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

 1 ```/* ``` ``` * BIRD Library -- Generic Bit Operations ``` ``` * ``` ``` * (c) 1998 Martin Mares ``` ``` * ``` ``` * 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 255 if the mask is invalid. ``` ``` */ ``` ```uint ``` ```u32_masklen(u32 x) ``` ```{ ``` ``` int l = 0; ``` ``` u32 n = ~x; ``` ``` if (n & (n+1)) return 255; ``` ``` 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; ``` ```} ```