Internet on FIRE

/*

 *        BIRD Library -- Generic Bit Operations

 *

 *        (c) 1998 Martin Mares <mj@ucw.cz>

 *

 *        Can be freely distributed and used under the terms of the GNU GPL.

 */

#include "nest/bird.h"

#include "bitops.h"

/**

 * u32_mkmask - create a bit mask

 * @n: number of bits

 *

 * u32_mkmask() returns an unsigned 32-bit integer which binary

 * representation consists of @n ones followed by zeroes.

 */

u32

u32_mkmask(uint n)

{

  return n ? ~((1 << (32 - n)) - 1) : 0;

}

/**

 * u32_masklen - calculate length of a bit mask

 * @x: bit mask

 *

 * This function checks whether the given integer @x represents

 * a valid bit mask (binary representation contains first ones, then

 * zeroes) and returns the number of ones or -1 if the mask is invalid.

 */

int

u32_masklen(u32 x)

{

  int l = 0;

  u32 n = ~x;

  if (n & (n+1)) return -1;

  if (x & 0x0000ffff) { x &= 0x0000ffff; l += 16; }

  if (x & 0x00ff00ff) { x &= 0x00ff00ff; l += 8; }

  if (x & 0x0f0f0f0f) { x &= 0x0f0f0f0f; l += 4; }

  if (x & 0x33333333) { x &= 0x33333333; l += 2; }

  if (x & 0x55555555) l++;

  if (x & 0xaaaaaaaa) l++;

  return l;

}

/**

 * u32_log2 - compute a binary logarithm.

 * @v: number

 *

 * This function computes a integral part of binary logarithm of given

 * integer @v and returns it. The computed value is also an index of the

 * most significant non-zero bit position.

 */

u32

u32_log2(u32 v)

{

  /* The code from http://www-graphics.stanford.edu/~seander/bithacks.html */

  u32 r, shift;

  r =     (v > 0xFFFF) << 4; v >>= r;

  shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;

  shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;

  shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;

  r |= (v >> 1);

  return r;

}