datum/stream/

source.rs

use super::flow::{self, FlowTransform};
use super::*;
use crate::Attributes;
use crate::context::SourceWithContext;

#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
pub struct NotUsed;

type CombinedSourceFactory<Out> =
    dyn Fn(&Materializer) -> StreamResult<BoxStream<Out>> + Send + Sync;

pub struct Keep;

impl Keep {
    pub fn left<Left, Right>(left: Left, _right: Right) -> Left {
        left
    }

    pub fn right<Left, Right>(_left: Left, right: Right) -> Right {
        right
    }

    pub fn both<Left, Right>(left: Left, right: Right) -> (Left, Right) {
        (left, right)
    }

    pub fn none<Left, Right>(_left: Left, _right: Right) -> NotUsed {
        NotUsed
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SourceCombineStrategy {
    Merge {
        eager_complete: bool,
    },
    Concat,
    Prioritized {
        priorities: Vec<usize>,
        eager_complete: bool,
    },
}

#[derive(Clone)]
pub struct MaybeHandle<T> {
    value: Arc<Mutex<Option<StreamResult<T>>>>,
}

impl<T> MaybeHandle<T> {
    #[must_use]
    pub fn is_completed(&self) -> bool {
        self.value.lock().expect("maybe handle poisoned").is_some()
    }

    pub fn complete(&self, item: T) -> StreamResult<()> {
        self.settle(Ok(item))
    }

    pub fn fail(&self, error: StreamError) -> StreamResult<()> {
        self.settle(Err(error))
    }

    fn settle(&self, result: StreamResult<T>) -> StreamResult<()> {
        let mut value = self.value.lock().expect("maybe handle poisoned");
        if value.is_some() {
            return Err(StreamError::Failed("maybe source already completed".into()));
        }
        *value = Some(result);
        Ok(())
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Demand(u64);

impl Demand {
    pub const ZERO: Self = Self(0);
    pub const ONE: Self = Self(1);
    pub const MAX: Self = Self(u64::MAX);

    #[must_use]
    pub const fn new(available: u64) -> Self {
        Self(available)
    }

    #[must_use]
    pub const fn available(self) -> u64 {
        self.0
    }

    #[must_use]
    pub const fn is_unbounded(self) -> bool {
        self.0 == u64::MAX
    }

    #[must_use]
    pub const fn is_empty(self) -> bool {
        self.0 == 0
    }

    #[must_use]
    pub const fn saturating_add(self, rhs: Self) -> Self {
        Self(self.0.saturating_add(rhs.0))
    }

    pub fn consume_one(&mut self) -> bool {
        match self.0 {
            0 => false,
            u64::MAX => true,
            _ => {
                self.0 -= 1;
                true
            }
        }
    }
}

pub trait PushOutlet<T>: Send {
    fn push(&mut self, item: T) -> StreamResult<Demand>;

    fn complete(&mut self) -> StreamResult<()> {
        Ok(())
    }

    fn fail(&mut self, cause: StreamError) -> StreamResult<()> {
        Err(cause)
    }
}

#[derive(Clone)]
pub struct Source<Out, Mat = NotUsed> {
    pub(crate) factory: Arc<dyn SourceFactory<Out, Mat>>,
    pub(super) hints: SourceHints,
    pub(super) attributes: Attributes,
    /// Set only for split-segment sources in fast mode. `None` for all other sources.
    /// Cleared by all composition operators (via/map/to_mat etc.).
    pub(crate) split_hook: Option<Arc<dyn SplitSegmentHookDyn>>,
}

impl<Out: Send + 'static> Source<Out, NotUsed> {
    pub(super) fn from_factory<F>(factory: F) -> Self
    where
        F: Fn() -> BoxStream<Out> + Send + Sync + 'static,
    {
        Self::from_factory_with_hints(factory, SourceHints::default())
    }

    fn from_factory_with_hints<F>(factory: F, hints: SourceHints) -> Self
    where
        F: Fn() -> BoxStream<Out> + Send + Sync + 'static,
    {
        Self::from_materialized_factory_with_hints(
            move |_materializer| Ok((factory(), NotUsed)),
            hints,
        )
    }

