Modelling a TLV (type length value)

[ckrenslehner]

Hi!

I have a very general question. How would you model a TLV? I am using binrw quite a lot and I want to learn about idiomatic usage of this amazing crate. :-)

The basic structure should be as follows: type (u8) ; length(u16) ; value

Particularly I want to model value as an enum but still want to be able to calculate the length similarly to the way I would calculate it, if value was a vector.

Here is a snippet for further illustration:

#[binrw]
#[br(import(r#type: u8))]
enum Value {
    #[br(assert(r#type == 1))]
    Temperature(f32),
    #[br(assert(r#type == 2))]
    Opened(
        #[br(map = |x: u8| x != 0)]
        #[bw(map = |x: &bool| if *x { 1 } else { 0 })]
        bool,
    ),
}

#[binrw]
struct TLV {
    r#type: u8,
    /// This value should be calculated by using the encoded size of `value`.
    /// If `value` was a `Vec<u8>` I could use `calc` to write the correct size.
    length: u16,
    #[br(args(r#type))]
    value: Value,
}

When reading a TLV it would be nice to validate, that the size encoded size of value indeed exactly matches the value of length. How would you tackle this aspect?
I am thinking of TakeSeek or using a custom parser function.

Ah and btw how do you model the encoding of bool?

Thanks and greetings,
Christian

[csnover]

Here is a snippet for further illustration:

Use pre_assert, not assert, for selecting an enum variant. For unknown types, specify a fallback variant as the last variant with no pre_assert with #[br(count = length)] Vec<u8> to collect data for an unknown type.

When reading a TLV it would be nice to validate, that the size encoded size of value indeed exactly matches the value of length. How would you tackle this aspect?

The simplest way is to write a function that returns the expected size of a given type and e.g. assert(length == TLV::size_of_type(r#type)). This particular thing of determining struct size has been discussed in some other issues/discussions, so you may do a search around.

Ah and btw how do you model the encoding of bool?

This depends on the file format you are writing. If it is a simple 0/1, Rust comes with impl From<bool> for {number} so you can just use map to map to the appropriate number type.