P25 - adds error detection and correction to the Link Control message in LDU1 voice messages and to the Encryption Sync message in LDU2 messages.

src/edac/Hamming10.java

@@ -0,0 +1,116 @@
+package edac;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import bits.BinaryMessage;
+
+public class Hamming10
+{
+	private final static Logger mLog = LoggerFactory.getLogger( Hamming10.class );
+	
+	private static int[] CHECKSUMS = new int[] { 0xE,0xD,0xB,0X7,0x3,0xC };
+
+	/**
+	 * Performs error detection and correction of any single-bit errors.  This 
+	 * is a truncated version of the Hamming15 class.
+	 * 
+	 * @param frame - binary frame containing a Hamming(10,6,3) protected field
+	 * 
+	 * @param startIndex - offset to the first bit of the field
+	 * 
+	 * @return - 0 = no errors 
+	 * 			 1 = a single-bit error was corrected
+	 */
+	public static int checkAndCorrect( BinaryMessage frame, int startIndex )
+	{
+		int syndrome = getSyndrome( frame, startIndex );
+
+		switch( syndrome )
+		{
+			case 0:
+				return 0;
+			case 1:
+				frame.flip( startIndex + 9 ); //Parity 1
+				return 1;
+			case 2:
+				frame.flip( startIndex + 8 ); //Parity 2
+				return 1;
+			case 3:
+				frame.flip( startIndex + 4 ); //Data 2
+				return 1;
+			case 4:
+				frame.flip( startIndex + 7 ); //Parity 4
+				return 1;
+			case 5:
+				return 2;
+			case 6:
+				return 2;
+			case 7:
+				frame.flip( startIndex + 3 ); //Data 3
+				return 1;
+			case 8:
+				frame.flip( startIndex + 6 ); //Parity 8
+				return 1;
+			case 9:
+				return 2;
+			case 10:
+				return 2;
+			case 11:
+				frame.flip( startIndex + 2 ); //Data 4
+				return 1;
+			case 12:
+				frame.flip( startIndex + 5 ); //Data 1
+				return 1;
+			case 13:
+				frame.flip( startIndex + 1 ); //Data 5
+				return 1;
+			case 14:
+				frame.flip( startIndex + 0 ); //Data 6
+				return 1;
+			case 15:
+				return 2;
+		}
+
+		/* We'll never get to here */
+		return 2;
+	}
+
+	/**
+	 * Calculates the checksum (Parity 8,4,2,1) for data (6 <> 1 ) bits.
+	 * @param frame - frame containing hamming(10) protected word
+	 * @param startIndex - start bit index of the hamming protected word
+	 * @return parity value, 0 - 15
+	 */
+	private static int calculateChecksum( BinaryMessage frame, int startIndex )
+	{
+		int calculated = 0; //Starting value
+
+		/* Iterate the set bits and XOR running checksum with lookup value */
+		for (int i = frame.nextSetBit( startIndex ); 
+				 i >= startIndex && i < startIndex + 6; 
+				 i = frame.nextSetBit( i+1 ) ) 
+		{
+			calculated ^= CHECKSUMS[ i - startIndex ];
+		}
+		
+		return calculated;
+	}
+
+	/**
+	 * Calculates the syndrome - xor of the calculated checksum and the actual
+	 * checksum.
+	 * 
+	 * @param frame - binary frame containing a hamming(15,11,4) protected word
+	 * @param startIndex - of bit 0 of the hamming protected word
+	 * @return - 0 (no errors) or 1 (single bit error corrected)
+	 */
+	private static int getSyndrome( BinaryMessage frame, int startIndex )
+	{
+		int calculated = calculateChecksum( frame, startIndex );
+		
+		int checksum = frame.getInt( startIndex + 6, startIndex + 9 );
+
+		return ( checksum ^ calculated );
+	}
+}