extern crate proc_macro;

use quote::quote;
use proc_macro::TokenStream;
use syn::*;
use syn::parse::{Parse, ParseStream};

#[derive(Debug)]
struct MetricDefinition {
    vis: Visibility,
    name: Ident,
    values: Vec<Ident>,
}

impl Parse for MetricDefinition {
    fn parse(input: ParseStream) -> Result<Self> {
        let vis = input.parse::<Visibility>()?;
        input.parse::<Token![struct]>()?;
        let name = input.parse::<Ident>()?;
        let content; braced!(content in input);
        let values = content
            .parse_terminated::<_, Token![,]>(Ident::parse)?
            .into_iter()
            .collect();
        Ok(Self { vis, name, values })
    }
}

#[proc_macro]
pub fn make_metrics(input: TokenStream) -> TokenStream {
    let data = parse_macro_input!(input as MetricDefinition);
    let vis = &data.vis;
    let name = &data.name;
    let values = &data.values;
    let values_str = data.values.iter().map(|ident| LitStr::new(
        &ident.to_string(),
        ident.span(),
    ));
    let expanded = quote! {
        #vis struct #name {
            #(#values: Counter,)*
        }
        impl #name {
            pub fn new() -> Self {
                Self {
                    #(#values: COUNTER.with_label_values(&[#values_str]),)*
                }
            }
        }
    };
    TokenStream::from(expanded)
}