Using a bytevector offset in the macro API

[vijaymarupudi]

Thank for very much for this library. I had a question about the macro API, and whether it is currently possible to use it with an offset into the bytevector. That is, when part of a bytevector follows the struct.

For example, let's say I define a bytestructure like this:

(define-bytestructure-accessors
  (bs:struct `((x ,int)
               (y ,int)))
  numbers-unwrap numbers-ref numbers-set!)

And I have a bytevector like this:

(define bv #vu8(1 2 3 4 5 6 7 8 9 10))

I would like to interpret this bytevector as the struct at offset 2. However, it seems like the macro API's -ref macro works like this.

(numbers-ref bytevector struct-index)

I can't seem to include an offset. If there's a way, I wonder if it could be made clearer in the documentation. If not, I wonder if it can be included. Currently, it seems like a lot of spurious allocation and copying needs to be done to achieve what I want (using the macro API).

[TaylanUB]

Hmm, good question.

You could use the unwrap form and add an offset to the result, but then you will have to specify the correct accessor function yourself every time.

Example:

(define-bytestructure-accessors
  (bs:struct `((x ,int)
               (y ,int)))
  numbers-unwrap numbers-ref numbers-set!)

(bytevector-s32-ref bv (+ offset (numbers-unwrap ...))) ;; assumes 32-bit int

That's quite sub-optimal...

I think the most user-friendly way of adding this feature properly would be:

Expand define-bytestructure-accessors to allow two more arguments, something like numbers-ref* and numbers-set!*, which take an offset argument after the bytevector argument and before the actual indices.

Example:

(define-bytestructure-accessors
  (bs:struct `((x ,int)
               (y ,int)))
  numbers-unwrap
  numbers-ref
  numbers-set!
  numbers-ref*
  numbers-set!*)

(numbers-ref* bv offset index)

WDYT?

[vijaymarupudi]

That would work really well! Thank you!

As a sidenote: I think it was not clear in the documentation what the macro signature of the macro API's unwrapper macro is. I had to figure it out via experimenting. I believe it's (unwrapper bv offset index ...), and returns (values bv offset). It might be useful to include this in the documentation.

[TaylanUB]

There's an example in the code snippet here:

https://github.com/TaylanUB/scheme-bytestructures#macro-based-api

But I guess I could document the generated macros more explicitly.

[vijaymarupudi]

Ah you're right! I think all of the indices being numbers confused me.