Asterix Item: Array with hidden (temporary) FX bit

[goto40]

Hi,

Thank you for this great crate!
I explored https://docs.rs/asterix/latest/asterix/ and your crate and I am preparing some documentation on it.

In discovered the possibility to implement arrays to be read until a flag is set: https://docs.rs/asterix/latest/src/asterix/data_item.rs.html#559-562

I would like to do the same, but

• to hide the FX flag (temp)
• and automate the computation of it (temp_value-like): only the last entry of the array gets a 0, else write a 1.

    //...
    #[derive(Debug, PartialEq, DekuRead, DekuWrite)]
    #[deku(endian = "big", bit_order = "msb")]
    struct Part2 {
        #[deku(bits = "4")]
        pub uint5_example: u8,
        #[deku(bits = "8")]
        pub sint7_example: i8,
        #[deku(bits = "3")]
        pub uint2_example: u8,
        #[deku(bits = "1")] // TODO hide! temp?
        pub fx: u8,
    }
    #[deku_derive(DekuRead, DekuWrite)]
    #[derive(Debug, PartialEq)]
    struct VariableLengthItem {
        part1: Part1,
        #[deku(bits = "1", temp, temp_value = "if (*part2).is_empty() {0} else {1}")]
        pub part1_fx: u8,
        #[deku(
            skip,
            cond = "*part1_fx==0",
            default = "Default::default()",
            until = "|codefx: &Part2| codefx.fx == 0" // TODO: hide fx, automate fx to be set to 0 only for the last element
        )]
        part2: Vec<Part2>,
    }

Do you think we can do this in the current state of deku, or do we need some extension?

[goto40]

Answer part 1 (hiding the FX field):

You can just make the fx-field non-public, then it is hidden.

Open: automatically set the fx field

[goto40]

Possible answer part 2 (update the FX fields):

You can use update to update the FX fields manually

// ...
        #[deku(
            skip,
            cond = "*part1_fx==0",
            default = "Default::default()",
            until = "|codefx: &Part2| codefx.fx == 0",
            update = "self.part2.iter_mut().rev().enumerate().map(|(index, entry)| { entry.fx = if index!=0 {1} else {0}; entry.clone() }).rev().collect::<Vec<_>>()"
        )]
        part2: Vec<Part2>,
// ...
    decoded.part2.push(Part2::new(2, -1, 3));
    decoded.update().unwrap();

I would prefer to have an "automatic-update" called before serialization. This would - however - require the FX field to have interior mutability, since serialization is a non-mutable operation.

What do you think of (a) allowing/enabling Cells (e.g., Cell<u8>) as data type and (b) adding a second update attribute (auto_update?) to be called before serialization?