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use proc_macro2::TokenStream;
use quote::{quote, ToTokens};
use syn::{parse_macro_input, DeriveInput, Ident};

#[proc_macro_derive(IsVariant)]
pub fn derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    // Parse the input tokens into a syntax tree
    let input = parse_macro_input!(input as DeriveInput);

    let enum_name = input.ident;
    let vis = input.vis.to_token_stream();

    let expanded: TokenStream = if let syn::Data::Enum(e) = input.data {
        e.variants
            .into_iter()
            .map(|v| {
                let ident = v.ident;
                let name = Ident::new(&format!("Is{ident}"), ident.span());
                let m = match v.fields {
                    syn::Fields::Unit => quote! { #enum_name::#ident },
                    _ => quote! { #enum_name::#ident { .. } },
                };

                quote! {
                    #vis struct #name;

                    impl bevy_mod_check_filter::Predicate<#enum_name> for #name {
                        fn test(test: &#enum_name) -> bool {
                            match *test {
                              #m => true,
                              _ => false,
                            }
                        }
                    }
                }
            })
            .collect()
    } else {
        panic!("IsVariant derive only works on Enums");
    };

    // Hand the output tokens back to the compiler
    proc_macro::TokenStream::from(expanded)
}