    #[must_use]
    pub fn empty() -> Self {
        Self::from_factory_with_hints(
            || Box::new(std::iter::empty()),
            SourceHints::with_inline_micro(0),
        )
    }

    #[must_use]
    pub fn never() -> Self {
        Self::from_materialized_factory_with_hints(
            move |materializer| {
                let state = Arc::clone(&materializer.inner.state);
                Ok((
                    Box::new(std::iter::from_fn(move || {
                        loop {
                            if state.shutdown.load(Ordering::SeqCst) {
                                return Some(Err(StreamError::AbruptTermination));
                            }
                            if super::runtime::current_stream_cancelled()
                                .as_ref()
                                .is_some_and(|cancelled| cancelled.load(Ordering::SeqCst))
                            {
                                return Some(Err(StreamError::Cancelled));
                            }
                            std::thread::park_timeout(Duration::from_millis(1));
                        }
                    })),
                    NotUsed,
                ))
            },
            SourceHints::default(),
        )
    }

    #[must_use]
    pub fn failed(error: StreamError) -> Self {
        Self::from_factory_with_hints(
            move || Box::new(std::iter::once(Err(error.clone()))),
            // max_success_items=0: failed() emits no Ok items before the Err.
            SourceHints::with_inline_micro(0),
        )
    }

    #[must_use]
    pub fn future<F, Fut>(future: F) -> Self
    where
        F: Fn() -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Out>> + Send + 'static,
    {
        let future = Arc::new(future);
        Self::from_factory(move || {
            let future = Arc::clone(&future);
            let mut emitted = false;
            Box::new(std::iter::from_fn(move || {
                if emitted {
                    return None;
                }
                emitted = true;
                Some(
                    catch_unwind_failed("source future factory", || future())
                        .and_then(flow::run_future_inline_or_spawn),
                )
            }))
        })
    }

    #[must_use]
    pub fn future_source<F, Fut>(future: F) -> Self
    where
        F: Fn() -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Source<Out>>> + Send + 'static,
    {
        let future = Arc::new(future);
        Self::from_materialized_factory(move |materializer| {
            let materializer = materializer.with_name_prefix(materializer.name_prefix().to_owned());
            let future = Arc::clone(&future);
            let mut current = None::<BoxStream<Out>>;
            let mut initialized = false;
            let mut terminated = false;
            Ok((
                Box::new(std::iter::from_fn(move || {
                    if terminated {
                        return None;
                    }

                    loop {
                        if let Some(stream) = current.as_mut() {
                            match stream.next() {
                                Some(item) => return Some(item),
                                None => {
                                    terminated = true;
                                    return None;
                                }
                            }
                        }

                        if initialized {
                            terminated = true;
                            return None;
                        }
                        initialized = true;
                        let source = match catch_unwind_failed("future_source factory", || future())
                            .and_then(flow::run_future_inline_or_spawn)
                        {
                            Ok(source) => source,
                            Err(error) => {
                                terminated = true;
                                return Some(Err(error));
                            }
                        };
                        current = Some(match Arc::clone(&source.factory).create(&materializer) {
                            Ok((stream, _)) => stream,
                            Err(error) => {
                                terminated = true;
                                return Some(Err(error));
                            }
                        });
                    }
                })) as BoxStream<Out>,
                NotUsed,
            ))
        })
    }

    #[must_use]
    pub fn cycle<F, I>(factory: F) -> Self
    where
        F: Fn() -> I + Send + Sync + 'static,
        I: IntoIterator<Item = Out>,
        I::IntoIter: Send + 'static,
    {
        let factory = Arc::new(factory);
        Self::from_factory(move || {
            let factory = Arc::clone(&factory);
            let mut current = None::<I::IntoIter>;
            let mut terminated = false;
            Box::new(std::iter::from_fn(move || {
                if terminated {
                    return None;
                }

                if let Some(iter) = current.as_mut()
                    && let Some(item) = iter.next()
                {
                    return Some(Ok(item));
                }

                let mut next = match catch_unwind_failed("cycle factory", || factory()) {
                    Ok(iterable) => iterable.into_iter(),
                    Err(error) => {
                        terminated = true;
                        return Some(Err(error));
                    }
                };
                match next.next() {
                    Some(item) => {
                        current = Some(next);
                        Some(Ok(item))
                    }
                    None => {
                        terminated = true;
                        Some(Err(StreamError::Failed("empty iterator".into())))
                    }
                }
            }))
        })
    }

