Scripting Helpers is winding down operations and is now read-only. More info→

Select range of bits from string?

Asked by

9 years ago

Okay, so you know how strings are made up of characters, and all those characters are made up of a single byte each. Assuming I'm using a string to represent a ton of bits, how can I get the values for a range of the bits? So say, I wanted the bits from 2 to 13 for some obscure reason, how can I take my string and extract all of the bits in that range?

As an example, "FF" is 0000111100001111 in binary. If I wanted to select bits 2 -> 13 in that, I'd get 000111100001. And then somewhere along the line somebody decides whether to pad the 0s to the left or to the right when converting it back to the string but that's not a problem.

Extra useful if an explanation includes how to turn the range into a boolean array (I have a solution for this bit but it's not particularly efficient).

If you're needing a use case, I'm planning on creating my own replication because Roblox networking sucks.

Once you get the binary value, you could use the :sub. For example, if it was in a string value: print(game.ServerStorage.IntValue.Value:sub(2,13)) Ethan_Waike 156 — 9y

Hm, I could convert it to binary, but that would require evaluating the values of the entire string and I'm hoping not to do that. User#6546 35 — 9y

I'm not sure how you would do this without first converting it to a binary format. I don't know of any built-in method that will extract such data from a variable/literal. BlackJPI 2658 — 9y

I can turn it into an array of bytes with {string.byte()} but that only gets me so far. User#6546 35 — 9y

1 answer

Answered by

ZarsBranchkin 885

9 years ago

Right, seems like I have came up with some seemingly efficient solution which returns the bits as table of booleans.

First thing you should do, is find the bytes that fall within the begin and end bit range, so no processing power is wasted converting unnecessary bytes to bits. After that is done, you can start converting the bytes into bits.

01function getBits( str, beginBit, endBit )
02    -- A reference table for bit values(reading from table is significantly faster than using power operator)
03    local byte_t = { 1, 2, 4, 8, 16, 32, 64, 128 }
04 
05    -- Finding out actual bytes that we are interested in
06    local beginChar = math.ceil( beginBit/8 )
07    local endChar = math.ceil( endBit/8 )
08    local range = str:sub( beginChar, endChar )
09 
10    -- I will convert and directly set the captured bits, so there is no reason to loop several times and what not
11    local bits = {}
12    -- Since I will directly store the result, I need to know from which bit should I start storing
13    local beginOffset = beginBit%8 - 1
14    -- Same applies to the end
15    local endOffset = endBit - beginOffset + 1
16 
17    for num = 1,#range do
18        local byte = str:sub( num, num ):byte()
19        -- Figure out how far in the bits table we are
20        local offset = (num-1)*8 - beginOffset
21 
22        -- I won't really go into details of byte to bit conversion, as I lack experience with these sort of operations in order to explain them properly
23        for bit=8, 1, -1 do
24            -- Some micro optimisation, probably no big performance gain, but might make a change on bigger captures
25            local sub = byte - byte_t[bit]
26            -- Current output bit
27            local pos = offset + bit
28 
29            -- If current bit is smaller than byte, subtract bit from it and set the current bit to true.
30            -- Otherwise simply set current output bit to false
31            if sub >= 0 then
32                -- Save only the bits in the capture range
33                if pos > 0 and pos < endOffset then
34                    bits[pos] = true
35                end
36 
37                -- But it's still necessary to subtract byte to continue converting it
38                byte = sub
39            else
40                if pos > 0 and pos < endOffset then
41                    bits[pos] = false
42                end
43            end
44        end
45    end
46 
47    return bits
48end
49 
50print( unpack( getBits( "Some content string, serving simply testing purpose", 56, 131  ) ) )
 View all 50 lines...

Tried my best to comment it, but I don't have much experience with these sort of manipulations and I'm too sleepy right now.

Amazing, thank you. User#6546 35 — 9y

Coming back to this after 5 months. What about just getting the value as a number of a range of bits? User#6546 35 — 8y

Select range of bits from string?

1 answer

Answer this question