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Annotation of embedaddon/bird2/lib/bitops.c, revision 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 255 if the mask is invalid.
        !            32:  */
        !            33: uint
        !            34: u32_masklen(u32 x)
        !            35: {
        !            36:   int l = 0;
        !            37:   u32 n = ~x;
        !            38: 
        !            39:   if (n & (n+1)) return 255;
        !            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: