Binary matching

Carsten Schultz <>
Mon Dec 20 16:24:29 CET 2004


Hi!

On Mon, Dec 20, 2004 at 12:39:03PM -0200, 
Inswitch Solutions - Erlang Evaluation wrote:
>  
> Does someone know why the following binary matching fails?
>  <<X:7/binary, Rest/binary>> = <<0>>
>  
> Is there an elegant way to group an octect binary in seven bits values?
> For example eight characters (1 character = 1 byte) in seven octects.

I think you have to align the Rest on a byte boundary yourself.  For
example the following code is working:

base64encode(B) ->
    base64encode(B, []).
base64encode(<<A:6,B:6,C:6,D:6,T/binary>>, Acc) -> 
						% 3+ bytes
    base64encode(T, [b64e(D),b64e(C),b64e(B),b64e(A)|Acc]);
base64encode(<<A:6,B:6,C:4>>, Acc) -> 
						% 2 bytes
    lists:reverse([$=,b64e(C bsl 2),b64e(B),b64e(A)|Acc]);
base64encode(<<A:6,B:2>>, Acc) ->
						% 1 byte
    lists:reverse([$=,$=,b64e(B bsl 4), b64e(A)|Acc]);
base64encode(<<>>, Acc) ->
						% 0 bytes
    lists:reverse(Acc).


b64e(X) when X >= 0, X < 26 -> 
    X + $A;
b64e(X) when X >= 26, X < 52 -> 
    X - 26 + $a;
b64e(X) when X >= 52, X < 62 ->
    X - 52 + $0;
b64e(62) ->
    $+;
b64e(63) ->
    $/.


HTH

Carsten

-- 
Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin
http://carsten.codimi.de/
PGP/GPG key on the pgp.net key servers, 
fingerprint on my home page.
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