chrome的base库中求最接近n的2的多少次幂，也就是k=log2(n); 求k的算法

最新推荐文章于 2025-07-22 08:59:39 发布

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本文介绍了一个实用的位操作工具库，其中包括两个核心函数：Log2Floor和Log2Ceiling。Log2Floor函数返回不大于输入整数的最大2的幂次方的指数，而Log2Ceiling则返回不小于输入整数的最小2的幂次方的指数。

// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file defines some bit utilities.

#ifndef BASE_BITS_H_
#define BASE_BITS_H_

#include "base/basictypes.h"
#include "base/logging.h"

namespace base
{

namespace bits
{

// Returns the integer i such as 2^i <= n < 2^(i+1)
inline int Log2Floor(uint32 n)
{
if (n == 0)
    return -1;

int log = 0;
uint32 value = n;

for (int i = 4; i >= 0; --i)
{
    int shift = (1 << i);

    uint32 x = value >> shift;

    if (x != 0)
{
      value = x;
      log += shift;
    }
}
DCHECK_EQ(value, 1u);
return log;
}

// Returns the integer i such as 2^(i-1) < n <= 2^i
inline int Log2Ceiling(uint32 n)
{
if (n == 0)
{
    return -1;
}
else
{
    // Log2Floor returns -1 for 0, so the following works correctly for n=1.
    return 1 + Log2Floor(n - 1);
}
}

} // namespace bits
} // namespace base

#endif // BASE_BITS_H_