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iof-bird / bird-2.0.1 / lib / bitops.c @ 6b3f1a54

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/*
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 *        BIRD Library -- Generic Bit Operations
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 *
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 *        (c) 1998 Martin Mares <mj@ucw.cz>
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 *
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 *        Can be freely distributed and used under the terms of the GNU GPL.
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 */
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#include "nest/bird.h"
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#include "bitops.h"
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/**
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 * u32_mkmask - create a bit mask
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 * @n: number of bits
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 *
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 * u32_mkmask() returns an unsigned 32-bit integer which binary
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 * representation consists of @n ones followed by zeroes.
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 */
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u32
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u32_mkmask(uint n)
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{
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  return n ? ~((1 << (32 - n)) - 1) : 0;
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}
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/**
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 * u32_masklen - calculate length of a bit mask
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 * @x: bit mask
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 *
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 * This function checks whether the given integer @x represents
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 * a valid bit mask (binary representation contains first ones, then
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 * zeroes) and returns the number of ones or 255 if the mask is invalid.
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 */
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uint
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u32_masklen(u32 x)
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{
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  int l = 0;
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  u32 n = ~x;
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  if (n & (n+1)) return 255;
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  if (x & 0x0000ffff) { x &= 0x0000ffff; l += 16; }
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  if (x & 0x00ff00ff) { x &= 0x00ff00ff; l += 8; }
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  if (x & 0x0f0f0f0f) { x &= 0x0f0f0f0f; l += 4; }
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  if (x & 0x33333333) { x &= 0x33333333; l += 2; }
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  if (x & 0x55555555) l++;
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  if (x & 0xaaaaaaaa) l++;
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  return l;
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}
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/**
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 * u32_log2 - compute a binary logarithm.
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 * @v: number
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 *
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 * This function computes a integral part of binary logarithm of given
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 * integer @v and returns it. The computed value is also an index of the
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 * most significant non-zero bit position.
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 */
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u32
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u32_log2(u32 v)
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{
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  /* The code from http://www-graphics.stanford.edu/~seander/bithacks.html */
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  u32 r, shift;
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  r =     (v > 0xFFFF) << 4; v >>= r;
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  shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
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  shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
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  shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
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  r |= (v >> 1);
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  return r;
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}
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