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use proc_macro2::{Ident, TokenStream};
use quote::quote;
use syn::{parse::Parse, Expr, ItemFn, Token};

pub fn parse(item: ItemFn, attr: TokenStream) -> syn::Result<TokenStream> {
    // grab threads if specified
    let MetricList { ptr, fn_name, data, scope } = syn::parse2(attr)?;

    let attrs = item.attrs;
    let vis = item.vis;
    let sig = item.sig;
    let block = item.block;

    if scope {
        Ok(quote!(
            #(#attrs)*
            #vis
            #sig
            {
                if let Some(metrics) = self.#ptr.clone() {
                    metrics.#fn_name(#(#data),*, || #block)
                } else {
                    #block
                }
            }
        ))
    } else {
        Ok(quote!(
            #(#attrs)*
            #vis
            #sig
            {
                let result = #block;
                self.#ptr.as_ref().inspect(|m| m.#fn_name(#(#data),*));
                result
            }
        ))
    }
}

pub struct MetricList {
    // ptr to metric in struct
    ptr:     Ident,
    scope:   bool,
    // recorder name
    fn_name: Ident,
    data:    Vec<Expr>,
}

impl Parse for MetricList {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        let ptr: Ident = input.parse()?;
        if ptr != "ptr" {
            return Err(syn::Error::new(ptr.span(), "first field must be ptr=location"))
        }
        input.parse::<Token![=]>()?;
        let ptr_value: Ident = input.parse()?;

        input.parse::<Token![,]>()?;
        let scope: Ident = input.parse()?;

        let (fn_name, scope) = if scope == "scope" {
            input.parse::<Token![,]>()?;
            let fn_name: Ident = input.parse()?;
            (fn_name, true)
        } else {
            (scope, false)
        };

        let mut data = Vec::new();
        // take out all args
        while input.peek(Token![,]) {
            input.parse::<Token![,]>()?;
            data.push(input.parse()?);
        }

        Ok(Self { ptr: ptr_value, fn_name, data, scope })
    }
}