You’ll problably need to store the number of bits padded in the file as well, otherwise you won’t be able to reconstruct the original bitstring from the file.
Small example
t = <<3::size(17)>>
# => <<0, 1, 1::size(1)>>
pad = 8 - (bit_size(t) - (bit_size(t) |> Integer.floor_div(8)) * 8)
# => 7
<<t::bitstring, 0::size(pad)>>
# => <<0, 1, 128>>
but if that is stored directly to a file, there is no way of knowing how many bits were padded.
So a convention could be to take the first 8-bits to mean the amount of padding that needs
to be removed (as well as the number itself).
