/* tnt: Hostmode Terminal for TNC
   Copyright (C) 1993 by Mark Wahl
   For license details see documentation
   Procedures for autobin-checksum (crc.c)
   created: Mark Wahl DL4YBG 94/01/17
   updated: Mark Wahl DL4YBG 94/01/17
*/

static int crcbit[8] = {
  0x9188,0x48c4,0x2462,0x1231,0x8108,0x4084,0x2042,0x1021
                       };
                       
static int bittab[8] = { 128,64,32,16,8,4,2,1 };

static int crctab[256];

void init_crc(void)
{
  int i,j;
  
  for (i = 0; i < 256; i++) {
    crctab[i] = 0;
    for (j = 0; j < 8; j++) {
      if ((bittab[j] & i) != 0) {
        crctab[i] = crctab[i] ^ crcbit[j];
      }
    }
  }
}

/* calculate checksum for autobin-protocol */
unsigned int calc_crc(unsigned char* buf, int n, unsigned crc)
{
  while (--n >= 0)
    crc = (crctab[(crc >> 8)] ^ ((crc << 8) | *buf++)) & 0xffff;
  return crc;
}