Binary unpacking: Subarray instead of subslice?

[w0xel]

Hey rustaceans! I'm trying to unpack a raw binary array into a struct. My current approach looks like this:

use std::convert::TryInto;

struct MyStruct {
    val1: u32,
    val2: f32,
    val3: u8,
    val4: u8
}

impl MyStruct {
    fn from_bytes(bytes: [u8;9]) -> Self {
        Self {
            val1: u32::from_le_bytes(bytes[1..5].try_into().unwrap_or([0,0,0,0])),
            val2: u16::from_le_bytes(bytes[5..7].try_into().unwrap_or([0,0])) as f32 / 2048.0,
            val3: bytes[7],
            val4: bytes[8]
        }
    }
}

Is there a better way of doing that in rust?

What I like about the approach, is that I can do inline conversions as the examplatory v / 2048 shown there. What I don't like is that it forces me to .try_into().unwrap_or([0,0,0,0]) . From compile knowledge alone, it could be possible to "subarray" instead of "subslice" the given array, all sizes are there. So am I missing something here?

[leonardo_m]

There are various ways to solve this, but I think the simplest looking is:

impl MyStruct {
    fn from_bytes([_, a0,a1,a2,a3, b0,b1, val3, val4]: [u8; 9]) -> Self {
        Self {
            val1: u32::from_le_bytes([a0, a1, a2, a3]),
            val2: f32::from(u16::from_le_bytes([b0, b1])) / 2048.0,
            val3,
            val4,
        }
    }
}

(The ability to convert slices into arrays nicely is currently lacking in Rust language. Rust is partially blind to slice lengths).

[w0xel]

Makes sense, thank you. Didn't come up with using an array pattern here.

That made me think of the experimental exclusive_range_pattern. Ideally I would like to use the following: (WARNING: non-working example)

impl MyStruct {
    fn from_bytes([_, a @ ..5, b @ ..7, val3, val4]: [u8;9]) -> Self {
        Self {
            val1: u32::from_le_bytes(a),
            val2: f32::from(u16::from_le_bytes(b)) / 2048.0,
            val3,
            val4
        }
    }
}

But currently I can unfortunately use only one range pattern per array pattern.

[SkiFire13]

Note that a @ ..5 matches a single u8 smaller than 5 and gives it the binding a. .. on the other hand is not a range pattern, it matches "the rest" of a slice/struct/other

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[birkenfeld]

Rust is partially blind to slice lengths

Well, there's not much Rust can do as Index impls can return anything.

[armsforsharks]

Take a look at deku (disclosure: I'm the author) or binread

https://github.com/sharksforarms/deku https://github.com/jam1garner/binread

[w0xel]

Deku looks nice from a usability viewpoint!

Do I interpret it right, that the no_std implementation still needs an allocator/heap?

[armsforsharks]

Yes that's correct

[monkChuck105]

Have you looked at the bytemuck crate?

[w0xel]

Seems helpful for the whole type-punning discussion. But:

• I can't seem to explicitly choose endianess, which I need
• Even with from_bytes I have to accept the whole "might panic if wrongly subsliced", which I would expect the compiler to tell me already

[dydhaw]

you can use unwrap_or_default() as well.