use quote::{ToTokens, quote};
use syn::spanned::Spanned;

pub struct All {
    name: syn::Ident,
    variants: Box<[syn::Ident]>,
}

impl All {
    pub fn parse(input: syn::DeriveInput) -> Result<Self, syn::Error> {
        let data = match input.data {
            syn::Data::Enum(enu) => enu,
            _ => {
                return Err(syn::Error::new(
                    input.span(),
                    "All can only be used on enums",
                ));
            }
        };
        let variants = data
            .variants
            .into_iter()
            .map(|v| match &v.fields {
                syn::Fields::Named(_) | syn::Fields::Unnamed(_) => Err(syn::Error::new(
                    v.ident.span(),
                    "All can only be used on enums with only unit fields",
                )),
                syn::Fields::Unit => Ok(v.ident),
            })
            .collect::<Result<Box<[_]>, _>>()?;
        let name = input.ident;
        Ok(Self { name, variants })
    }
}

impl ToTokens for All {
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
        let name = &self.name;
        let variants = &self.variants;
        let count = self.variants.len();
        tokens.extend(quote! {
            impl #name {
                pub const ALL: [#name; #count] = [#(#name::#variants),*];
            }
        });
    }
}