    #[must_use]
    pub fn unfold<State, F>(initial: State, f: F) -> Self
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State) -> Option<(State, Out)> + Send + Sync + 'static,
    {
        let f = Arc::new(f);
        Self::from_factory(move || {
            let f = Arc::clone(&f);
            let mut state = Some(initial.clone());
            let mut terminated = false;
            Box::new(std::iter::from_fn(move || {
                if terminated {
                    return None;
                }
                let current = state.take().expect("unfold state present");
                match catch_unwind_failed("unfold function", || f(current)) {
                    Ok(Some((next_state, item))) => {
                        state = Some(next_state);
                        Some(Ok(item))
                    }
                    Ok(None) => {
                        terminated = true;
                        None
                    }
                    Err(error) => {
                        terminated = true;
                        Some(Err(error))
                    }
                }
            }))
        })
    }

    #[must_use]
    pub fn unfold_async<State, F, Fut>(initial: State, f: F) -> Self
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Option<(State, Out)>>> + Send + 'static,
    {
        let f = Arc::new(f);
        Self::from_factory(move || {
            let f = Arc::clone(&f);
            let mut state = Some(initial.clone());
            let mut terminated = false;
            Box::new(std::iter::from_fn(move || {
                if terminated {
                    return None;
                }
                let current = state.take().expect("unfold_async state present");
                match catch_unwind_failed("unfold_async factory", || f(current))
                    .and_then(flow::run_future_inline_or_spawn)
                {
                    Ok(Some((next_state, item))) => {
                        state = Some(next_state);
                        Some(Ok(item))
                    }
                    Ok(None) => {
                        terminated = true;
                        None
                    }
                    Err(error) => {
                        terminated = true;
                        Some(Err(error))
                    }
                }
            }))
        })
    }

    #[must_use]
    pub fn unfold_resource<Resource, Create, Read, Close>(
        create: Create,
        read: Read,
        close: Close,
    ) -> Self
    where
        Resource: Send + 'static,
        Create: Fn() -> StreamResult<Resource> + Send + Sync + 'static,
        Read: Fn(&mut Resource) -> StreamResult<Option<Out>> + Send + Sync + 'static,
        Close: Fn(Resource) -> StreamResult<()> + Send + Sync + 'static,
    {
        let create = Arc::new(create);
        let read = Arc::new(read);
        let close = Arc::new(close);
        Self::from_factory(move || {
            Box::new(UnfoldResourceStream {
                create: Arc::clone(&create),
                read: Arc::clone(&read),
                close: Arc::clone(&close),
                resource: None,
                created: false,
                terminated: false,
                _marker: PhantomData,
            })
        })
    }

    #[must_use]
    pub fn unfold_resource_async<Resource, Create, CreateFut, Read, ReadFut, Close, CloseFut>(
        create: Create,
        read: Read,
        close: Close,
    ) -> Self
    where
        Resource: Send + 'static,
        Create: Fn() -> CreateFut + Send + Sync + 'static,
        CreateFut: Future<Output = StreamResult<Resource>> + Send + 'static,
        Read: Fn(&mut Resource) -> ReadFut + Send + Sync + 'static,
        ReadFut: Future<Output = StreamResult<Option<Out>>> + Send + 'static,
        Close: Fn(Resource) -> CloseFut + Send + Sync + 'static,
        CloseFut: Future<Output = StreamResult<()>> + Send + 'static,
    {
        let create = Arc::new(create);
        let read = Arc::new(read);
        let close = Arc::new(close);
        Self::from_factory(move || {
            Box::new(UnfoldResourceAsyncStream {
                create: Arc::clone(&create),
                read: Arc::clone(&read),
                close: Arc::clone(&close),
                resource: None,
                created: false,
                terminated: false,
                _marker: PhantomData,
            })
        })
    }

