How to best create bitstrings / binary data with known values with mixed types

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,
1

Hi,

I have a general question about building bitstring/binary data. I have been following some articles that look at parsing binary data, so I feel like I have some handle on matching a binary, and bringing parts to variables, but I am having some difficulty creating binary data from known data with non-binary types.

# this makes sense to me
iex> <<first::binary-size(1), rest::binary>> = "hello world"
"hello world"
iex> first                                                  
"h"

# This also makes sense
iex> first                                               
"h"
iex> rest                                                
"ello world"
iex> <<first::binary, rest::binary>>                   
"hello world"

# But how now do I encode the integer 1 as a 4-or-8-byte block into the bitstring
iex> n = 1024                          
1024
iex> <<first::binary, _______, rest::binary>>

What I would hope to find out, is there a way for me to perform this in that form, or is the only way to do it is to go about it the long way:

iex(48)> first                                               
"h"
iex(49)> rest                                                
"ello world"
iex> n = 1024
1024
iex> n_as_bin = 
...>   n 
...>   |> :binary.encode_unsigned(:big) 
...>   |> :binary.bin_to_list() 
...>   |> (fn b -> List.duplicate(0, 4-length(b)) ++ b end).() 
...>   |> :binary.list_to_bin
<<0, 0, 4, 0>>
iex> <<first::binary, n_as_bin::binary, rest::binary>>                                                                                                    
<<104, 0, 0, 4, 0, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100>>

Thanks much in advance.

1 Like
2

I think you want this:

iex(1)> first = "h"
"h"
iex(2)> n = 1024
1024
iex(3)> rest = "ello"
"ello"

# specify the number of bits as your binary segment decorator
iex(4)> <<first::binary, n::32, rest::binary>>
<<104, 0, 0, 4, 0, 101, 108, 108, 111>>
iex(5)> <<first::binary, n::64, rest::binary>>
<<104, 0, 0, 0, 0, 0, 0, 4, 0, 101, 108, 108, 111>>

# note that you can pattern match out of it too.
iex(6)> <<_first::binary-size(1), res::64, _rest::binary>> = v
<<104, 0, 0, 0, 0, 0, 0, 4, 0, 101, 108, 108, 111>>
iex(7)> res
1024

You can also do ‘crazy things with bitbashing’, those numbers don’t have to be multiples of 8.

iex(9)> <<1::1, 2::4, 1::3>> 
<<145>>
4 Likes
3

That is exactly what I am looking for! I just couldn’t see the tree through the forest, thanks so much!

1 Like