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1 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 // This file defines some bit utilities.
6 
7 #ifndef BASE_BITS_H_
8 #define BASE_BITS_H_
9 
10 #include "base/basictypes.h"
11 #include "base/logging.h"
12 
13 namespace base {
14 namespace bits {
15 
16 // Returns the integer i such as 2^i <= n < 2^(i+1)
Log2Floor(uint32 n)17 inline int Log2Floor(uint32 n) {
18   if (n == 0)
19     return -1;
20   int log = 0;
21   uint32 value = n;
22   for (int i = 4; i >= 0; --i) {
23     int shift = (1 << i);
24     uint32 x = value >> shift;
25     if (x != 0) {
26       value = x;
27       log += shift;
28     }
29   }
30   DCHECK_EQ(value, 1u);
31   return log;
32 }
33 
34 // Returns the integer i such as 2^(i-1) < n <= 2^i
Log2Ceiling(uint32 n)35 inline int Log2Ceiling(uint32 n) {
36   if (n == 0) {
37     return -1;
38   } else {
39     // Log2Floor returns -1 for 0, so the following works correctly for n=1.
40     return 1 + Log2Floor(n - 1);
41   }
42 }
43 
44 }  // namespace bits
45 }  // namespace base
46 
47 #endif  // BASE_BITS_H_
48