/* START LIBRARY DESCRIPTION *********************************************
CRC16.LIB
    Copyright (c) 1996, Robust DataComm.

DESCRIPTION: Calc CRC-16 per Modbus using XMODEM type table

SUPPORT LIB'S: 
END DESCRIPTION **********************************************************/

// History:

unsigned int crc16_uCalcBuf( unsigned char *ucpBuf, int nByteCount);
unsigned int crc16_uCalcByte( unsigned char uc, unsigned int uCRC);
void   crc16_Init( void);              // pre-seed Table[] (must call to init table!)

ushort crc16_Table[256];               // table for partial CRC seeds


/* START FUNCTION DESCRIPTION ********************************************
crc16_uCalcBuf               <CRC16.LIB>

SYNTAX: unsigned int crc16_uCalcBuf( uchar *ucpBuf, int nByteCount);

DESCRIPTION: Given buffer ptr & byte count, calc new CRC-16

RETURN VALUE: return CRC-16 (swap bytes for Modbus)
END DESCRIPTION **********************************************************/
unsigned int crc16_uCalcBuf( uchar *ucpBuf, int nByteCount)
{
   ushort crc_data;                       // holder of byte during calc
   ushort crc_crc;                        // holder of CRC
   int    crc_i;                          // loop counter

   crc_crc = 0xFFFF;                       // start with all 1's for reverse CRC
   // printf( " <*> crc%x\n", crc_crc);
   for( crc_i = 0; crc_i < nByteCount; ++crc_i) {  // process each byte
      crc_data = crc_crc ^ ucpBuf[crc_i];
      crc_crc = (crc_crc >> 8) ^ crc16_Table[crc_data & 0x00FF];
      // printf( " crc%d:%x", crc_i, crc_crc);
   }
   return( crc_crc);
}
/* Modbus CRC are stored MSByte in lo byte - you must swap bytes */

/* START FUNCTION DESCRIPTION ********************************************
crc16_uCalcByte              <CRC16.LIB>

SYNTAX: unsigned int crc16_uCalcByte( uchar uc, ushort uCRC);

DESCRIPTION: accumulate CRC 1 byte at a time (use 0xFFFF for first call)

RETURN VALUE: none
END DESCRIPTION **********************************************************/
unsigned int crc16_uCalcByte( uchar uc, ushort uCRC)
{
   static ushort crc_data;   // holder of byte during calc

   crc_data = uCRC ^ uc;
   uCRC = (uCRC >> 8) ^ crc16_Table[crc_data & 0x00FF];
   return( uCRC);
}

/* crc16_Init() - Initialize the CRC-16 table (crc16_Table[])
 */
void crc16_Init( void)
{
   ushort crc_data;   // holder of byte during calc
   ushort crc_crc;    // holder of CRC
   int    crc_i;      // loop counter
   char   j;          // loop counters

   for( crc_i = 0; crc_i < 256; crc_i++) {
      crc_data = (crc_i << 1);
      crc_crc = 0;
      for( j = 8; j > 0; j--) {
         crc_data >>= 1;
         if( (crc_data ^ crc_crc) & 0x0001)
            crc_crc = (crc_crc >> 1) ^ 0xA001;
         else
            crc_crc >>= 1;
      } // endfor j
      crc16_Table[crc_i] = crc_crc;
      // printf( " %d:%x", crc_i, crc16_Table[crc_i]);
   } // endfor crc_i

   return;
}