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(uint 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 255 if the mask is invalid.
  32  */
  33 uint
  34 u32_masklen(u32 x)
  35 {
  36   int l = 0;
  37   u32 n = ~x;
  38
  39   if (n & (n+1)) return 255;
  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 }
  48
  49 /**
  50  * u32_log2 - compute a binary logarithm.
  51  * @v: number
  52  *
  53  * This function computes a integral part of binary logarithm of given
  54  * integer @v and returns it. The computed value is also an index of the
  55  * most significant non-zero bit position.
  56  */
  57
  58 u32
  59 u32_log2(u32 v)
  60 {
  61   /* The code from http://www-graphics.stanford.edu/~seander/bithacks.html */
  62   u32 r, shift;
  63   r =     (v > 0xFFFF) << 4; v >>= r;
  64   shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
  65   shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
  66   shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
  67   r |= (v >> 1);
  68   return r;
  69 }
  70