Question

I just started my path on Erlang and I’m facing a problem I can’t sort out a solution about:

I wrote a metod to take a domain espressed as a binary string, i.e. <<“www.404pagenotfound.com”>> and convert it in the domain format as required for DNS protocol, so: <<3,”www”,15,”pagenotfound”,3,”com”>>.

In the following the code (I rewroted it many times in different ways):

domainbyte(Bin) ->
    if byte_size(Bin) > 0 ->
        Res =  binary:split(Bin, <<".">>),
        [Chunk|[RestList]] = Res, 
        ChunkSize = byte_size(Chunk),   
        if length(RestList) > 0 -> 
            Rest = domainbyte(RestList),  %% <- Got "bad argument" here!
            <<ChunkSize/binary,Chunk,Rest>>;
        true ->
            <<ChunkSize/binary,Chunk>>
        end
    end.

Thx in advance for any clues.

PS.

Thx to comments I’ve found the error in the code abobe:

if length(RestList) > 0 -> %% here RestList is binary data so length throw "bad argument" error.

I’ve rewroted the method this way, but still with no luck:

**NOTE: I was able to fix the code in the following, problem is that if you have a binary chunk and you want to use it in another binary string, you must specify /binary on it: something not obvious to me.

I.e.: consider this small code snip:

**

TT = <<“com”>>,
SS = <<3, TT, 0>> %% <- you get error: bad argument

**
must be fixed this way:
**

TT = <<“com”>>,
SS = <<3, TT/binary, 0>>

domainbyte(Bin) ->
    if byte_size(Bin) > 0 ->
        Res =  binary:split(Bin, <<".">>),
                if length(Res) > 1 -> 
                    [Chunk|[RestList]] = Res, 
                    ChunkSize = byte_size(Chunk),
                    Rest = domainbyte(RestList),
                    <<ChunkSize,Chunk,Rest>>;
                true -> 
                    [Chunk] = Res,
                    ChunkSize = byte_size(Chunk), 
                    <<ChunkSize,Chunk>>
                end
    end.

MdP

Answer 1

I think the easiest solution is to define the function using a binary comprehension:

domainbyte(Bin) ->
    Chunks = binary:split(Bin, <<".">>, [global]),      %A list of all the chunks
    << <<byte_size(C),C/binary>> || C <- Chunks >>.     %Build output binary

It might be slightly faster to build the output binary as a list of segments in a separate function then put them together in an iolist_to_binary/1. Note that if a ‘.’ occurs outermost in the binary then this code will take that as an empty segment of length 0. If these should be discarded then you nee to add the option trim to binary:split/3. Note also that the size will occupy only one byte.

@Alnitak has the separate function but builds the binary one segment at time so it is not more efficient than the binary comprehension which does the same thing.

N.B. that if you have a binary segment Chunk/binary when constructing a binary this means that Chunk IS a binary, not that it should become one. Binaries are flat structures, think byte arrays, so everything becomes a binary. Or rather the binary.

EDIT: Though I see I missed the 0 which should be at the end. That is left as an exercise to the reader.

EDIT: Being in teaching mode, apart from the constructing binaries, a key to writing good Erlang code is understanding pattern matching. You use a bit but could do it more:

domainbyte(Bin) ->
    case binary:split(Bin, <<".">>) of
        [Chunk,Rest] ->                       %There was a '.'
            RestBin = domainbyte(Rest),
            Size = byte_size(Chunk),
            <<Size,Chunk/binary,RestBin/binary>>;
        [Chunk] ->                            %This was the last chunk
            Size = byte_size(Chunk),
            <<Size,Chunk/binary,0>>           %Add terminating 0
end.

This is basically doing the same as your code but we are using pattern matching to select a clause, not only to pull apart a known structure. Pattern matching is the basic method for control, not just in case as here but also in functions and receive. This results in if being used quite sparingly.

Enough from me for now.