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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;

/** This class contains some utility methods to convert unsigned integers to signed integers using two's complement.
 * They are used internally in various implementation classes in this package.
 * 
 * @author Pierre America
 * @author Pierre van de Laar
 */
public class UtilsTwosComplement {

	/** Converts an 8-bit unsigned integer value to a signed value using 2's complement.
	 * @param bits an (8-bit) unsigned integer value
	 * @return a signed value
	 */
	public static int convertFrom8Bits (int bits)
	{
		assert( (bits & 0xFF) == bits);
		
		int retval = bits;
		if( bits >= 0x80 )
	    {
	        retval -= 0x100;
	    } 
		return retval;
	}

	/** Converts a 17-bit unsigned integer value to a signed value using 2's complement.
	 * @param bits a (17-bit) unsigned integer value
	 * @return a signed value
	 */
	public static int convertFrom17Bits (int bits)
	{
		assert( (bits & 0x1FFFF) == bits);
		
		int retval = bits;
		if( bits >= 0x10000 )
	    {
	        retval -= 0x20000;
	    } 
		return retval;
	}

	/** Converts an 18-bit unsigned integer value to a signed value using 2's complement.
	 * @param bits an (18-bit) unsigned integer value
	 * @return a signed value
	 */
	public static int convertFrom18Bits (int bits)
	{
		assert( (bits & 0x3FFFF) == bits);
		
		int retval = bits;
		if( bits >= 0x20000 )
	    {
	        retval -= 0x40000;
	    } 
		return retval;
	}

	/** Converts a 27-bit unsigned integer value to a signed value using 2's complement.
	 * @param bits a (27-bit) unsigned integer value
	 * @return a signed value
	 */
	public static int convertFrom27Bits (int bits)
	{
		assert( (bits & 0x7FFFFFF) == bits);
		
		int retval = bits;
		if( bits >= 0x4000000 )
	    {
	        retval -= 0x8000000;
	    } 
		return retval;
	}

	/** Converts a 28-bit unsigned integer value to a signed  value using 2's complement.
	 * @param bits a (28-bit) unsigned integer value
	 * @return a signed value
	 */
	public static int convertFrom28Bits (int bits)
	{
		assert( (bits & 0xFFFFFFF) == bits);
		
		int retval = bits;
		if( bits >= 0x8000000 )
	    {
	        retval -= 0x10000000;
	    } 
		return retval;
	}
}