +
 *                           (maximum) number of bits    144
 */
class AISMessage16Impl extends AISMessageImpl implements AISMessage16 {
	public static final int LENGTH_ONEDESTINATION = 96;
	public static final int LENGTH_TWODESTINATIONS = 144;
	
	public static boolean validLength(int length)
	{
		return (length == LENGTH_ONEDESTINATION) ||
		       (length == LENGTH_TWODESTINATIONS) ;
	}
	/**
	 * 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
	 * @return
	 */
	public static int differenceInBits(Sixbit sb)
	{
		final int available = sb.available();
		if (available >= LENGTH_TWODESTINATIONS)
		{
			return available - LENGTH_TWODESTINATIONS;
		}
		else
		{
			return available - LENGTH_ONEDESTINATION;
		}
	}

	private static final int SPARE1_FROM = 39;
	private static final int SPARE1_TO = 40;

	private int spare1;
	/** spare1
	 * @return int value of spare1 (2 bits [39,40])
	 */
	public int getSpare1() { return spare1; }

	private static final int UNITA_FROM = 41;
	private static final int UNITB_FROM = 93;

	private AISMessage16Unit[] units;
	/** units
	 * @return Array of AISMessage16Unit containing all units in the AIS Message
	 */
	public AISMessage16Unit[] getUnits() { return units; }

	
	private static final int SPARE2_FROM = 93;
	private static final int SPARE2_TO = 96;

	private int spare2;
	/** spare2
	 * @return int value of spare2 (0 or 4 bits {4 bits at [93,96]})
	 */
	public int getSpare2() { return spare2; }

  
	/** AISMessage 16 constructor
	 * @param content AIS content
	 * @param prov the provenance of the message
	 * @precondition validLength(content.length()) && AISMessageBase.getMessageId(content)==16
	 */
	public AISMessage16Impl(Sixbit content, VDMMessageProvenance prov)
	{
		super(content, prov);
		assert(validLength(content.length()));
		assert(getMessageID() == 16);

		spare1 = content.getIntFromTo(SPARE1_FROM,SPARE1_TO);
	    if (!UtilsSpare.isSpareSemanticallyCorrect(spare1))
	    {
	    	annotations.add(new AISIllegalValueAnnotation("getSpare1", spare1, UtilsSpare.range));
	    }
		if(content.length() == LENGTH_ONEDESTINATION)
		{
			units = new AISMessage16Unit[1];
			units[0] = new AISMessage16UnitImpl(UNITA_FROM, content);
			spare2 = content.getIntFromTo(SPARE2_FROM,SPARE2_TO);
		}
		else
		{
			assert(content.length() == LENGTH_TWODESTINATIONS);
			units = new AISMessage16Unit[2];
			units[0] = new AISMessage16UnitImpl(UNITA_FROM, content);
			units[1] = new AISMessage16UnitImpl(UNITB_FROM, content);
			spare2 = 0;
		}
	    if (!UtilsSpare.isSpareSemanticallyCorrect(spare2))
	    {
	    	annotations.add(new AISIllegalValueAnnotation("getSpare2", spare2, UtilsSpare.range));
	    }
	}
	
	/** Generates a String representing the AIS message. 
	 * Format: all fields are shown in the order as specified by the standard separated by the SEPARATOR string.
	 */
	@Override
	public String toString() {
		String result = super.toString();
		for(AISMessage16Unit unit : units)
		{
			result += SEPARATOR + unit.toString();
		}
		return result;
	}
}