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bird 1.6.3

    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 -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: }
   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: