P25 audio decoder integration.

Auto-discovers the jmbe ( https://github.com/DSheirer/jmbe ) library at startup and provides decode P25 Phase 1 audio

src/edac/Golay23.java

@@ -0,0 +1,128 @@
+package edac;
+
+import bits.BinaryMessage;
+
+public class Golay23
+{
+	public static final int MAX_CORRECTABLE_ERRORS = 3;
+	
+	public static final int[] CHECKSUMS = new int[]
+	{
+	    0x63A, 0x31D, 0x7B4, 0x3DA, 0x1ED, 0x6CC, 0x366, 0x1B3, 
+	    0x6E3, 0x54B, 0x49F, 0x475, 0x400, 0x200, 0x100, 0x080, 
+	    0x040, 0x020, 0x010, 0x008, 0x004, 0x002, 0x001 
+	};
+
+	/**
+	 * Implements Golay(23,11,7) error detection and correction.  Returns the
+	 * number of detected errors.  If the error count is less than or equal to
+	 * the max correctable errors (3), then the error bits are corrected.  
+	 * Otherwise the message is left intact and an error count greater than 3
+	 * is returned.
+	 * 
+	 * @param frame - message frame bitset
+	 * @param startIndex - first bit index of the golay protected bit sequence
+	 * 
+	 * @return - number of detected errors
+	 */
+	public static int checkAndCorrect( BinaryMessage frame, int startIndex )
+	{
+		int syndrome = getSyndrome( frame, startIndex );
+		
+		/* No errors */
+		if( syndrome == 0 )
+		{
+			return 0;
+		}
+		
+		BinaryMessage copy = frame.getSubMessage( startIndex, startIndex + 23 );
+
+		int index = -1;
+		int syndromeWeight = MAX_CORRECTABLE_ERRORS;
+		int errors = 0;
+		
+		while( index < 23 )
+		{
+			if( index != -1 )
+			{
+				/* restore the previous flipped bit */
+				if( index > 0 )
+				{
+					copy.flip( index - 1 );
+				}
+				
+				copy.flip( index );
+				
+				syndromeWeight = MAX_CORRECTABLE_ERRORS - 1;
+			}
+			
+			syndrome = getSyndrome( copy, 0 );
+			
+			if( syndrome > 0 )
+			{
+				for( int i = 0; i < 23; i++ )
+				{
+					errors = Integer.bitCount( syndrome );
+					
+					if( errors <= syndromeWeight )
+					{
+						copy.xor( 12, 11, syndrome );
+						
+						copy.rotateRight( i, 0, 22 );
+
+						if( index >= 0 )
+						{
+							errors ++;
+						}
+
+						int corrected = copy.getInt( 0, 22 );
+						int original = frame.getInt( startIndex, startIndex + 22 );
+						
+						int errorCount = Integer.bitCount( original ^ corrected );
+						
+						if( errorCount <= 3 )
+						{
+							frame.load( startIndex, 23, corrected );
+						}
+
+						return errorCount;
+					}
+					else
+					{
+						copy.rotateLeft( 0, 22 );
+						syndrome = getSyndrome( copy, 0 );
+					}
+				}
+				
+				index++;
+			}
+		}
+
+		/* Return an error count greater than 3 to indicate failed correction attempt */
+		return 4;
+	}
+
+	private static int getSyndrome( BinaryMessage frame, int startIndex )
+	{
+		int calculated = calculateChecksum( frame, startIndex );
+		
+		int checksum = frame.getInt( startIndex + 12, startIndex + 22 );
+
+		return ( checksum ^ calculated );
+	}
+	
+	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 + 12; 
+				 i = frame.nextSetBit( i+1 ) ) 
+		{
+			calculated ^= CHECKSUMS[ i - startIndex ];
+		}
+		
+		return calculated;
+	}
+}