    #[must_use]
    pub fn lazy_single<F>(create: F) -> Self
    where
        F: Fn() -> Out + Send + Sync + 'static,
    {
        let create = Arc::new(create);
        Self::from_factory(move || {
            let create = Arc::clone(&create);
            let mut emitted = false;
            Box::new(std::iter::from_fn(move || {
                if emitted {
                    return None;
                }
                emitted = true;
                Some(catch_unwind_failed("lazy_single factory", || create()))
            }))
        })
    }

    #[must_use]
    pub fn lazy_future<F, Fut>(create: F) -> Self
    where
        F: Fn() -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Out>> + Send + 'static,
    {
        let create = Arc::new(create);
        Self::from_factory(move || {
            let create = Arc::clone(&create);
            let mut emitted = false;
            Box::new(std::iter::from_fn(move || {
                if emitted {
                    return None;
                }
                emitted = true;
                Some(
                    catch_unwind_failed("lazy_future factory", || create())
                        .and_then(flow::run_future_inline_or_spawn),
                )
            }))
        })
    }

    #[must_use]
    pub fn lazy_source<InnerMat, F>(create: F) -> Source<Out, StreamCompletion<InnerMat>>
    where
        InnerMat: Send + 'static,
        F: Fn() -> Source<Out, InnerMat> + Send + Sync + 'static,
    {
        let create = Arc::new(create);
        Source::from_materialized_factory(move |materializer| {
            let (sender, receiver) = oneshot::channel();
            Ok((
                Box::new(LazySourceStream {
                    create: Arc::clone(&create),
                    materializer: materializer
                        .with_name_prefix(materializer.name_prefix().to_owned()),
                    current: None,
                    mat_sender: Some(sender),
                    initialized: false,
                    terminated: false,
                }) as BoxStream<Out>,
                StreamCompletion::from_receiver(receiver, None),
            ))
        })
    }

    #[must_use]
    pub fn lazy_future_source<InnerMat, F, Fut>(
        create: F,
    ) -> Source<Out, StreamCompletion<InnerMat>>
    where
        InnerMat: Send + 'static,
        F: Fn() -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Source<Out, InnerMat>>> + Send + 'static,
    {
        let create = Arc::new(create);
        Source::from_materialized_factory(move |materializer| {
            let (sender, receiver) = oneshot::channel();
            Ok((
                Box::new(LazyFutureSourceStream {
                    create: Arc::clone(&create),
                    materializer: materializer
                        .with_name_prefix(materializer.name_prefix().to_owned()),
                    current: None,
                    mat_sender: Some(sender),
                    initialized: false,
                    terminated: false,
                    _marker: PhantomData,
                }) as BoxStream<Out>,
                StreamCompletion::from_receiver(receiver, None),
            ))
        })
    }

    #[must_use]
    pub fn from_fn_iter<F, I>(factory: F) -> Self
    where
        F: Fn() -> I + Send + Sync + 'static,
        I: IntoIterator<Item = Out>,
        I::IntoIter: Send + 'static,
    {
        Self::from_factory(move || Box::new(factory().into_iter().map(Ok)))
    }
}

impl<Out: Send + 'static, Mat: Send + 'static> Source<Out, Mat> {
    fn from_materialized_factory_with_hints<F>(factory: F, hints: SourceHints) -> Self
    where
        F: Fn(&Materializer) -> StreamResult<(BoxStream<Out>, Mat)> + Send + Sync + 'static,
    {
        Self {
            factory: Arc::new(FnSourceFactory(factory)),
            hints,
            attributes: Attributes::default(),
            split_hook: None,
        }
    }

    pub(crate) fn from_materialized_factory<F>(factory: F) -> Self
    where
        F: Fn(&Materializer) -> StreamResult<(BoxStream<Out>, Mat)> + Send + Sync + 'static,
    {
        Self::from_materialized_factory_with_hints(factory, SourceHints::default())
    }

