有人可以解释 redis setbit 命令吗? [英] Can someone explain redis setbit command?

问题描述

>setbit mykey 1 1>setbit mykey 7 1

当我将字符串值 1 和 7 存储到mykey"中时，redis 中究竟存储了什么?以及 getbit 在 redis 中是如何工作的?

有人试图循环这个值内的位吗?我知道 bitcount 会给我数字 2，但我也想从中获得确切的字符串值 1 和 7，这可能吗?

--

我做了一些实验，使用 erlang redis 客户端读取输出.

>setbit mykey 1 1

erlang 输出:

<<<"@">>

然后我删除这个条目:

>德尔我的钥匙

我做同样的事情来偏移2 4 8，在这里你可以看到映射:

当offset为1时，输出<<<"@">;当 offset 为 2 时，输出为 <<" ">>;当offset为4时，输出为<<"">；当 offset 为 8 时，输出为 <<0,128>;

老实说，我现在更糊涂了.

或者有人可以解释这个"bitops.c"

-- 更新 ---

也许我应该提一下我想这样做的原因，以使问题更清楚.我们都知道使用位图来存储在线用户会很酷.我想要做的是从 redis 存储的内容中获取在线的确切用户 ID.

刚刚完成了一个快速版本来提取偏移量Redis.请随时改进它.

解决方案

顾名思义，SETBIT 允许您执行位操作 - 即在给定的位置将给定的位设置为 0 或 1位偏移，对于给定的键.

重要的是要了解结果并不总是只包含可打印的字符.这就是Redis使用自定义函数sdscatrepr的原因 格式化 CLI 输出:

<块引用>

附加到 sds 字符串 "s" 一个转义的字符串表示形式，其中所有不可打印的字符(用 isprint() 测试)都变成了 " a...." 或 "形式的转义字符x".

话虽如此，让我们从一个简单的例子开始.如果考虑十六进制数 0x7F (= 127)，其 8 位二进制表示为:

pos: 0 1 2 3 4 5 6 7位:0 1 1 1 1 1 1 1^ ^||最高有效位最低有效位

您通常可以使用 SETBIT 来存储这个值，记住偏移量 0 是 MSB，偏移量 7 是 LSB:

redis>SETBIT myval 0 0(整数)0Redis>SETBIT myval 1 1(整数)0Redis>SETBIT myval 2 1(整数)0Redis>SETBIT 米瓦尔 3 1(整数)0Redis>SETBIT myval 4 1(整数)0Redis>SETBIT 米瓦尔 5 1(整数)0Redis>SETBIT 米瓦尔 6 1(整数)0Redis>SETBIT 米瓦尔 7 1(整数)0

在以下情况下检查您的价值:

redis>获取 myval"x7f"

现在多字节会发生什么?假设您要存储 0x52 (= 82)，它对应于 ASCII 中的字符 R.8 位表示是 01010010 位位置 (8, 9, ..., 15) 因为我们希望它在第一个值之后立即存储:

redis>SETBIT myval 8 0(整数)0Redis>SETBIT 米瓦尔 9 1(整数)0Redis>SETBIT myval 10 0(整数)0Redis>SETBIT 米瓦尔 11 1(整数)0Redis>SETBIT myval 12 0(整数)0Redis>SETBIT myval 13 0(整数)0Redis>SETBIT 米瓦尔 14 1(整数)0Redis>SETBIT myval 15 0(整数)0

你会得到:

redis>获取 myval"x7fR"

这里Redis CLI能够表示可打印字符R.

<块引用>

当我将字符串值 1 和 7 存储到mykey"中时

它对应于 01000001 等于 65 和 0x41 十六进制.它对应于ASCII字符A.这样做:

redis>SETBIT mykey 1 1(整数)0Redis>SETBIT mykey 7 1(整数)0

给出:

redis>得到我的钥匙一个"

<块引用>

getbit 在 redis 中是如何工作的?

