crc32.c

CRC校验采用多项式编码方法。多项式乘除法运算过程与普通代数多项式的乘除法相同。多项式的加减法运算以2为模

#include "digest.h"
#include "crc32.h"

static algo_t crc32 = {
    CRC32_DIGEST_LENGTH,
    sizeof(CRC32_CTX),
    (hash_init_func_t)CRC32_Init,
    (hash_update_func_t)CRC32_Update,
    (hash_end_func_t)CRC32_End,
    (hash_final_func_t)CRC32_Final,
    (hash_equal_func_t)CRC32_Equal,
};

static unsigned long crc_table[256];
 
static void crc32_init(void)                /* build the crc table */
{
  unsigned long crc, poly;
  int i, j;
  
  poly = 0xEDB88320L;
  for (i = 0; i < 256; i++)
    {
      crc = i;
      for (j = 8; j > 0; j--)
	{
	  if (crc & 1)
	    crc = (crc >> 1) ^ poly;
	  else
	    crc >>= 1;
	}
      crc_table[i] = crc;
    }
}

void
CRC32_Init(CRC32_CTX *pms)
{
    pms->count[0] = pms->count[1] = 0;
    pms->state[0] = 0xFFFFFFFF;
}

void
CRC32_Update(CRC32_CTX *pms, const uint8_t *data, size_t nbytes)
{
    register uint32_t crc = pms->state[0];
    register const uint8_t *p = data;
    uint32_t nbits = (uint32_t)(nbytes << 3);

    if (nbytes <= 0)
      return;

    /* Update the message length. */
    pms->count[1] += nbytes >> 29;
    pms->count[0] += nbits;
    if (pms->count[0] < nbits)
      pms->count[1]++;

    /* Update */
    while (nbytes-- > 0)
      crc = ((crc>>8) & 0x00FFFFFF) ^ crc_table[ (crc^(*p++)) & 0xFF ];
    
    pms->state[0] = crc;
}

void
CRC32_Final(uint8_t *digest, CRC32_CTX *pms)
{
  register uint32_t crc = pms->state[0] ^ 0xFFFFFFFF;
  register int i;

  for (i = 3; i >= 0; i--) 
    {
      digest[i] = (uint8_t)(crc & 0xFF);
      crc = crc >> 8;
    }
}

void
CRC32_End(CRC32_CTX *pctx, uint8_t *hexdigest)
{
  unsigned char digest[4];
  size_t i;

  CRC32_Final(digest, pctx);

  for (i = 0; i < 4; i++)
    sprintf(hexdigest + i * 2, "%02x", digest[i]);
}

int CRC32_Equal(CRC32_CTX* pctx1, CRC32_CTX* pctx2) {
  return memcmp(pctx1->count, pctx2->count, sizeof(pctx1->count)) == 0
    && memcmp(pctx1->state, pctx2->state, sizeof(pctx1->state)) == 0;
}

void
Init_crc32_in_c()
{
    VALUE mDigest, cDigest_Base, cDigest_CRC32;

    crc32_init();

    rb_require("digest.so");

    mDigest = rb_path2class("Digest");
    cDigest_Base = rb_path2class("Digest::Base");

    cDigest_CRC32 = rb_define_class_under(mDigest, "CRC32InC", cDigest_Base);

    rb_cvar_set(cDigest_CRC32, rb_intern("metadata"),
		Data_Wrap_Struct(rb_cObject, 0, 0, &crc32), Qtrue);
}