    #[must_use]
    pub fn as_source_with_context<Ctx, F>(
        self,
        extract_context: F,
    ) -> SourceWithContext<Out, Ctx, Mat>
    where
        Ctx: Send + 'static,
        F: Fn(&Out) -> Ctx + Send + Sync + 'static,
    {
        SourceWithContext::from_source(self.map(move |item| {
            let context = extract_context(&item);
            (item, context)
        }))
    }

    #[must_use]
    pub fn via<Next, FlowMat>(self, flow: Flow<Out, Next, FlowMat>) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        FlowMat: Send + 'static,
    {
        self.via_mat(flow, Keep::left)
    }

    #[must_use]
    pub fn via_mat<Next, FlowMat, Combined, F>(
        self,
        flow: Flow<Out, Next, FlowMat>,
        combine: F,
    ) -> Source<Next, Combined>
    where
        Next: Send + 'static,
        FlowMat: Send + 'static,
        Combined: Send + 'static,
        F: Fn(Mat, FlowMat) -> Combined + Send + Sync + 'static,
    {
        let source = self.factory;
        let transform = flow.transform;
        let materialize_flow = flow.materialize;
        let hints = self.hints.after_flow(flow.hints);
        let combine = Arc::new(combine);
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (stream, source_mat) = Arc::clone(&source).create(materializer)?;
                let flow_mat = materialize_flow()?;
                let stream = match &transform {
                    FlowTransform::Pure(transform) => transform(stream),
                    FlowTransform::Runtime(transform) => transform(stream, materializer)?,
                };
                Ok((stream, combine(source_mat, flow_mat)))
            },
            hints,
        )
    }

    #[must_use]
    pub fn via_mat_with<Next, FlowMat, Combined, F>(
        self,
        flow: Flow<Out, Next, FlowMat>,
        combine: F,
    ) -> Source<Next, Combined>
    where
        Next: Send + 'static,
        FlowMat: Send + 'static,
        Combined: Send + 'static,
        F: Fn(Mat, FlowMat) -> Combined + Send + Sync + 'static,
    {
        self.via_mat(flow, combine)
    }

    #[must_use]
    pub fn map<Next, F>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Next + Send + Sync + 'static,
    {
        Source {
            factory: Arc::new(MapSourceFactory {
                source: self.factory,
                stage: f,
                _marker: PhantomData,
            }),
            hints: self.hints,
            attributes: self.attributes,
            split_hook: None,
        }
    }

    #[must_use]
    pub fn attributes(&self) -> &Attributes {
        &self.attributes
    }

    #[must_use]
    pub fn with_attributes(mut self, attributes: Attributes) -> Self {
        self.attributes = attributes;
        self
    }

    #[must_use]
    pub fn add_attributes(mut self, attributes: Attributes) -> Self {
        self.attributes = self.attributes.and(attributes);
        self
    }

    #[must_use]
    pub fn named(self, name: impl Into<String>) -> Self {
        self.add_attributes(Attributes::named(name))
    }

    #[must_use]
    pub fn map_result<Next, F>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<Next> + Send + Sync + 'static,
    {
        self.via(Flow::identity().map_result(f))
    }

    #[must_use]
    pub fn map_result_with_supervision<Next, F>(
        self,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<Next> + Send + Sync + 'static,
    {
        self.via(Flow::identity().map_result_with_supervision(f, decider))
    }

    #[must_use]
    pub fn filter<F>(self, predicate: F) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter(predicate))
    }

    #[must_use]
    pub fn filter_result<F>(self, predicate: F) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> StreamResult<bool> + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_result(predicate))
    }

    #[must_use]
    pub fn filter_result_with_supervision<F>(
        self,
        predicate: F,
        decider: SupervisionDecider,
    ) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> StreamResult<bool> + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_result_with_supervision(predicate, decider))
    }

    #[must_use]
    pub fn filter_not<F>(self, predicate: F) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_not(predicate))
    }

