/*
    CRC-8 for Dallas iButton products


    From Maxim/Dallas AP Note 27

    "Understanding and Using Cyclic Redundancy Checks with 
    Dallas Semiconductor iButton Products"

    The Ap note describes the CRC-8 algorithm used in the 
    iButton products. Their implementation involves a 256 byte
    CRC table. This algorithm is implemented here. In addition
    two other algorithms are shown. One uses nibble arrays and
    the other uses boolean arithmetic.

    
    18JAN03 - T. Scott Dattalo
  */



/*
  crc array from the Maxim ApNote
*/
unsigned char crc_array[256] = {

0x00, 0x5e, 0xbc, 0xe2, 0x61, 0x3f, 0xdd, 0x83, 
0xc2, 0x9c, 0x7e, 0x20, 0xa3, 0xfd, 0x1f, 0x41, 
0x9d, 0xc3, 0x21, 0x7f, 0xfc, 0xa2, 0x40, 0x1e, 
0x5f, 0x01, 0xe3, 0xbd, 0x3e, 0x60, 0x82, 0xdc, 
0x23, 0x7d, 0x9f, 0xc1, 0x42, 0x1c, 0xfe, 0xa0, 
0xe1, 0xbf, 0x5d, 0x03, 0x80, 0xde, 0x3c, 0x62, 
0xbe, 0xe0, 0x02, 0x5c, 0xdf, 0x81, 0x63, 0x3d, 
0x7c, 0x22, 0xc0, 0x9e, 0x1d, 0x43, 0xa1, 0xff, 
0x46, 0x18, 0xfa, 0xa4, 0x27, 0x79, 0x9b, 0xc5, 
0x84, 0xda, 0x38, 0x66, 0xe5, 0xbb, 0x59, 0x07, 
0xdb, 0x85, 0x67, 0x39, 0xba, 0xe4, 0x06, 0x58, 
0x19, 0x47, 0xa5, 0xfb, 0x78, 0x26, 0xc4, 0x9a, 
0x65, 0x3b, 0xd9, 0x87, 0x04, 0x5a, 0xb8, 0xe6, 
0xa7, 0xf9, 0x1b, 0x45, 0xc6, 0x98, 0x7a, 0x24, 
0xf8, 0xa6, 0x44, 0x1a, 0x99, 0xc7, 0x25, 0x7b, 
0x3a, 0x64, 0x86, 0xd8, 0x5b, 0x05, 0xe7, 0xb9, 
0x8c, 0xd2, 0x30, 0x6e, 0xed, 0xb3, 0x51, 0x0f, 
0x4e, 0x10, 0xf2, 0xac, 0x2f, 0x71, 0x93, 0xcd, 
0x11, 0x4f, 0xad, 0xf3, 0x70, 0x2e, 0xcc, 0x92, 
0xd3, 0x8d, 0x6f, 0x31, 0xb2, 0xec, 0x0e, 0x50, 
0xaf, 0xf1, 0x13, 0x4d, 0xce, 0x90, 0x72, 0x2c, 
0x6d, 0x33, 0xd1, 0x8f, 0x0c, 0x52, 0xb0, 0xee, 
0x32, 0x6c, 0x8e, 0xd0, 0x53, 0x0d, 0xef, 0xb1, 
0xf0, 0xae, 0x4c, 0x12, 0x91, 0xcf, 0x2d, 0x73, 
0xca, 0x94, 0x76, 0x28, 0xab, 0xf5, 0x17, 0x49, 
0x08, 0x56, 0xb4, 0xea, 0x69, 0x37, 0xd5, 0x8b, 
0x57, 0x09, 0xeb, 0xb5, 0x36, 0x68, 0x8a, 0xd4, 
0x95, 0xcb, 0x29, 0x77, 0xf4, 0xaa, 0x48, 0x16, 
0xe9, 0xb7, 0x55, 0x0b, 0x88, 0xd6, 0x34, 0x6a, 
0x2b, 0x75, 0x97, 0xc9, 0x4a, 0x14, 0xf6, 0xa8, 
0x74, 0x2a, 0xc8, 0x96, 0x15, 0x4b, 0xa9, 0xf7, 
0xb6, 0xe8, 0x0a, 0x54, 0xd7, 0x89, 0x6b, 0x35, 
};

