/*******************************************************************************
* SDR Trunk
* Copyright (C) 2014 Dennis Sheirer
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>
******************************************************************************/
package edac;
import java.util.BitSet;
import bits.BinaryMessage;
/**
* Passport CRC checksum utility
*
* Passport message blocks are 68 bits in length as follows:
* 0 - 8: Sync
* 9 - 59: message bits
* 60 - 67: CRC-7 check bits plus 1 parity
*
* Passport uses a CRC-7 (0x44) with an initial fill of 0x44 plus parity set.
*
* CRC-7 Generating Polynomial: x6 + x2 + 1 (0x44)
*/
public class CRCPassport
{
private static final byte sFILL_00 = (byte)0x00;
private static byte[] sCHECKSUMS = new byte[]
{
(byte)0x6E, //DCC 1
(byte)0xBF, //DCC 0
(byte)0xD6, //LCN 10
(byte)0xE3, //LCN 9
(byte)0xF8, //LCN 8
(byte)0x7C, //LCN 7
(byte)0x3E, //LCN 6
(byte)0x97, //LCN 5
(byte)0xC2, //LCN 4
(byte)0xE9, //LCN 3
(byte)0x75, //LCN 2
(byte)0x3B, //LCN 1
(byte)0x94, //LCN 0
(byte)0x4A, //SITE 6
(byte)0xAD, //SITE 5
(byte)0x57, //SITE 4
(byte)0xA2, //SITE 3
(byte)0xD9, //SITE 2
(byte)0x6D, //SITE 1
(byte)0x37, //SITE 0
(byte)0x92, //GROUP 15
(byte)0xC1, //GROUP 14
(byte)0x61, //GROUP 13
(byte)0x31, //GROUP 12
(byte)0x19, //GROUP 11
(byte)0x0D, //GROUP 10
(byte)0x07, //GROUP 9
(byte)0x8A, //GROUP 8
(byte)0xCD, //GROUP 7
(byte)0x67, //GROUP 6
(byte)0xBA, //GROUP 5
(byte)0xD5, //GROUP 4
(byte)0x6B, //GROUP 3
(byte)0xBC, //GROUP 2
(byte)0x5E, //GROUP 1
(byte)0xA7, //GROUP 0
(byte)0xDA, //TYPE 3
(byte)0xE5, //TYPE 2
(byte)0x73, //TYPE 1
(byte)0xB0, //TYPE 0
(byte)0x58, //FREE 10
(byte)0x2C, //FREE 9
(byte)0x16, //FREE 8
(byte)0x83, //FREE 7
(byte)0xC8, //FREE 6
(byte)0x64, //FREE 5
(byte)0x32, //FREE 4
(byte)0x91, //FREE 3
(byte)0x49, //FREE 2
(byte)0x25, //FREE 1
(byte)0x13 //FREE 0
};
/**
* Determines if message bits 9 - 59 pass the CRC checksum
* contained in bits 60 - 68, using a lookup table of CRC checksum values
* derived from the CRC-7 value and the final parity bit
*/
public static CRC check( BitSet msg )
{
CRC crc = CRC.UNKNOWN;
byte calculated = 0x0; //Starting value for an OSW
//Iterate bits that are set and XOR running checksum with lookup value
for (int i = msg.nextSetBit( 9 ); i >= 9 && i <= 59; i = msg.nextSetBit( i+1 ) )
{
calculated ^= sCHECKSUMS[ i - 9 ];
}
//Apply the message checksum to derive the residual
calculated ^= getChecksum( msg );
switch( (byte)calculated )
{
case sFILL_00:
crc = CRC.PASSED;
break;
default:
crc = CRC.FAILED_CRC;
}
return crc;
}
public static byte getResidual( BitSet msg )
{
byte calculated = 0x0; //Initial fill of zero
//Iterate bits that are set and XOR running checksum with lookup value
for (int i = msg.nextSetBit( 9 ); i >= 9 && i <= 59; i = msg.nextSetBit( i+1 ) )
{
calculated ^= sCHECKSUMS[ i - 9 ];
}
calculated ^= getChecksum( msg );
return calculated;
}
public static byte[] getChecks()
{
return sCHECKSUMS;
}
/**
* Returns the integer value of the 7 bit crc checksum
*/
public static byte getChecksum( BitSet msg )
{
byte retVal = 0x0;
for( int x = 0; x < 8; x++ )
{
if( msg.get( x + 60 ) )
{
retVal ^= (byte)( 1<<( 7 - x ) );
}
}
return retVal;
}
/**
* Initial take on error correction. The majority of the errors detected
* are in the final 2 bits, owing to the forward looking soft bit detection
* used in the decoder. For right now, correct any messages where the
* residual indicates that the final 1 or 2 bits are bad.
*/
public static BinaryMessage correct( BinaryMessage msg )
{
int residual = (int)( 0xFF & getResidual( msg ) );
switch( residual )
{
case 1:
case 136:
msg.flip( 67 );
break;
case 3:
case 138:
msg.flip( 66 );
msg.flip( 67 );
break;
case 2:
case 139:
msg.flip( 66 );
break;
case 140:
msg.flip( 65 );
msg.flip( 67 );
break;
case 153:
msg.flip( 67 );
break;
case 142:
msg.flip( 65 );
msg.flip( 66 );
msg.flip( 67 );
break;
}
return msg;
}
/**
* Performs an even parity check on the bitset. If the number of bits that
* are set is an even number, then it passes the even parity check
*/
public static boolean isEvenParity( BinaryMessage bits )
{
return ( bits.cardinality() % 2 == 0 );
}
}