Read a u16 from one or 2 bytes

[lvauvillier]

Hi ,

I'm trying to parse a file that have an integer field encoded as u8 for old formats and u16 for new formats.
How to achieve this ?

#[binread]
#[derive(Debug)]
#[br(little, import(format_version: u8))]
struct RecordFrame {
    record_type: u8,
    // i want to read here 1 or 2 bytes depending on arg format_version and cast it into a u16 in both cases
    #[br(count = if format_version <= 12 { 1 } else { 2 })] // can i use something like count ???
    length: u16,
    #[br(count = length)]
    data: Vec<u8>,
    end: u8,
}

[lvauvillier]

I finally got it to work with a custom parser, but I'm wondering if there is a solution using built-in directives.

#[binrw::parser(reader)]
pub fn parse_record_length(two_bytes: bool) -> BinResult<u16> {
    let mut bytes = [0; 2];
    reader.read_exact(&mut bytes[if two_bytes { 0..=1 } else { 0..=0 }])?;
    Ok(u16::from_le_bytes(bytes))
}

and

#[binread]
#[derive(Debug)]
#[br(little)]
#[br(import(format_version: u8))]
struct RecordFrame {
    record_type: u8,
    #[br(args(format_version > 12), parse_with = parse_record_length)]
    length: u16,
    #[br(count = length)]
    data: Vec<u8>,
    end: u8,
}

[kitlith]

In general, I think the way you work with choosing to read using one format or another using built-in directives would be to use enums. So, you'd have an enum:

#[binread]
#[derive(Debug)]
#[br(little, import(format_version: u8))]
enum RecordLength {
    #[br(pre_assert(format_version > 12))]
    New(u16),
    Old(u8),
}

And then you'd use it in your struct:

// ...
length: RecordLength,
// ...

or map it:

// ...
#[br(map = |l: RecordLength| l.into())] // assuming an into implementation
length: u16
// ...

[lvauvillier]

Oh, thanks for the tips. The enum + map looks good. I'll give a try.