[Coco] IP packets on my coco

[Barry Nelson]

Oops, open mouth, insert foot. Here is some code that uses a 256 byte table to accelerate a 32 bit CRC calculation.

/* This does table lookup to calulate a 32 bit CRC. First the CRC table itself
   is calculated. Not counting the table setup when compiled to Cyclops with
   GCC, this function executes in 7 + 13n instructions, where n is the number
   of bytes in the input message. It should be doable in 4 + 9n instructions.
   In any case, two of the 13 or 9 instrucions are load byte. */

static unsigned int crctable[256];

void setupcrc32c() {
   unsigned int byte, crc, mask;
   int j;

   /* Set up the table. */

   for (byte = 0; byte <= 255; byte++) {
      crc = byte;
      for (j = 7; j >= 0; j--) {    // Do eight times.
         mask = -(crc & 1);
         crc = (crc >> 1) ^ (0xEDB88320 & mask); 
         /* 0xEDB88320 is 0x04C11DB7 reversed */
      }
      crctable[byte] = crc;
   }
}

unsigned int crc32c(unsigned char *message) {
   int i=0;
   unsigned int byte, crc;


   /* Calculate the CRC. */

   crc = 0xFFFFFFFF;
   while ((byte = message[i]) != 0) {
      crc = (crc >> 8) ^ crctable[(crc ^ byte) & 0xFF];
      i = i + 1;
   }
   return ~crc;
}

On Jun 8, 2016, at 1:57 AM, Barry Nelson <barry.nelson at amobiledevice.com> wrote:

>  The problem with that approach is that an 8 bit CRC requires a 256 byte lookup table or 2^8 bytes. A 32 bit CRC would need a 2^32 byte table or 4294967296 bytes, that is a 4Gb lookup table.