extern crate proc_macro;

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};

#[proc_macro_derive(BinarySerializable)]
pub fn derive_binary_serializable(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    let name = input.ident;

    let fields = match input.data {
        syn::Data::Struct(s) => s.fields,
        _ => panic!("#[derive(BinarySerializable)] only works on structs"),
    };

    let field_writes = fields.iter().map(|f| {
        let ident = f.ident.as_ref().unwrap();
        quote! {
            BinarySerializable::write_to(&self.#ident, writer)?;
        }
    });

    let expanded = quote! {
        impl BinarySerializable for #name {
            fn write_to<W: std::io::Write>(&self, writer: &mut W) -> std::io::Result<()> {
                #(#field_writes)*
                return Ok(());
            }
        }
    };

    TokenStream::from(expanded)
}