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extern crate proc_macro;
use proc_macro::TokenStream;

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

#[proc_macro_attribute]
pub fn display(args: TokenStream, input: TokenStream) -> TokenStream {
    assert!(args.is_empty());

    let item = parse_macro_input!(input as syn::Item);

    display_impl(item).unwrap_or_else(|e| e.to_compile_error()).into()
}

fn display_impl(item: syn::Item) -> Result<proc_macro2::TokenStream, syn::Error> {
    let f = is_fn(item)?;
    let f = is_main(f)?;
    let f = to_termination_main(f);

    Ok(quote! {
        #f

        #[allow(unreachable_code)]
        fn main() -> ! {
            ::std::process::exit(
                ::termination::Termination::report(
                    termination_main()
                )
            );
        }
    })
}

fn is_fn(item: syn::Item) -> Result<syn::ItemFn, syn::Error> {
    if let syn::Item::Fn(f) = item {
        Ok(f)
    } else {
        Err(syn::Error::new(item.span(), "expected 'fn'"))
    }
}

fn is_main(f: syn::ItemFn) -> Result<syn::ItemFn, syn::Error> {
    if f.sig.ident == "main" {
        Ok(f)
    } else {
        Err(syn::Error::new(f.sig.ident.span(), "expected 'main'"))
    }
}

fn to_termination_main(mut f: syn::ItemFn) -> syn::ItemFn {
    f.sig.ident = syn::Ident::new("termination_main", proc_macro2::Span::call_site());
    f
}