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CRC32 Calculation

liquidspark Apr 7th, 2019 110 Never
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  1. const int order = 32;
  2. const unsigned long polynom = 0x4c11db7;
  3. const int direct = 1;
  4. const unsigned long crcinit = 0xffffffff;
  5. const unsigned long crcxor = 0xffffffff;
  6. const int refin = 1;
  7. const int refout = 1;
  8. unsigned long crcmask;
  9. unsigned long crchighbit;
  10. unsigned long crcinit_direct;
  11. unsigned long crcinit_nondirect;
  12. unsigned long crctab[256];
  13.  
  14. unsigned long halo_crc32_reflect (unsigned long crc, int bitnum) {
  15.    
  16.     // reflects the lower 'bitnum' bits of 'crc'
  17.    
  18.     unsigned long i, j=1, crcout=0;
  19.    
  20.     for (i=(unsigned long)1<<(bitnum-1); i; i>>=1) {
  21.         if (crc & i) crcout|=j;
  22.         j<<= 1;
  23.     }
  24.     return (crcout);
  25. }
  26.  
  27. void halo_crc32_generate_crc_table() {
  28.    
  29.     // make CRC lookup table used by table algorithms
  30.    
  31.     int i, j;
  32.     unsigned long bit, crc;
  33.    
  34.     for (i=0; i<256; i++) {
  35.        
  36.         crc=(unsigned long)i;
  37.         if (refin) crc=halo_crc32_reflect(crc, 8);
  38.         crc<<= order-8;
  39.        
  40.         for (j=0; j<8; j++) {
  41.            
  42.             bit = crc & crchighbit;
  43.             crc<<= 1;
  44.             if (bit) crc^= polynom;
  45.         }
  46.        
  47.         if (refin) crc = halo_crc32_reflect(crc, order);
  48.         crc&= crcmask;
  49.         crctab[i]= crc;
  50.     }
  51. }
  52.  
  53. unsigned long halo_crctablefast (unsigned char* p, unsigned long len) {
  54.    
  55.     // fast lookup table algorithm without augmented zero bytes, e.g. used in pkzip.
  56.     // only usable with polynom orders of 8, 16, 24 or 32.
  57.    
  58.     unsigned long crc = crcinit_direct;
  59.    
  60.     if (refin) crc = halo_crc32_reflect(crc, order);
  61.    
  62.     if (!refin) while (len--) crc = (crc << 8) ^ crctab[ ((crc >> (order-8)) & 0xff) ^ *p++];
  63.     else while (len--) crc = (crc >> 8) ^ crctab[ (crc & 0xff) ^ *p++];
  64.    
  65.     if (refout^refin) crc = halo_crc32_reflect(crc, order);
  66.     crc^= crcxor;
  67.     crc&= crcmask;
  68.    
  69.     crc = ~crc & 0xffffffff;    // Halo has an inverted CRC32 value and we also want to mask to strip unusable bytes from unsigned long to unsigned int
  70.    
  71.     return(crc);
  72. }
  73.  
  74. uint32_t halo_crc32 (const void* p, unsigned long len)
  75. {
  76.     int i;
  77.     unsigned long bit, crc;
  78.    
  79.     // at first, compute constant bit masks for whole CRC and CRC high bit
  80.    
  81.     crcmask = ((((unsigned long)1<<(order-1))-1)<<1)|1;
  82.     crchighbit = (unsigned long)1<<(order-1);
  83.    
  84.     // check parameters
  85.    
  86.     if (order < 1 || order > 32) {
  87.         printf("ERROR, invalid order, it must be between 1..32.\n");
  88.         return(0);
  89.     }
  90.    
  91.     if (polynom != (polynom & crcmask)) {
  92.         printf("ERROR, invalid polynom.\n");
  93.         return(0);
  94.     }
  95.    
  96.     if (crcinit != (crcinit & crcmask)) {
  97.         printf("ERROR, invalid crcinit.\n");
  98.         return(0);
  99.     }
  100.    
  101.     if (crcxor != (crcxor & crcmask)) {
  102.         printf("ERROR, invalid crcxor.\n");
  103.         return(0);
  104.     }
  105.    
  106.     // generate lookup table
  107.    
  108.     halo_crc32_generate_crc_table();
  109.    
  110.     // compute missing initial CRC value
  111.    
  112.     if (!direct) {
  113.        
  114.         crcinit_nondirect = crcinit;
  115.         crc = crcinit;
  116.         for (i=0; i<order; i++) {
  117.            
  118.             bit = crc & crchighbit;
  119.             crc<<= 1;
  120.             if (bit) crc^= polynom;
  121.         }
  122.         crc&= crcmask;
  123.         crcinit_direct = crc;
  124.     }
  125.    
  126.     else {
  127.        
  128.         crcinit_direct = crcinit;
  129.         crc = crcinit;
  130.         for (i=0; i<order; i++) {
  131.            
  132.             bit = crc & 1;
  133.             if (bit) crc^= polynom;
  134.             crc >>= 1;
  135.             if (bit) crc|= crchighbit;
  136.         }
  137.         crcinit_nondirect = crc;
  138.     }
  139.    
  140.     uint32_t result = (uint32_t)halo_crctablefast((unsigned char *)p, len);
  141.     return result;
  142. }
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