recommended method for keeping a `Digest` in a struct w/ 3.0 API

[james-rms]

Hello,

I'm trying to make a wrapper for a std::io::Write object that keeps a running CRC. This looks something like this:

struct Wrapper<W> {
   w: W,
   crc: Crc<u32>,
}

impl <W: std::io::Write> std::io::Write for Wrapper {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        self.crc.digest().update(buf);
        self.w.write(buf)
    }
}

However this is wrong, because the running state of the digest gets discarded on every write call (i think). I feel like what I want is crc.update(), but it's not pub. I could also figure out a way to store both the crc and the digest in the struct, but that would involve internal references, which are a pain to work with.

Is there some recommended way to do this? Is this a good case for making Crc::update pub?

[akhilles]

Can Crc be initialized at compile-time?

pub const CASTAGNOLI: Crc<u32> = Crc::<u32>::new(&CRC_32_ISCSI);

Then, Digest will be Digest<'static, u32> and you won't have to deal with internal references.

Crc::new is quite slow because it generates the tables, so you typically don't want to do it at runtime.

[james-rms]

Oh nice, I didn't think of that at all. I'm happy to make a PR to put an example of that in the docs somewhere.

[akhilles]

Oh nice, I didn't think of that at all. I'm happy to make a PR to put an example of that in the docs somewhere.

Sure, that'd be great.

[plwalsh]

@akhilles The Rust docs state that "constants are essentially inlined." Does that mean that by setting Crc::new() to a const, it will be evaluated (i.e. table generated) every time the constant is accessed/referenced? Or will it still only be evaluated once? And does it make any difference if the const is defined as an "associated constant" inside an impl, as opposed to being at the module level?

[akhilles]

It will only be evaluated at compile-time and it doesn't matter if it's an associated const. The output of Crc::new() is what gets inlined (in some cases).

So, if Crc::new() is assigned to a const, table generation will not occur at runtime. Not even once.

[plwalsh]

Makes sense, thanks! Just wasn't sure what happened under the hood.