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use quote::quote;
use syn::{DeriveInput, Data, Fields};
use quote::format_ident;

#[proc_macro_derive(EnumConstValue)]
pub fn enum_const_value(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let derive_input: DeriveInput = syn::parse(input).unwrap();
    let original_enum_type_name = &derive_input.ident;
    let const_enum_type_ident = format_ident!("{}ConstValue", &original_enum_type_name);
    let mut generate_const_enum = false;
    let (idents_and_matcher, original_matcher): (Vec<_>, Vec<_>) = match derive_input.data {
        Data::Enum(e) => {
            e
                .variants
                .into_iter()
                .enumerate()
                .map( |(index, variant) | {
                let index = index as i32;
                let variant_name = variant.ident;
                let tokens = match variant.fields {
                    Fields::Unit => quote! {},
                    Fields::Unnamed(..) => {
                        generate_const_enum = true;
                        quote! {
                            (..)
                        }
                    }
                    Fields::Named(..) => {
                        generate_const_enum = true;
                        quote! {
                            {..}
                        }
                    }
                };

                let original_enum_matcher = quote! {
                        &#original_enum_type_name::#variant_name#tokens => #index
                    };
                let const_enum_matcher = quote! {
                        &#const_enum_type_ident::#variant_name => #index
                    };

                ((variant_name, const_enum_matcher), original_enum_matcher)
            })
            .unzip()
        }
        _ => panic!("Only enums are supported")
    };
    let (variant_names, const_matcher): (Vec<_>, Vec<_>) = idents_and_matcher.into_iter().unzip();
    let mut tokens = quote! {
        impl #original_enum_type_name {
            pub fn const_value(&self) -> i32 {
                match self {
                    #(#original_matcher),*
                }
            }
        }
    };

    if generate_const_enum {
        tokens.extend(quote! {
            pub enum #const_enum_type_ident {
                #(#variant_names),*
            }

            impl #const_enum_type_ident {
                pub fn value_for_variant(variant: &#const_enum_type_ident) -> i32 {
                    match variant {
                        #(#const_matcher),*
                    }
                }

                pub fn const_value(&self) -> i32 {
                    #const_enum_type_ident::value_for_variant(self)
                }
            }
        });
    }

    tokens.into()
}