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extern crate proc_macro;
#[macro_use]
extern crate quote;
extern crate syn;

use proc_macro::TokenStream;
use syn::{Body, DeriveInput, Ident, VariantData};

#[proc_macro_derive(EnumIter)]
pub fn enum_iter(input: TokenStream) -> TokenStream {
    let source = input.to_string();

    let ast = syn::parse_derive_input(&source).unwrap();

    let expanded = enum_iter_impl(&ast);

    expanded.parse().unwrap()
}

fn enum_iter_impl(ast: &DeriveInput) -> quote::Tokens {
    let name = &ast.ident;

    let variants = match ast.body {
        Body::Enum(ref variants) => variants,
        _ => panic!(
            "`EnumIter` can be applied only to the enums, {} is not an enum",
            &name
        ),
    };

    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
    let iter_impl = Ident::from(format!("_EnumIterator{}", name));
    let vis = &ast.vis;
    let match_variants = variants.iter().enumerate().map(|(idx, variant)| {
        let id = &variant.ident;
        match variant.data {
            VariantData::Unit => quote!{
                #idx => #name::#id,
            },
            VariantData::Tuple(ref fields) => {
                let types = fields.iter().map(|f| &f.ty);
                quote! {
                    #idx => #name::#id( #(#types::default(),)* ),
                }
            }
            VariantData::Struct(ref fields) => {
                let items = fields.iter().map(|f| {
                    let ident = &f.ident;
                    let ty = &f.ty;

                    quote! {
                        #ident: #ty::default()
                    }
                });

                quote! {
                    #idx => #name::#id{ #(#items,)* },
                }
            }
        }
    });

    quote!{
        #[derive(Default)]
        #vis struct #iter_impl {
            count: usize,
        }

        impl ::std::iter::Iterator for #iter_impl {
            type Item = #name;

            fn next(&mut self) -> Option<Self::Item> {
                let item = match self.count {
                    #(#match_variants)*
                    _ => return None,
                };
                self.count += 1;
                Some(item)
            }
        }

        impl #impl_generics #name #ty_generics #where_clause {
            pub fn iter() -> #iter_impl {
                #iter_impl::default()
            }
        }
    }
}