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:
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