use darling::FromMeta;
use proc_macro2::TokenStream;
use quote::{format_ident, quote};

use crate::util::ctxt::Ctxt;
use crate::util::path::ModulePrefix;

#[derive(Debug, FromMeta)]
struct Args {
    #[darling(default)]
    pool_size: Option<usize>,
    #[darling(default)]
    embassy_prefix: ModulePrefix,
}

pub fn run(args: syn::AttributeArgs, f: syn::ItemFn) -> Result<TokenStream, TokenStream> {
    let args = Args::from_list(&args).map_err(|e| e.write_errors())?;

    let embassy_prefix = args.embassy_prefix.append("embassy");
    let embassy_path = embassy_prefix.path();

    let pool_size: usize = args.pool_size.unwrap_or(1);

    let ctxt = Ctxt::new();

    if f.sig.asyncness.is_none() {
        ctxt.error_spanned_by(&f.sig, "task functions must be async");
    }
    if !f.sig.generics.params.is_empty() {
        ctxt.error_spanned_by(&f.sig, "task functions must not be generic");
    }
    if pool_size < 1 {
        ctxt.error_spanned_by(&f.sig, "pool_size must be 1 or greater");
    }

    let mut arg_names = Vec::new();
    let mut fargs = f.sig.inputs.clone();

    for arg in fargs.iter_mut() {
        match arg {
            syn::FnArg::Receiver(_) => {
                ctxt.error_spanned_by(arg, "task functions must not have receiver arguments");
            }
            syn::FnArg::Typed(t) => match t.pat.as_mut() {
                syn::Pat::Ident(id) => {
                    arg_names.push(id.ident.clone());
                    id.mutability = None;
                }
                _ => {
                    ctxt.error_spanned_by(
                        arg,
                        "pattern matching in task arguments is not yet supported",
                    );
                }
            },
        }
    }

    ctxt.check()?;

    let task_ident = f.sig.ident.clone();
    let task_inner_ident = format_ident!("__{}_task", task_ident);

    let mut task_inner = f;
    let visibility = task_inner.vis.clone();
    task_inner.vis = syn::Visibility::Inherited;
    task_inner.sig.ident = task_inner_ident.clone();

    let result = quote! {
        // This is the user's task function, renamed.
        // We put it outside the #task_ident fn below, because otherwise
        // the items defined there (such as POOL) would be in scope
        // in the user's code.
        #task_inner

        #visibility fn #task_ident(#fargs) -> #embassy_path::executor::SpawnToken<impl ::core::future::Future + 'static> {
            use ::core::future::Future;
            use #embassy_path::executor::SpawnToken;
            use #embassy_path::executor::raw::TaskPool;

            type Fut = impl Future + 'static;

            static POOL: TaskPool<Fut, #pool_size> = TaskPool::new();

            // Opaque type laundering, to obscure its origin!
            // Workaround for "opaque type's hidden type cannot be another opaque type from the same scope"
            // https://github.com/rust-lang/rust/issues/96406
            fn launder_tait(token: SpawnToken<impl Future+'static>) -> SpawnToken<impl Future+'static> {
                token
            }

            launder_tait(POOL.spawn(move || #task_inner_ident(#(#arg_names,)*)))
        }
    };

    Ok(result)
}