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use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, Data, DeriveInput};
#[proc_macro_derive(PhysicsLayer)]
pub fn derive_layer(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
let enum_ident = input.ident;
let variants = match &input.data {
Data::Enum(data) => &data.variants,
_ => panic!("Only enums can automatically derive Layer"),
};
assert!(variants.len() <= 32, "Reached the maximum of 32 layers");
let to_bits = variants.iter().enumerate().map(|(index, variant)| {
let bits: u32 = 1 << index;
assert!(
variant.fields.is_empty(),
"Can only derive Layer for enums without fields"
);
let ident = &variant.ident;
quote! { #enum_ident::#ident => #bits, }
});
let all_bits: u32 = if variants.len() == 32 {
0xffffffff
} else {
(1 << variants.len()) - 1
};
let expanded = quote! {
impl heron::PhysicsLayer for #enum_ident {
fn all_bits() -> u32 {
#all_bits
}
fn to_bits(&self) -> u32 {
match self {
#(#to_bits)*
}
}
}
};
TokenStream::from(expanded)
}