/* FILE: crc.c */
#include "crc.h"

CRC_TYPE crc( unsigned char *ptr, register int len )
{
static int tblInitialized = 0;
static CRC_TYPE crcReg, crcTbl[ 256 ];
unsigned long i, j, k;

   if( !tblInitialized ){   /* init crc lookup table only once */

      #ifdef CRC8
      for( i=0; i < 256; i++ ){
         j = i;
         for( k=8; k--; ){
            j = j & 0x80? (j << 1) ^ POLY : j << 1;
         }
         crcTbl[ i ] = (CRC_TYPE)j;
      }
      #endif

      #ifdef CRC10
      for( i=0; i < 256; i++ ){
         j = i << 2;
         for( k=8; k--; ){
            if( (j <<= 1) & 0x0400 ){
               j ^= POLY;
            }
         }
         crcTbl[ i ] = (CRC_TYPE)j;
      }
      #endif

      #ifdef XMODEM
      for( i=0; i < 256; i++ ){
         j = i << 8;
         for( k=8; k--; ){
            j = j & 0x8000? (j << 1) ^ POLY : j << 1;
         }
         crcTbl[ i ] = (CRC_TYPE)j;
      }
      #endif

      #if defined X25 || CRC16 || MILSTD188 || AUTODINII
      for( i=0; i < 256; i++ ){
         j = i;
         for( k=8; k--; ){
            j = j & 1? (j >> 1) ^ POLY : j >> 1;
         }
         crcTbl[ i ] = (CRC_TYPE)j;
      }
      #endif

      tblInitialized = 1;
   } /* end of if(!tblInitialized) */



   crcReg = CRC_INIT;     /* always init register */



   /* calc CRC */
   for( ;len--; ){
      #ifdef CRC8
      crcReg = crcTbl[ crcReg ^ *ptr++ ];
      #endif

      #ifdef CRC10
      crcReg = (CRC_TYPE)(((crcReg << 8) & 0x3ff) ^  
                             crcTbl[ ((crcReg >> 2) & 0x00ff) ] ^ *ptr++ );
      #endif

      #ifdef XMODEM
      crcReg = (CRC_TYPE)((crcReg << 8) ^  
                             crcTbl[ ((crcReg >> 8) & 0x00ff) ^ *ptr++ ]);
      #endif

      #if defined X25 || CRC16 || MILSTD188 || AUTODINII
      crcReg = (CRC_TYPE)((crcReg >> 8) ^ 
                             crcTbl[ (crcReg ^ *ptr++) & 0x00ff ]);
      #endif
   }


   /* finish up and return */
   #ifdef CRC10
   /* flush out with two NULL bytes */
   for( len=2; len--; ){
      crcReg = (CRC_TYPE)(((crcReg << 8) & 0x3ff) ^  
                          crcTbl[ ((crcReg >> 2) & 0x00ff) ] );
   }
   return( crcReg );
   #endif
   #ifdef X25
   /* complement and swap bytes */
   return( ~(((crcReg & 0xff) << 8) | (crcReg >> 8)) );
   #endif

   #ifdef AUTODINII
   /* complement without swap */
   return( ~crcReg );
   #endif

   #if defined CRC8 || CRC16 || XMODEM || MILSTD188
   return( crcReg );
   #endif

} /* end of crc() */