skalibs

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bitarray_count.c (559B)

      1 /* ISC license. */
      2 
      3 #include <skalibs/bitarray.h>
      4 
      5 #define B0(n) n, n+1, n+1, n+2
      6 #define B1(n) B0(n), B0(n+1), B0(n+1), B0(n+2)
      7 #define B2(n) B1(n), B1(n+1), B1(n+1), B1(n+2)
      8 #define B3(n) B2(n), B2(n+1), B2(n+1), B2(n+2)
      9 
     10 size_t bitarray_countones (unsigned char const *c, size_t n)
     11 {
     12   static unsigned char const table[256] = { B3(0) } ;
     13   size_t len = bitarray_div8(n) ;
     14   size_t total = 0 ;
     15   size_t i = 0 ;
     16   if (n & 7) len-- ;
     17   for (; i < len ; i++) total += table[c[i]] ;
     18   if (n & 7) total += table[c[i] & ((1 << (n & 7)) - 1)] ;
     19   return total ;
     20 }