C언어] CRC32 구하기 버전업 (코드 최적화): CRC32.cpp

unsigned long getFileCRC(FILE *);
unsigned long calcCRC (const unsigned char *, signed long, unsigned long, unsigned long *);
void makeCRCtable(unsigned long *, unsigned long);




unsigned long getFileCRC(FILE *s) {
  unsigned char buf [32768];
  unsigned long CRC = 0;
  unsigned long table[256];
  size_t len;


  makeCRCtable(table, 0xEDB88320);

  while ( (len = fread(buf, 1, sizeof(buf), s)) != NULL )
    CRC = calcCRC(buf, (unsigned long) len, CRC, table);

  return CRC;
}


unsigned long calcCRC(const unsigned char *mem, signed long size, unsigned long CRC, unsigned long *table) {
  CRC = ~CRC;

  while(size--)
    CRC = table[(CRC ^ *(mem++)) & 0xFF] ^ (CRC >> 8);

  return ~CRC;
}


void makeCRCtable(unsigned long *table, unsigned long id) {
  unsigned long i, j, k;

  for(i = 0; i < 256; ++i) {
    k = i;
    for(j = 0; j < 8; ++j) {
      if (k & 1) k = (k >> 1) ^ id;
      else k >>= 1;
    }
    table[i] = k;
  }
}