base/bits.h

   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)
  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
  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_