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18c8241a MM |
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 | ||
7722938d MM |
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 | */ | |
18c8241a | 19 | u32 |
ae80a2de | 20 | u32_mkmask(uint n) |
18c8241a MM |
21 | { |
22 | return n ? ~((1 << (32 - n)) - 1) : 0; | |
23 | } | |
24 | ||
7722938d MM |
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 | |
fe9f1a6d | 31 | * zeroes) and returns the number of ones or 255 if the mask is invalid. |
7722938d | 32 | */ |
fe9f1a6d | 33 | uint |
18c8241a MM |
34 | u32_masklen(u32 x) |
35 | { | |
36 | int l = 0; | |
37 | u32 n = ~x; | |
38 | ||
fe9f1a6d | 39 | if (n & (n+1)) return 255; |
18c8241a MM |
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 | } | |
b1a597e0 OZ |
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 | |
c60cdd8c | 55 | * most significant non-zero bit position. |
b1a597e0 OZ |
56 | */ |
57 | ||
58 | u32 | |
59 | u32_log2(u32 v) | |
60 | { | |
c60cdd8c | 61 | /* The code from http://www-graphics.stanford.edu/~seander/bithacks.html */ |
b1a597e0 OZ |
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 | } | |
c60cdd8c | 70 |