lib/bitops.c

   1 /*
   2  *      BIRD Library -- Generic Bit Operations
   3  *
   4  *      (c) 1998 Martin Mares <mj@ucw.cz>
   5  *
   6  *      Can be freely distributed and used under the terms of the GNU GPL.
   7  */
   8
   9 #include "nest/bird.h"
  10 #include "bitops.h"
  11
  12 /**
  13  * u32_mkmask - create a bit mask
  14  * @n: number of bits
  15  *
  16  * u32_mkmask() returns an unsigned 32-bit integer which binary
  17  * representation consists of @n ones followed by zeroes.
  18  */
  19 u32
  20 u32_mkmask(unsigned n)
  21 {
  22   return n ? ~((1 << (32 - n)) - 1) : 0;
  23 }
  24
  25 /**
  26  * u32_masklen - calculate length of a bit mask
  27  * @x: bit mask
  28  *
  29  * This function checks whether the given integer @x represents
  30  * a valid bit mask (binary representation contains first ones, then
  31  * zeroes) and returns the number of ones or -1 if the mask is invalid.
  32  */
  33 int
  34 u32_masklen(u32 x)
  35 {
  36   int l = 0;
  37   u32 n = ~x;
  38
  39   if (n & (n+1)) return -1;
  40   if (x & 0x0000ffff) { x &= 0x0000ffff; l += 16; }
  41   if (x & 0x00ff00ff) { x &= 0x00ff00ff; l += 8; }
  42   if (x & 0x0f0f0f0f) { x &= 0x0f0f0f0f; l += 4; }
  43   if (x & 0x33333333) { x &= 0x33333333; l += 2; }
  44   if (x & 0x55555555) l++;
  45   if (x & 0xaaaaaaaa) l++;
  46   return l;
  47 }