/**
* 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;
}
/**
* 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;
}