[erlang-questions] Real basic binary manipulation question

Steve Davis <>
Sat Jan 17 18:44:59 CET 2009


Oh wow!

I guess I should at least be thankful I'm beginning to spot where my 
erlang is plain ugly!

I can now see many places where I could improve my code using this 
approach. I feel a big refactor coming on.

Thanks, both of you!

/s

Per Gustafsson wrote:
> Or even more straight-forward:
> 
> -module(zrle).
> -export([decompress/1]).
> 
> decompress(Bin) when is_binary(Bin) ->
>    decompress(Bin, <<>>).
> 
> decompress(<<0, Count, Tail/binary>>, Acc) ->
>    decompress(Tail, <<Acc/binary, 0:(Count * 8)>>);
> decompress(<<Head, Tail/binary>>, Acc) ->
>    decompress(Tail, <<Acc/binary, Head>>);
> decompress(<<>>, Acc) ->
>    Acc.
> 
> If you are using at least R12 the performance for this should be ok.
> 
> Per
> 
> Juan Jose Comellas wrote:
>> In your function it wasn't clear to me why you were adding the 
>> 'expand' atom to the destination list T. Maybe something like this 
>> could do:
>>
>>
>> -module(zrle).
>> -export([decompress/1]).
>>
>> decompress(Bin) when is_binary(Bin) ->
>>     decompress(Bin, []).
>>
>> decompress(<<0, Count, Tail/binary>>, Acc) ->
>>     decompress(Tail, [<<0:(Count * 8)>> | Acc]);
>> decompress(<<Head, Tail/binary>>, Acc) ->
>>     decompress(Tail, [Head | Acc]);
>> decompress(<<>>, Acc) ->
>>     list_to_binary(lists:reverse(Acc)).
>>
>>
>> e.g.
>>
>> 1> zrle:decompress(<<1, 0, 5, 2, 3, 4, 0, 3>>).
>> <<1,0,0,0,0,0,2,3,4,0,0,0>>
>>
>>
>>
>> On Sat, Jan 17, 2009 at 8:46 AM, Steve Davis 
>> < 
>> <mailto:>> wrote:
>>
>>     Problem - to decompress a binary where zeros are run-length encoded.
>>     The following seems to work but also seems ugly and causes 
>> unnecessary
>>     overhead.
>>
>>     %% Decompress binary where Zeros are Run-Length Encoded
>>     decompress(Bin) when is_binary(Bin) ->
>>            L = binary_to_list(Bin),
>>            F = fun(X, [H|T]) when H =:= expand -> lists:duplicate(X,
>>     0) ++ T;
>>                            (X, Acc) when X =:= 0 -> [expand] ++ Acc;
>>                            (X, Acc) -> [X] ++ Acc
>>                    end,
>>            Decompressed = lists:foldl(F, [], L),
>>            list_to_binary(lists:reverse(lists:flatten(Decompressed))).
>>
>>     Any bids on an improvement?
>>
>>     Thanks for any attention.
>>
>>     Side note: the erlang bit syntax is *amazing* when you actually get
>>     down to doing something with IP packets
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>>
>>
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