1 files changed, 113 insertions, 0 deletions
diff --git a/src/stream_select.rs b/src/stream_select.rs
new file mode 100644
index 0000000..9927b53
--- /dev/null
+++ b/src/stream_select.rs
@@ -0,0 +1,113 @@
+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)),*)
+
+        }
+    })
+}