[erlang-questions] idea: service pack one

Juan Jose Comellas <>
Thu Nov 15 15:20:38 CET 2007


Thanks for the tips. I'm going to start using this idiom now.

BTW, is the Erlang VM capable of expanding a binary without copying data if
there is a contiguous block of memory available? If a feature like this one
could be added, it would be possible to efficiently append data to a binary
without resorting to using a list as an intermediate value. Another option
could be to make the binaries preallocate memory.

Just a thought...



On 15 Nov 2007 07:53:26 +0100, Bjorn Gustavsson <>
wrote:
>
> "Juan Jose Comellas" <> writes:
>
> > One thing that I was always doing when iterating over binaries is doing
> > matches like the following one:
> >
> > <<Head:Offset/binary, Char, Tail/binary>>
>
> You don't have to do that anymore in R12B.
>
> Take for instance your index/2 function:
>
> index(Str, Char) when is_binary(Str), is_integer(Char) ->
>     index(Str, Char, 0).
> index(Str, _Char, N) when N > size(Str) ->
>     0;
> index(Str, Char, N) ->
>     case Str of
>         <<_Head:N/binary, Char, _Tail/binary>> ->
>             N + 1;
>         <<>> ->
>             0;
>         _ ->
>             index(Str, Char, N + 1)
>     end.
>
> In R12B, you can write it simply like:
>
> index(Str, Char) when is_binary(Str), is_integer(Char) ->
>     index(Str, Char, 1).
>
> index(<<Char,T/binary>>, Char, N) ->
>     N;
> index(<<_,T/binary>>, Char, N) ->
>     index(T, Char, N+1);
> index(<<>>, _, _) -> 0.
>
> On my Sparc workstation this simpler program is more than twice as fast
> in R12B.
>
> Note: Your index function can be written speeded a bit.
>
> index(Str, Char) when is_binary(Str), is_integer(Char) ->
>     index(Str, Char, 0).
> %% index(Str, _Char, N) when N > size(Str) ->
> %%     0;
> index(Str, Char, N) ->
>     case Str of
>         <<_Head:N/binary, Char, _Tail/binary>> ->
>             N + 1;
>         <<_Head:N/binary>> ->
>             0;
>         _ ->
>             index(Str, Char, N + 1)
>     end.
>
> Still, even after this improvement, in R12B the simpler version is about
> twice
> as fast.
>
> /Bjorn
> --
> Björn Gustavsson, Erlang/OTP, Ericsson AB
>
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