    #[must_use]
    pub fn filter_map<Next, F>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Option<Next> + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_map(f))
    }

    #[must_use]
    pub fn filter_map_result<Next, F>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<Option<Next>> + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_map_result(f))
    }

    #[must_use]
    pub fn filter_map_result_with_supervision<Next, F>(
        self,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<Option<Next>> + Send + Sync + 'static,
    {
        self.via(Flow::identity().filter_map_result_with_supervision(f, decider))
    }

    #[must_use]
    pub fn map_concat<Next, F, I>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> I + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().map_concat(f))
    }

    #[must_use]
    pub fn map_concat_result<Next, F, I>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<I> + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().map_concat_result(f))
    }

    #[must_use]
    pub fn map_concat_result_with_supervision<Next, F, I>(
        self,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> StreamResult<I> + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().map_concat_result_with_supervision(f, decider))
    }

    #[must_use]
    pub fn stateful_map<State, Next, F>(self, seed: State, f: F) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> Next + Send + Sync + 'static,
    {
        self.via(Flow::identity().stateful_map(seed, f))
    }

    #[must_use]
    pub fn stateful_map_result<State, Next, F>(self, seed: State, f: F) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> StreamResult<Next> + Send + Sync + 'static,
    {
        self.via(Flow::identity().stateful_map_result(seed, f))
    }

    #[must_use]
    pub fn stateful_map_result_with_supervision<State, Next, F>(
        self,
        seed: State,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> StreamResult<Next> + Send + Sync + 'static,
    {
        self.via(Flow::identity().stateful_map_result_with_supervision(seed, f, decider))
    }

    #[must_use]
    pub fn stateful_map_concat<State, Next, F, I>(self, seed: State, f: F) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> I + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().stateful_map_concat(seed, f))
    }

    #[must_use]
    pub fn stateful_map_concat_result<State, Next, F, I>(
        self,
        seed: State,
        f: F,
    ) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> StreamResult<I> + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().stateful_map_concat_result(seed, f))
    }

    #[must_use]
    pub fn stateful_map_concat_result_with_supervision<State, Next, F, I>(
        self,
        seed: State,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        State: Clone + Send + Sync + 'static,
        Next: Send + 'static,
        F: Fn(&mut State, Out) -> StreamResult<I> + Send + Sync + 'static,
        I: IntoIterator<Item = Next>,
        I::IntoIter: Send + 'static,
    {
        self.via(Flow::identity().stateful_map_concat_result_with_supervision(seed, f, decider))
    }

    #[must_use]
    pub fn map_async<Next, F, Fut>(self, parallelism: usize, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Next>> + Send + 'static,
    {
        self.via(Flow::identity().map_async(parallelism, f))
    }

    #[must_use]
    pub fn map_async_with_supervision<Next, F, Fut>(
        self,
        parallelism: usize,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Next>> + Send + 'static,
    {
        self.via(Flow::identity().map_async_with_supervision(parallelism, f, decider))
    }

    #[must_use]
    pub fn map_async_unordered<Next, F, Fut>(self, parallelism: usize, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Next>> + Send + 'static,
    {
        self.via(Flow::identity().map_async_unordered(parallelism, f))
    }

    #[must_use]
    pub fn map_async_unordered_with_supervision<Next, F, Fut>(
        self,
        parallelism: usize,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        F: Fn(Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Next>> + Send + 'static,
    {
        self.via(Flow::identity().map_async_unordered_with_supervision(parallelism, f, decider))
    }

    #[must_use]
    pub fn map_async_partitioned<Key, Next, Partition, F, Fut>(
        self,
        parallelism: usize,
        per_partition: usize,
        partition: Partition,
        f: F,
    ) -> Source<Next, Mat>
    where
        Key: Clone + Eq + Hash + Send + 'static,
        Next: Send + 'static,
        Partition: Fn(&Out) -> Key + Send + Sync + 'static,
        F: Fn(Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Next>> + Send + 'static,
    {
        self.via(Flow::identity().map_async_partitioned(parallelism, per_partition, partition, f))
    }

