iofbird / bird2.0.1 / 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 32bit 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 nonzero 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://wwwgraphics.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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