Add const generic support

[v1gnesh]

Hello @Licenser!

Thank you for sharing your excellent work.
I'm delighted to see that this crate shaves off about half the run time when parsing a huge binary file into structs, then into JSON.

As a novice programmer in general, I'm quite stuck by the below error.

When you have time, could you please help?

Code to reproduce:

use std::io::Cursor;
use binrw::BinRead;
use simd_json_derive::Serialize;

pub fn funk(bytes: &[u8]) -> String {
    String::from_utf8(Vec::from(bytes)).unwrap()
}

#[derive(Debug, PartialEq, BinRead, Serialize)]
pub struct EString<const N: usize>(
    #[br(map = |bytes: [u8; N]| funk(&bytes))]
    String
);

#[derive(Debug, PartialEq, BinRead, Serialize)]
#[br(big)]
pub struct Header {
    len: u16,
    _field: [u8; 12],
    // want: EString<4>,
    want: u32,
}

fn main() {

    let rec = vec![0x01, 0xB0, 0x00, 0x00, 0x1e, 0x0e, 0x00, 0x37, 0x93, 0xfa, 0x01, 0x21,
                   0x23, 0x0f, 0xe2, 0xf0, 0xe6, 0xf1];

    let value = Header::read(&mut Cursor::new(rec)).unwrap();
    // value.json_string().unwrap()
    println!("\n{}", value.json_string().unwrap());
}

Happy output:
When the EString struct is commented out, and when Header.want of u32 is used instead of EString<4>.

{"len":432,"_field":[0,0,30,14,0,55,147,250,1,33,35,15],"want":3807438577}

Sad output:

error: unexpected `const` parameter declaration
  --> src/main.rs:10:20
   |
10 | pub struct EString<const N: usize>(
   |                    ^^^^^^^^^^^^^^ expected a `const` expression, not a parameter declaration

error: proc-macro derive produced unparsable tokens
 --> src/main.rs:9:37
  |
9 | #[derive(Debug, PartialEq, BinRead, Serialize)]
  |                                     ^^^^^^^^^

error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
  --> src/main.rs:10:20
   |
10 | pub struct EString<const N: usize>(
   |                    ^^^^^^^^^^^^^^ unconstrained const parameter
   |
   = note: expressions using a const parameter must map each value to a distinct output value
   = note: proving the result of expressions other than the parameter are unique is not supported

error: aborting due to 3 previous errors

[Licenser]

Heya,

sorry for the late reply. Const generics are not implemented in the derive but the EString is fairly simple you can implement Serialize like this:

impl<const N: usize> Serialize for EString<N> {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
    where
        W: std::io::Write,
    {
        self.0.json_write(writer)
    }
}

Adding it to the derive, while not impossible I think, might be a bit tricky and I don't have tons of time at the moment :(

NOTE:

I ran this example and it gives UTF8 Error, is the string valid UTF8, it fails while decoding from what I can tell?

[v1gnesh]

Thanks for getting back!
While I'm excited at the prospect of having a workaround for this, I'm getting the same error as before 😕.

EDIT:
I've changed the body of the funk function, and it now prints a valid String.
Also replaced _field with

    _field1: u64,
    _field2: u32,

.. to just print some val, and not an array.

Header {
    len: 432,
    _field1: 33045482017786,
    _field2: 18948879,
    want: EString(
        "S0W1",
    ),
}

[Licenser]

This is the whole code that compiles for me, but the inout seems to be invalid as the funk function crashes:

use binrw::BinRead;
use simd_json_derive::Serialize;
use std::io::Cursor;

pub fn funk(bytes: &[u8]) -> String {
    String::from_utf8(Vec::from(bytes)).unwrap()
}

#[derive(Debug, PartialEq, BinRead)]
pub struct EString<const N: usize>(#[br(map = |bytes: [u8; N]| funk(&bytes))] String);

impl<const N: usize> Serialize for EString<N> {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
    where
        W: std::io::Write,
    {
        self.0.json_write(writer)
    }
}

#[derive(Debug, PartialEq, BinRead, Serialize)]
#[br(big)]
pub struct Header {
    len: u16,
    _field: [u8; 12],
    want: EString<4>,
    // want: u32,
}

fn main() {
    let rec = vec![
        0x01, 0xB0, 0x00, 0x00, 0x1e, 0x0e, 0x00, 0x37, 0x93, 0xfa, 0x01, 0x21, 0x23, 0x0f, 0xe2,
        0xf0, 0xe6, 0xf1,
    ];

    let value = Header::read(&mut Cursor::new(rec)).unwrap();
    dbg!(&value);
    // value.json_string().unwrap()
    println!("\n{}", value.json_string().unwrap());
}

[v1gnesh]

Oh, didn't notice that EString shouldn't have a derive(Serialize) at the top.

#[derive(Debug, PartialEq, BinRead)]
pub struct EString<const N: usize>(#[br(map = |bytes: [u8; N]| funk(&bytes))] String);

Thank you, this is amazing!

[Licenser]

nice glad we got that working and it's really cool to hear this could cut the execution time in half :D

[v1gnesh]

Now for a dumb question - say I have loads of these -
impl Serialize for CustomField {

Can I do something like - impl Serialize for CustomField1, CustomField2 { to avoid repetition... if they're all just unit structs where I want to print out the String?

[Licenser]

if they're all the same do they need to be different sturcts? (could you give examples of how CustmField1 and 2 are looking?)

[v1gnesh]

I'm invoking different map functions on them though:

#[derive(Debug, PartialEq, BinRead)]
pub struct CustomType1 (
    #[br(map = yyyyddd)]
    String,
);

#[derive(Debug, PartialEq, BinRead)]
pub struct CustomType2(
    #[br(map = hhmmss)]
    String
);

... and in the main Header struct, they end up like this:

    tme: CustomType1,
    dte: CustomType2,

... so the below is repetitive:

impl Serialize for CustomType1 {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
        where
            W: std::io::Write,
    {
        self.0.json_write(writer)
    }
}

impl Serialize for CustomType2 {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
        where
            W: std::io::Write,
    {
        self.0.json_write(writer)
    }
}

[Licenser]

Ah I see, one way to deal with that is making a macro for it something like (the syntax might be slightly off):

macro_rules! string_type {
    ( $x:ident ) => {
      impl Serialize for $x {
          fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
              where
                  W: std::io::Write,
          {
              self.0.json_write(writer)
          }
      }
    };
}
string_type!(CustomType1);
string_type!(CustomType2);

[v1gnesh]

If I implement a wrapper struct like this:

pub struct CustomStuff {
    #[br(map=funk1)
    tme: Custom1,
    #[br(map=funk2)
    dte: Custom2,
}

and then in Header can I refer to them as

    tme: CustomStuff.Custom1,
    dte: CustomStuff.Custom2,

so I can

impl Serialize for CustomStuff {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
        where
            W: std::io::Write,
    {
        self.tme.0.json_write(writer)
    }
}

... note that it has to have tme.0, dte.0 etc

[Licenser]

that would be something that needs flattening and be implemented in the upper level, so you'd either need to hand roll the entire serializer to not use the wrapper

[v1gnesh]

Can't say I understand fully but I'm glad the macro you've shared works.
Thanks again for your excellent, excellent work with Rust in general (incl. tremor).

[Licenser]

Thank you :)! I'll rename the issue if you don't mind to better capture that support or const generics would be nice.

[v1gnesh]

Const generics are not implemented in the derive but the EString is fairly simple you can implement Serialize like this:

impl<const N: usize> Serialize for EString<N> {
    fn json_write<W>(&self, writer: &mut W) -> simd_json_derive::Result
    where
        W: std::io::Write,
    {
        self.0.json_write(writer)
    }
}

Adding it to the derive, while not impossible I think, might be a bit tricky and I don't have tons of time at the moment :(

@Licenser Similar to the Serialize impl in this comment, I attempted to impl for a unit struct with a field that uses const generic like so:

pub struct OString<const L: usize>(
    #[br(map = fenk)]
    String<L>
);

... where String type is from the heapless crate.

And when I try to impl, it says Only traits defined in the current crate can be implemented for arbitrary types [E0117].

So... I think this comment is still relevant to this new issue name (const generic support).
This one, I can't work around though :(

PS: Another item in the wishlist - Serialize impl for types from the speedate crate.
Can I open another issue for this one, if you will get time to spare, at least sometime this year? :)