Read N bytes into larger data primitive

[ARizzo35]

Is it possible to easily parse a number of bytes into a larger data primitive? I'm trying to do something like so:

pub struct MyStruct {
    pub data1: u32,
    pub data2: u16,
    pub data3: u16,
    pub data4: i16,
}

Then, the data I am trying to parse would have the first 3 bytes stored as data1 (MSB = 0), 2 bytes as data2, 2 bytes as data3, and then the last byte stored in data4, for a total of only 8 bytes parsed. Also, the last value stored in the i16 would need to be manipulated (e.g. subtract 50 from u8 value read). I'm guessing I can write a custom parser/writer for the last byte but can also be done post parsing into the struct.

[jam1garner]

#[binread]
#[br(big)]
pub struct MyStruct {
    #[br(reader = parse_u24)]
    pub data1: u32,
    pub data2: u16,
    pub data3: u16,

    #[br(map = |x: u8| x as i16)]
    pub data4: i16,
}

/// A parser that reads a 24-bit unsigned integer into a `u32`.
#[binrw::parser(reader, endian)]
pub fn parse_u24() -> BinResult<u32> {
    let mut bytes = [0; 4];
    let (range, from_bytes): (_, fn(_) -> _) = match endian {
        Endian::Big => (1..=3, u32::from_be_bytes),
        Endian::Little => (0..=2, u32::from_le_bytes),
    };
    reader.read_exact(&mut bytes[range])?;
    Ok(from_bytes(bytes))
}

apologies if I've made any mistakes, as I'm on mobile. I have not checked the endians match what you describe. lmk if there's any issues and I'll try to correct the code accordingly

[octylFractal]

You can also do the manipulation for data4 in the map, e.g. for the "subtract 50" do map = |x: u8| x as i16 - 50.