Discussion on: [Challenge] Add numbers without (+-*/)

[shekohex]

const fn add<const A: usize, const B: usize>() -> usize {
    struct _Add<const A: usize, const B: usize> {
        _a: [u8; A],
        _b: [u8; B],
    }
    core::mem::size_of::<_Add<A, B>>()
}

#[test]
fn it_works() {
    assert_eq!(add::<40, 2>(), 42);
}

Compile time execution goes brrrrrr 😂

Here is the full code with other solutions: https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=3fd17ceb138f1e2c230987bd93d93f25

[Aurélien Delogu]

Can you explain the trick with size?

[shekohex]

The memory size of the struct _Add here would be the size of _a and _b combined, in other words for any type T and length of n, [T; n] has the size of n * size_of::<T>(), and since here T is u8 and its size is just 1 byte; hence [u8; n] is just n bytes. Using this; the size of _Add would be the size of [u8; A] (A bytes) and the size of [u8; B] (B bytes); will result in total is A+B bytes.

[Aurélien Delogu]

Yes but this is where I'm lost. Wouldn't be the returned value 2 in every case?

[shekohex]

No, in the test case, A = 40 means [0u8; 40] that also means 40 bytes, and B = 2 which is [0u8; 2] that's 2 bytes, which is total of 42 bytes.

[Aurélien Delogu]

Ah indeed, thanks! I misunderstood the syntax.

edit : that's a really smart solution btw, thanks for sharing

[Keff]

Nice, that's really interesting! I like how the compiler replaces the Iterator example with a + b lol 🤣

[shekohex]

Crazy!
(for anyone wonder what iterators, check out the rust playground link I've posted, there is other solution that uses iterators instead of const generics)

[Denis]

Rust gang lesssgooo