unsigned char nibbleTable[16] = {
0x00, 0x5e, 0xbc, 0xe2, 0x61, 0x3f, 0xdd, 0x83, 
0xc2, 0x9c, 0x7e, 0x20, 0xa3, 0xfd, 0x1f, 0x41, 
};

unsigned char elbaTelbbin[16] = {  /* bit reversed elements of nibbleTable */

0x00, 0x9d, 0x23, 0xbe, 0x46, 0xdb, 0x65, 0xf8,
0x8c, 0x11, 0xaf, 0x32, 0xca, 0x57, 0xe9, 0x74

};

/*
  crc - global crc stored here
*/

unsigned char crc;

/*===========================================================*/

unsigned char crc_bits(int data)
{
  int i = (data ^ crc) & 0xff;

  crc = 0;

  if(i & 1)
    crc ^= 0x5e;
  if(i & 2)
    crc ^= 0xbc;
  if(i & 4)
    crc ^= 0x61;
  if(i & 8)
    crc ^= 0xc2;
  if(i & 0x10)
    crc ^= 0x9d;
  if(i & 0x20)
    crc ^= 0x23;
  if(i & 0x40)
    crc ^= 0x46;
  if(i & 0x80)
    crc ^= 0x8c;

  return crc;
}

/*===========================================================*/

unsigned char crc_table(int data)
{
  int i = (data ^ crc) & 0xff;

  crc  = crc_array[(data ^ crc)&0xff];

  return crc;
}

/*===========================================================*/
unsigned char crc_nibbles(unsigned char data)
{
  unsigned char b = data ^ crc;
  return crc = nibbleTable[b&0xf] ^ elbaTelbbin[b>>4];
}
/*===========================================================*/

/* crc8 - test all three crc algorithms
*/
unsigned char crc8(int data)
{
  unsigned char c, resTable, resBits, resNibbles;

  c = crc;
  resTable = crc_table(data);

  crc = c;
  resBits = crc_bits(data);
  
  crc = c;
  resNibbles = crc_nibbles(data);
  if (resNibbles != resBits || resBits != resTable)
    printf(
"CRC-8 Algorithm ERROR:  resNibbles=0x%2x, resBits=0x%2x, resTable=0x%2x\n",
          resNibbles, resBits, resTable);

  return crc;
}
/*===========================================================*/

int main(int argc, char **argv)
{
  int i;

  crc = 0xff;

  /*
    Standard check for the string "123456789"
  */
  
  for(i='1'; i<='9'; i++) {
    crc8(0);
    printf(" %d 0x%02x\n",i,crc);
  }
  
  printf("\n----\n\n");

  /* 
     Orthogonal check
     Set the crc =0 and data = 2^n and compute new crc
  */

  crc = 0x00;
  crc8(1);
  crc = 0x00;
  crc8(2);
  crc = 0x00;
  crc8(4);
  crc = 0x00;
  crc8(8);
  crc = 0x00;
  crc8(16);
  crc = 0x00;
  crc8(32);
  crc = 0x00;
  crc8(64);
  crc = 0x00;
  crc8(128);

  /*
    CRC example in the Dallas APNote
  */
  crc=0;
  i = 0x02;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0x1c;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0xb8;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0x01;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0x00;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0x00;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0x00;
  printf(" %3x 0x%02x\n",i,crc8(i));
  
  i = 0xa2;
  printf(" %3x 0x%02x\n",i,crc8(i));

  i = crc;
  printf(" %3x 0x%02x\n",i,crc8(i));

  return 0;
}