New feature: support for variable length fields

[mmindenhall]

Hi Igor,

It would be helpful to specify the "extra data" portion of a field with an expression. If you agree it would be useful, I'd be happy to take a first cut and submit another pull request.

Here's the reason for the request. BCD is something that belongs at the field level...it's a single entity within the binary message. However, the length of the field varies, and in some message formats there's a length field prior to the BCD bytes. For example:

{<bcd length ubyte>, <bcd field byte 0>, ... <bcd field byte (length-1)>}

I'd like to be able to specify that with the DSL...something like this:

ubyte length;
bcd(length) bcd_field;

Or for an array:

ubyte field_count;
ubyte field_length;
bcd(field_length) [field_count] bcd_fields;

The exact syntax doesn't really matter, as long as it's possible to specify the field size via an expression.

Thoughts?

[raydac]

I have added support of expressions for 'extra field number field', so now it is possible to write

ubyte arrlen; ubyte bitlen; bit:(bitlen) [arrlen] bitarray;

[mmindenhall]

Wow, thanks! This is a holiday weekend in the US, so I might not be able to check it out until Tuesday.

[mmindenhall]

Hello,

Thanks again...this is working as expected with my BCD JBBPCustomFieldTypeProcessor implementation.