71 lines
1.7 KiB
C
71 lines
1.7 KiB
C
/*
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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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