CODEWITHSUNDEEP
stringredishex
Swapnanil Saha
Swapnanil Saha

Reputation: 134

Converting Redis output string to bit string

127.0.0.1:6379> setbit mykey 1 1
(integer) 0
127.0.0.1:6379> setbit mykey 23 1
(integer) 0
127.0.0.1:6379> setbit mykey 345 1
(integer) 0
127.0.0.1:6379> get mykey
"@\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00@"
127.0.0.1:6379>

I want to parse the above output string ( as a string of bits, like "100000000011...") to get the indexes of the set bits, how would I do it?

Upvotes: 2

Views: 1212

Answers (1)

GuangshengZuo
GuangshengZuo

Reputation: 4677

You know the setbit is just set the bit of a string, so the mykey is holding a string. when you use the command get mykey, it just return the string value.

And the \x00 is a also a char, you could refer this ASCII TAble.

Every char has eight bits, and the bitmap is just composed of all bits of every char of the string. its order is that every 8 bits's order is char's order in string, and in this every 8 bits, the order of bit is from the high bit to low bit. for example, if the string is @\x00, then the length of string is 2, the bits length is 16. The ascii value of @ is 64 in decimal, and the ascii of \x00 is 0.

So its bitmap is :

0100 0000 0000 0000

So their offsets is from 0 to 15 from the left to right.

If you have a doubt about this, you could execute this commands in redis-cli to test:

setbit test 1 1
setbit test 15 0
get test

It will be @\x00.

So the relation between the bitmap and string is clear, we could use the string to get the bitmap we want.

just use a simple python script:

s = "@\x00"
bitmap = ""
for c in s:
    x = ord(c)
    str = bin(x).split('b')[1]
    if len(str) < 8 : 
       str = '0' * (8-len(str)) + str
    bitmap += str
print bitmap

In this script , it uses the ord to get ascii value of char, and use to bin to convert the dec to bin, and add 0 to high bit it length is smaller than 8

the output is :

0100000000000000

Upvotes: 5

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