crc16.c

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/*  
 * crc16.c
 * 
 * Computes a 16-bit CRC 
 * 
 */

#include <stdio.h>


#define GP 0x11021    /* x^16 + x^12 + x^5 + 1 */
#define DI 0x1021    


static short crc16_table[256];     /* 8-bit table */
static int made_table=0;

static void init_crc16()
     /*
      * Should be called before any other crc function.  
      */
{
  int i,j;
  short crc;
    
  if (!made_table) {
    for (i=0; i<256; i++) {
      crc = (i << 8);
      for (j=0; j<8; j++)
        crc = (crc << 1) ^ ((crc & 0x8000) ? DI : 0);
      crc16_table[i] = crc & 0xFFFF;
    }
    made_table=1;
  }
}


void crc16(short *crc, unsigned char m)
     /*
      * For a byte array whose accumulated crc value is stored in *crc, computes
      * resultant crc obtained by appending m to the byte array
      */
{
  if (!made_table)
    init_crc16();
  *crc = crc16_table[(((*crc) >> 8) ^ m) & 0xFF] ^ ((*crc) << 8);
  *crc &= 0xFFFF;
}

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