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use proc_macro2::TokenStream;
use quote::{format_ident, quote, ToTokens};
use syn::{parse::Parser, punctuated::Punctuated, Expr, Index, Token};
/// The `stream_select!` macro.
pub(crate) fn stream_select(input: TokenStream) -> Result<TokenStream, syn::Error> {
let args = Punctuated::<Expr, Token![,]>::parse_terminated.parse2(input)?;
if args.len() < 2 {
return Ok(quote! {
compile_error!("stream select macro needs at least two arguments.")
});
}
let generic_idents = (0..args.len()).map(|i| format_ident!("_{}", i)).collect::<Vec<_>>();
let field_idents = (0..args.len()).map(|i| format_ident!("__{}", i)).collect::<Vec<_>>();
let field_idents_2 = (0..args.len()).map(|i| format_ident!("___{}", i)).collect::<Vec<_>>();
let field_indices = (0..args.len()).map(Index::from).collect::<Vec<_>>();
let args = args.iter().map(|e| e.to_token_stream());
Ok(quote! {
{
#[derive(Debug)]
struct StreamSelect<#(#generic_idents),*> (#(Option<#generic_idents>),*);
enum StreamEnum<#(#generic_idents),*> {
#(
#generic_idents(#generic_idents)
),*,
None,
}
impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamEnum<#(#generic_idents),*>
where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
{
type Item = ITEM;
fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
match self.get_mut() {
#(
Self::#generic_idents(#generic_idents) => ::std::pin::Pin::new(#generic_idents).poll_next(cx)
),*,
Self::None => panic!("StreamEnum::None should never be polled!"),
}
}
}
impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamSelect<#(#generic_idents),*>
where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
{
type Item = ITEM;
fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
let Self(#(ref mut #field_idents),*) = self.get_mut();
#(
let mut #field_idents_2 = false;
)*
let mut any_pending = false;
{
let mut stream_array = [#(#field_idents.as_mut().map(|f| StreamEnum::#generic_idents(f)).unwrap_or(StreamEnum::None)),*];
__futures_crate::async_await::shuffle(&mut stream_array);
for mut s in stream_array {
if let StreamEnum::None = s {
continue;
} else {
match __futures_crate::stream::Stream::poll_next(::std::pin::Pin::new(&mut s), cx) {
r @ __futures_crate::task::Poll::Ready(Some(_)) => {
return r;
},
__futures_crate::task::Poll::Pending => {
any_pending = true;
},
__futures_crate::task::Poll::Ready(None) => {
match s {
#(
StreamEnum::#generic_idents(_) => { #field_idents_2 = true; }
),*,
StreamEnum::None => panic!("StreamEnum::None should never be polled!"),
}
},
}
}
}
}
#(
if #field_idents_2 {
*#field_idents = None;
}
)*
if any_pending {
__futures_crate::task::Poll::Pending
} else {
__futures_crate::task::Poll::Ready(None)
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
let mut s = (0, Some(0));
#(
if let Some(new_hint) = self.#field_indices.as_ref().map(|s| s.size_hint()) {
s.0 += new_hint.0;
// We can change this out for `.zip` when the MSRV is 1.46.0 or higher.
s.1 = s.1.and_then(|a| new_hint.1.map(|b| a + b));
}
)*
s
}
}
StreamSelect(#(Some(#args)),*)
}
})
}