使用按位运算符在一个int中打包多个值 [英] use of the bitwise operators to pack multiple values in one int

问题描述

低级别位操作从来都不是我的强项。我将非常感谢理解以下bitwise运算符的用例.Consider ...

Low level bit manipulation has never been my strong point. I will appreciate some help in understanding the following use case of bitwise operators.Consider...

int age, gender, height, packed_info;

. . .   // Assign values 

// Pack as AAAAAAA G HHHHHHH using shifts and "or"
packed_info = (age << 8) | (gender << 7) | height;

// Unpack with shifts and masking using "and"
height = packed_info & 0x7F;   // This constant is binary ...01111111
gender = (packed_info >> 7) & 1;
age    = (packed_info >> 8);

我不确定这段代码是完成的以及如何完成？为什么要使用幻数0x7F？如何完成包装和拆包？

I am not sure what this code is accomplishing and how? Why use the magic number 0x7F ? How is the packing and unpacking accomplished?

来源

推荐答案

正如评论所说，我们将把年龄，性别和身高打包成15位，格式为：

As the comment says, we're going to pack the age, gender and height into 15 bits, of the format:

AAAAAAAGHHHHHHH

让我们从这一部分开始：

Let's start with this part:

(age << 8)

首先，年龄有以下格式：

To start with, age has this format:

age           = 00000000AAAAAAA

其中每个A可以是0或1。

where each A can be 0 or 1.

<< 8 将位移到左侧8位，并用零填充间隙。所以你得到：

<< 8 moves the bits 8 places to the left, and fills in the gaps with zeroes. So you get:

(age << 8)    = AAAAAAA00000000

同样：

gender        = 00000000000000G
(gender << 7) = 0000000G0000000
height        = 00000000HHHHHHH

现在我们要将这些组合成一个变量。 | 运算符通过查看每个位来工作，如果任一输入中的位为1，则返回1。所以：

Now we want to combine these into one variable. The | operator works by looking at each bit, and returning 1 if the bit is 1 in either of the inputs. So:

0011 | 0101 = 0111

如果一个输入中的位为0，则从另一个输入中获取该位。查看（年龄<<< 8），（性别<<<<<<<<<<<<<<<<<<<<<&n; c> height ，您会看到，如果其中一个位为1，则其他位为0。所以：

If a bit is 0 in one input, then you get the bit from the other input. Looking at (age << 8), (gender << 7) and height, you'll see that, if a bit is 1 for one of these, it's 0 for the others. So:

packed_info = (age << 8) | (gender << 7) | height = AAAAAAAGHHHHHHH

现在我们要解包这些位。让我们从高度开始吧。我们想得到最后7位，并忽略前8位。为此，我们使用& 运算符，只有当两个输入位都是1.所以：

Now we want to unpack the bits. Let's start with the height. We want to get the last 7 bits, and ignore the first 8. To do this, we use the & operator, which returns 1 only if both of the input bits are 1. So:

0011 & 0101 = 0001

所以：

packed_info          = AAAAAAAGHHHHHHH
0x7F                 = 000000001111111
(packed_info & 0x7F) = 00000000HHHHHHH = height

为了获得年龄，我们可以将所有8个位置向右推，我们留下 0000000AAAAAAAA 。所以年龄=（packed_info>> 8）。

To get the age, we can just push everything 8 places to the right, and we're left with 0000000AAAAAAAA. So age = (packed_info >> 8).

最后，为了获得性别，我们推动一切7个地方向右摆脱高度。然后我们只关心最后一点：

Finally, to get the gender, we push everything 7 places to the right to get rid of the height. We then only care about the last bit:

packed_info            = AAAAAAAGHHHHHHH
(packed_info >> 7)     = 0000000AAAAAAAG
1                      = 000000000000001
(packed_info >> 7) & 1 = 00000000000000G

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