Pointer: jump to position, parse/write struct, and come back

[marnunez]

In construct, there's a wrapper called Pointer, which is really useful for binary files that have jumptables. It uses a field as an offset, jumps to the position, parses/writes the content of the structure, and jumps back to position. Would something like this be feasible in Deku?

[sharksforarms]

I don't see why not! Possibly a seek and peek attribute. Do you have an example protocol or format for reference?

[marnunez]

There's a nice example of case of use in the construct gallery for the PE/COFF format.

[kitlith]

So, I went to implement something like this as part of my own project, and ran into the fact that currently, an implementation of DekuRead has no way to perform an absolute offset operation -- you'd have to pass in the original BitSlice as context. so, that indicates to me that some breaking changes to deku are likely necessary to make any sort of file pointer ergonomic, or else some very careful additions to the derive macro to thread the starting position through to whatever needs it.

[sharksforarms]

@kitlith Breaking changes are fine with me. This crate is still pre 1.0.0 :)

[CJKay]

For anybody looking for an interim solution, I hacked up this absolute nightmare:

fn read_offset<'a, C, T, O>(
    rest: &'a BitSlice<Msb0, u8>,
    input: &'a BitSlice<Msb0, u8>,
    ctx: C,
    offset: O,
) -> Result<(&'a BitSlice<Msb0, u8>, T), DekuError>
where
    C: Copy,
    T: DekuRead<'a, C>,
    O: Into<usize>,
{
    let offset = offset.into();
    let subslice = input.get(offset..).ok_or_else(|| {
        let need = NeedSize::new(rest.len() - offset);

        DekuError::Incomplete(need)
    })?;

    T::read(subslice, ctx)
}

fn read_offset_count<'a, C, T, U, O, N>(
    rest: &'a BitSlice<Msb0, u8>,
    input: &'a BitSlice<Msb0, u8>,
    ctx: C,
    offset: O,
    count: N,
) -> Result<(&'a BitSlice<Msb0, u8>, T), DekuError>
where
    C: Copy,
    T: DekuRead<'a, (Limit<U, fn(&U) -> bool>, C)>,
    U: Copy + DekuRead<'a, C>,
    O: Into<usize>,
    N: Into<usize>,
{
    read_offset(rest, input, (Limit::new_count(count.into()), ctx), offset)
}

Usage example:

#[derive(PartialEq, Debug, DekuRead)]
#[deku(endian = "little", ctx = "endian: deku::ctx::Endian")]
pub struct MorrowindArchive {
    ...

    /// File name hashes.
    #[deku(reader = "read_offset_count(deku::rest, deku::input_bits, endian, ((*file_name_hashes_offset as usize) + 8) * 8, *file_count as usize)")]
    file_name_hashes: Vec<u64>,
}