Ejemplo n.º 1
0
	/**
	 * Determines if message bits 0 - 47 pass the Fleetsync CRC checksum 
	 * contained in bits 48 - 63, using a lookup table of CRC checksum values
	 * derived from the CRC-15 value, and verifies the message has even parity
	 */
	public static CRC check( RadioLog.Common.SafeBitArray msg )
	{
		CRC crc = CRC.UNKNOWN;
		
		int calculated = 1; //Starting value

		//Check even parity
		if( msg.Cardinality() % 2 == 0 )
		{
			//Iterate bits that are set and XOR running checksum with lookup value
			for (int i = msg.nextSetBit( 0 ); i >= 0 && i < 48; i = msg.nextSetBit( i+1 ) ) 
			{
				calculated ^= sCHECKSUMS[ i ];
			}
			
			if( calculated == getChecksum( msg ) )
			{
				crc = CRC.PASSED;
			}
			else
			{
				crc = CRC.FAILED_CRC;
			}
		}
		else
		{
			crc = CRC.FAILED_PARITY;
		}
		
		return crc;
	}
Ejemplo n.º 2
0
    /**
     * Determines the errant bit positions and returns them in an array of 
     * integer bit positions.
     * 
     * Note: currently only detects single-bit errors
     * 
     * @param msg to be checked for errors
     * @return - array of integer positions of bits that need flipped
     */
    public static int[] findBitErrors( RadioLog.Common.SafeBitArray msg )
    {
    	int[] retVal = null;
    	
    	int checksum = getChecksum( msg );
    	
		//Remove the initial fill value (1)
		checksum ^= 1;
		
		//Iterate set message bits, removing their respective checksum value
    	//from the transmitted checksum, to arrive at the remainder
		for (int i = msg.nextSetBit( 0 ); i >= 0 && i < 48; i = msg.nextSetBit( i+1 ) ) 
		{
			checksum ^= sCHECKSUMS[ i ];
		}
		
		//If at this point the checksum is 0, then we have a parity bit error
		if( checksum == 0 )
		{
			retVal = new int[ 1 ];
			retVal[ 0 ] = 63;
		}
		//Otherwise, try to lookup the syndrome for a single bit error
		else
		{
			for( int x = 0; x < 63; x++ )
			{
				if( checksum == sCHECKSUMS[ x ] )
				{
					//return this bit position
					retVal = new int[ 1 ];
					retVal[ 0 ] = x;
				}
			}
		}

		return retVal;
    }