crc32.c

The main purpose of this project is to add a new scheduling algorithm to GeekOS and to implement a s

/* Copyright abandoned; this code is in the public domain. */
/* Provided to GNUnet by [e-mail] */

/*
 * Adapted for GeekOS by David Hovemeyer
 * Code downloaded from OVM (http://www.ovmj.org)
 */

#include <geekos/crc32.h>
#include <geekos/kassert.h>

#define POLYNOMIAL (ulong_t)0xedb88320
static ulong_t crc_table[256];

/*
 * This routine writes each crc_table entry exactly once,
 * with the correct final value.  Thus, it is safe to call
 * even on a table that someone else is using concurrently.
 */
void Init_CRC32(void) {
  unsigned int i, j;
  ulong_t h = 1;
  crc_table[0] = 0;
  for (i = 128; i; i >>= 1) {
    h = (h >> 1) ^ ((h & 1) ? POLYNOMIAL : 0);
    /* h is now crc_table[i] */
    for (j = 0; j < 256; j += 2*i)
      crc_table[i+j] = crc_table[j] ^ h;
  }
}

/*
 * This computes the standard preset and inverted CRC, as used
 * by most networking standards.  Start by passing in an initial
 * chaining value of 0, and then pass in the return value from the
 * previous crc32() call.  The final return value is the CRC.
 * Note that this is a little-endian CRC, which is best used with
 * data transmitted lsbit-first, and it should, itself, be appended
 * to data in little-endian byte and bit order to preserve the
 * property of detecting all burst errors of length 32 bits or less.
 */
ulong_t crc32(ulong_t crc, char const *buf, size_t len) {
  KASSERT(crc_table[255] != 0);
  crc ^= 0xffffffff;
  while (len--)
    crc = (crc >> 8) ^ crc_table[(crc ^ *buf++) & 0xff];
  return crc ^ 0xffffffff;
}

/* end of crc32.c */