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Annotation of embedaddon/bird/lib/bitops.c, revision 1.1.1.1

1.1       misho       1: /*
                      2:  *     BIRD Library -- Generic Bit Operations
                      3:  *
                      4:  *     (c) 1998 Martin Mares <mj@ucw.cz>
                      5:  *
                      6:  *     Can be freely distributed and used under the terms of the GNU GPL.
                      7:  */
                      8: 
                      9: #include "nest/bird.h"
                     10: #include "bitops.h"
                     11: 
                     12: /**
                     13:  * u32_mkmask - create a bit mask
                     14:  * @n: number of bits
                     15:  *
                     16:  * u32_mkmask() returns an unsigned 32-bit integer which binary
                     17:  * representation consists of @n ones followed by zeroes.
                     18:  */
                     19: u32
                     20: u32_mkmask(uint n)
                     21: {
                     22:   return n ? ~((1 << (32 - n)) - 1) : 0;
                     23: }
                     24: 
                     25: /**
                     26:  * u32_masklen - calculate length of a bit mask
                     27:  * @x: bit mask
                     28:  *
                     29:  * This function checks whether the given integer @x represents
                     30:  * a valid bit mask (binary representation contains first ones, then
                     31:  * zeroes) and returns the number of ones or -1 if the mask is invalid.
                     32:  */
                     33: int
                     34: u32_masklen(u32 x)
                     35: {
                     36:   int l = 0;
                     37:   u32 n = ~x;
                     38: 
                     39:   if (n & (n+1)) return -1;
                     40:   if (x & 0x0000ffff) { x &= 0x0000ffff; l += 16; }
                     41:   if (x & 0x00ff00ff) { x &= 0x00ff00ff; l += 8; }
                     42:   if (x & 0x0f0f0f0f) { x &= 0x0f0f0f0f; l += 4; }
                     43:   if (x & 0x33333333) { x &= 0x33333333; l += 2; }
                     44:   if (x & 0x55555555) l++;
                     45:   if (x & 0xaaaaaaaa) l++;
                     46:   return l;
                     47: }
                     48: 
                     49: /**
                     50:  * u32_log2 - compute a binary logarithm.
                     51:  * @v: number
                     52:  *
                     53:  * This function computes a integral part of binary logarithm of given
                     54:  * integer @v and returns it. The computed value is also an index of the
                     55:  * most significant non-zero bit position.
                     56:  */
                     57: 
                     58: u32
                     59: u32_log2(u32 v)
                     60: {
                     61:   /* The code from http://www-graphics.stanford.edu/~seander/bithacks.html */
                     62:   u32 r, shift;
                     63:   r =     (v > 0xFFFF) << 4; v >>= r;
                     64:   shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
                     65:   shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
                     66:   shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
                     67:   r |= (v >> 1);
                     68:   return r;
                     69: }
                     70: