eagle/src/lib.rs

/*
Eagle - A library for creating protocols for full-stack Rust applications
Copyright (c) 2024 KodiCraft

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, spanned::Spanned, DeriveInput, Field, Ident};

#[proc_macro_derive(Protocol)]
pub fn derive_protocol(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    // Must be on an enum
    let enum_ = match &input.data {
        syn::Data::Enum(e) => e,
        _ => {
            return syn::Error::new(input.span(), "Protocol can only be derived on enums")
                .to_compile_error()
                .into()
        }
    };
    let name = &input.ident;
    let vis = &input.vis;

    let mut server_trait = Vec::new();
    let mut client_impl = Vec::new();

    for variant in &enum_.variants {
        // Every variant must have 2 fields
        // The first field is the question (serverbound), the second field is the answer (clientbound)
        if variant.fields.len() != 2 {
            return syn::Error::new(
                variant.span(),
                "Every variant on a protocol must have exactly 2 fields",
            )
            .to_compile_error()
            .into();
        }

        let var_name = ident_to_snake_case(&variant.ident);
        let mut variant_fields = variant.fields.iter();
        let question_field = variant_fields.next().unwrap();
        let question_args = field_to_args(question_field);
        let answer_type = variant_fields.next().unwrap().ty.clone();

        server_trait.push(quote! {
            fn #var_name(&mut self, #question_args) -> #answer_type;
        });
        client_impl.push(quote! {
            pub fn #var_name(&mut self, #question_args) -> #answer_type {
                ::std::unimplemented!()
            }
        })
    }

    // Create a trait which the server will have to implement
    let server_trait_name = Ident::new(&format!("{}Server", name), name.span());
    let server_trait = quote! {
        #vis trait #server_trait_name {
            #(#server_trait)*
        }
    };
    let client_struct_name = Ident::new(&format!("{}Client", name), name.span());
    let client_struct = quote! {
        #vis struct #client_struct_name; // TODO: This struct will have some fields to handle the actual connection
        impl #client_struct_name {
            #(#client_impl)*
        }
    };

    let expanded = quote! {
        #server_trait
        #client_struct
    };
    expanded.into()
}

fn ident_to_snake_case(ident: &Ident) -> Ident {
    let ident = ident.to_string();
    let mut out = String::new();
    for (i, c) in ident.chars().enumerate() {
        if c.is_uppercase() {
            if i != 0 {
                out.push('_');
            }
            out.push(c.to_lowercase().next().unwrap());
        } else {
            out.push(c);
        }
    }
    Ident::new(&out, ident.span())
}

fn field_to_args(field: &Field) -> proc_macro2::TokenStream {
    let type_ = &field.ty;
    if let syn::Type::Tuple(tuple) = type_ {
        let mut args = Vec::new();
        for (i, elem) in tuple.elems.iter().enumerate() {
            let arg = Ident::new(&format!("arg{}", i), elem.span());
            args.push(quote! { #arg: #elem });
        }
        quote! { #( #args ), * }
    } else {
        quote! { arg: #type_ }
    }
}