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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.AISMessage24PartA;
import nl.esi.metis.aisparser.Sixbit;
import nl.esi.metis.aisparser.UtilsString;
import nl.esi.metis.aisparser.provenance.VDMMessageProvenance;
/* ESI AIS Parser
 * 
 * Copyright 2011/2012 by Pierre van de Laar (Embedded Systems Institute)
 * Copyright 2008 by Brian C. Lane 
 * All Rights Reserved
 * 
 */

/** This class implements AIS messages of type 24 Part A.
 *  * Field Nr     Field Name                              Nr of Bits (from, to )
 * ------------------------------------------------------------------------
 *  1           messageID                                        6 (   1,   6)
 *  2           repeatIndicator                                  2 (   7,   8)
 *  3           userID                                          30 (   9,  38)
 *  4           partNumber                                       2 (  39,  40)
 *  5           name                                           120 (  41, 160)
 *                                                            ---- +
 *                                 (maximum) number of bits    160
 * 
 * @author Pierre van de Laar
 * @author Pierre America
 * @author Brian C. Lane
 */
class AISMessage24PartAImpl extends AISMessage24Impl implements AISMessage24PartA {

	/** The length of a valid message of type 24 Part A. */
	public static final int LENGTHPartA = 160;

	/** Checks whether the content is a valid message of type 24 Part A. (length & Part number)
	 * @param content possible message type 24 A
	 * @return true if this could be a valid message of type 24 Part A
	 */
	public static boolean valid(Sixbit content)
	{
		return ( (content.length() == LENGTHPartA) && (content.getIntFromTo(PARTNUMBER_FROM,PARTNUMBER_TO) == 0) );
	}
	
	public static boolean isTypeA(Sixbit content)
	{
		return AISMessage24Impl.valid(content) && (content.getIntFromTo(PARTNUMBER_FROM,PARTNUMBER_TO) == 0);
	}
	
	/**
	 * Return the difference in available and needed bits to parse this sixbit as an AisMessagePositionReport
	 * A positive difference indicates that there are more bits available than needed by the standard.
	 * @param sb
	 * @return
	 */
	public static int differenceInBits(Sixbit sb)
	{	
		return (sb.available() - LENGTHPartA);
	}

	/** The number of the first bit of the name field */ 
	private static final int NAME_FROM = 41;

	/** The number of the last bit of the name field */ 
	private static final int NAME_TO = 160;

	/** The name field */
	private String name;

	/** Returns the name field.
	 * @return the String value of the name field (120 bits [41,160])
	 */
	public String getName() { return name; }

	/** Returns the part number.
	 * @return the part number, which is always 0 for Part A.
	 */
	public int getPartNumber() {
		return 0;
	}

	/** Constructs an AIS Message 24 Part A.
	 * @param content the content of an AIS message
	 * @param prov the provenance of the message
	 * @precondition valid(content)
	 */
	public AISMessage24PartAImpl(Sixbit content, VDMMessageProvenance prov)
	{
		super(content, prov);
		assert(valid(content));

		/* Parse 24A */
		/* Get the Ship Name, convert to ASCII */
		name = UtilsString.stripAtSigns(content.getStringFromTo(NAME_FROM,NAME_TO));
	}
	
	/** 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() {
		return super.toString() + SEPARATOR + "A" + SEPARATOR + getName(); 
	}
}