它只是返回给定位置的位值.这里:

redis>获取我的密钥 1(整数)1

但位 0 尚未设置(默认为 0)因此:

redis>获取我的密钥 0(整数)0

> setbit mykey 1 1
> setbit mykey 7 1

When I store string value 1 and 7 into "mykey", what was exactly stored in redis? And how the getbit works inside redis?

Does anyone try to loop the bit inside this value? I know bitcount will give me number 2, but I also want to get the exact string value 1 and 7 from it, is it possible?

--

I doing some experiment by using erlang redis client to read the output.

> setbit mykey 1 1

erlang output:

<<"@">>

Then I delete this entry:

> del mykey

I do the same thing to offset 2 4 8, here you can see the mapping:

When offset is 1, the output is <<"@">>;
When offset is 2, the output is <<" ">>;
When offset is 4, the output is <<"">>;
When offset is 8, the output is <<0,128>>;

Honestly, I am more confused now.

Or someone can explain this "bitops.c"

-- updates ---

Maybe I should mention the reason why I want to do this to make the question more clear. We all know it will be cool to use bitmap to store online users. What I am trying to do is get the exactly user id who is online from what redis stored.

Just finished a quick version to extract offsets from redis. Please feel free to improve it.

解决方案

As its name implies, SETBIT allows you to perform bit operations - namely set a given bit to 0 or 1, at a given bit offset, for a given key.

What is important to understand is that the result not always includes only printable characters. This is why Redis uses a custom function sdscatrepr to format the CLI output:

Append to the sds string "s" an escaped string representation where all the non-printable characters (tested with isprint()) are turned into escapes in the form " a...." or "x".

That being said let's start with a simple example. If you consider the hex number 0x7F (= 127) its binary representation on 8-bit is:

pos: 0 1 2 3 4 5 6 7
bit: 0 1 1 1 1 1 1 1
     ^             ^
     |             |
     MSB           LSB

You can typically use SETBIT to store this value, keeping in mind that offset 0 is MSB and offset 7 is LSB:

redis> SETBIT myval 0 0
(integer) 0
redis> SETBIT myval 1 1
(integer) 0
redis> SETBIT myval 2 1
(integer) 0
redis> SETBIT myval 3 1
(integer) 0
redis> SETBIT myval 4 1
(integer) 0
redis> SETBIT myval 5 1
(integer) 0
redis> SETBIT myval 6 1
(integer) 0
redis> SETBIT myval 7 1
(integer) 0

The get your value to inspect if:

redis> GET myval
"x7f"

Now what happens with multi bytes? Let's say you want to store 0x52 (= 82) which corresponds to character R in ASCII. The 8-bit representation is 01010010 with bit positions (8, 9, ..., 15) since we want it to be stored right after the first value:

redis> SETBIT myval 8 0
(integer) 0
redis> SETBIT myval 9 1
(integer) 0
redis> SETBIT myval 10 0
(integer) 0
redis> SETBIT myval 11 1
(integer) 0
redis> SETBIT myval 12 0
(integer) 0
redis> SETBIT myval 13 0
(integer) 0
redis> SETBIT myval 14 1
(integer) 0
redis> SETBIT myval 15 0
(integer) 0

And you get:

redis> GET myval
"x7fR"

Here Redis CLI is able to represent the printable character R.

When I store string value 1 and 7 into "mykey"

It corresponds to 01000001 which is equal to 65 and 0x41 in hex. It corresponds to ASCII character A. So doing:

redis> SETBIT mykey 1 1
(integer) 0
redis> SETBIT mykey 7 1
(integer) 0

Gives:

redis> GET mykey
"A"

how the getbit works inside redis?

It simply returns the value of the bit at the given position. Here:

redis> GETBIT mykey 1
(integer) 1

But bit 0 has not been set (it is 0 by default) thus:

redis> GETBIT mykey 0
(integer) 0

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