[erlang-questions] Offset calculation in bit syntax

[Ben Hood]

Hi,

I have to parse the following byte sequence where the first byte  
indicates the payload length if the
payload length in 15 bytes or less, e.g. the 3 byte payload

x01 x02 x03

will be encoded as

0x23 x01 x02 x03

with 0x23 - 0x20 = 3, the length of the payload.

So when I parse it, I'd like to use a bit syntax similar to following:

decode(<< Len:8/unsigned, Rest:(Len - 16#20)/unsigned>>) ->
	<<Rest>>.

But this is an illegal bit size.

Is there an elegant way around this?

Thx,

Ben
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