    #[must_use]
    pub fn prefix_and_tail(self, n: usize) -> Source<(Vec<Out>, Source<Out>), Mat> {
        self.via(Flow::identity().prefix_and_tail(n))
    }

    #[must_use]
    pub fn flat_map_prefix<Next, FlowMat, F>(self, n: usize, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        FlowMat: Send + 'static,
        F: Fn(Vec<Out>) -> Flow<Out, Next, FlowMat> + Send + Sync + 'static,
        Out: Clone,
    {
        self.via(Flow::identity().flat_map_prefix(n, f))
    }

    #[must_use]
    pub fn group_by<Key, F>(
        self,
        max_substreams: usize,
        f: F,
        allow_closed_substream_recreation: bool,
    ) -> Source<Source<Out>, Mat>
    where
        Key: Clone + Eq + Hash + Send + 'static,
        F: Fn(&Out) -> Key + Send + Sync + 'static,
        Out: Clone,
    {
        self.via(Flow::identity().group_by(max_substreams, f, allow_closed_substream_recreation))
    }

    #[must_use]
    pub fn split_when<F>(self, predicate: F) -> Source<Source<Out>, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
        Out: Clone,
    {
        self.via(Flow::identity().split_when(predicate))
    }

    #[must_use]
    pub fn split_after<F>(self, predicate: F) -> Source<Source<Out>, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
        Out: Clone,
    {
        self.via(Flow::identity().split_after(predicate))
    }

    #[must_use]
    pub fn flat_map_concat<Next, NextMat, F>(self, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        NextMat: Send + 'static,
        F: Fn(Out) -> Source<Next, NextMat> + Send + Sync + 'static,
    {
        self.via(Flow::identity().flat_map_concat(f))
    }

    #[must_use]
    pub fn flat_map_merge<Next, NextMat, F>(self, breadth: usize, f: F) -> Source<Next, Mat>
    where
        Next: Send + 'static,
        NextMat: Send + 'static,
        F: Fn(Out) -> Source<Next, NextMat> + Send + Sync + 'static,
    {
        self.via(Flow::identity().flat_map_merge(breadth, f))
    }

    #[must_use]
    pub fn take(self, n: usize) -> Source<Out, Mat> {
        self.via(Flow::identity().take(n))
    }

    #[must_use]
    pub fn drop(self, n: usize) -> Source<Out, Mat> {
        self.via(Flow::identity().drop(n))
    }

    #[must_use]
    pub fn take_while<F>(self, predicate: F) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
    {
        self.via(Flow::identity().take_while(predicate))
    }

    #[must_use]
    pub fn drop_while<F>(self, predicate: F) -> Source<Out, Mat>
    where
        F: Fn(&Out) -> bool + Send + Sync + 'static,
    {
        self.via(Flow::identity().drop_while(predicate))
    }

    #[must_use]
    pub fn limit(self, max: u64) -> Source<Out, Mat> {
        self.via(Flow::identity().limit(max))
    }

    #[must_use]
    pub fn grouped(self, size: usize) -> Source<Vec<Out>, Mat> {
        self.via(Flow::identity().grouped(size))
    }

    #[must_use]
    pub fn scan<State, F>(self, seed: State, f: F) -> Source<State, Mat>
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State, Out) -> State + Send + Sync + 'static,
    {
        self.via(Flow::identity().scan(seed, f))
    }

    #[must_use]
    pub fn scan_async<State, F, Fut>(self, seed: State, f: F) -> Source<State, Mat>
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State, Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<State>> + Send + 'static,
    {
        self.via(Flow::identity().scan_async(seed, f))
    }

    #[must_use]
    pub fn scan_result<State, F>(self, seed: State, f: F) -> Source<State, Mat>
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State, Out) -> StreamResult<State> + Send + Sync + 'static,
    {
        self.via(Flow::identity().scan_result(seed, f))
    }

    #[must_use]
    pub fn scan_result_with_supervision<State, F>(
        self,
        seed: State,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<State, Mat>
    where
        State: Clone + Send + Sync + 'static,
        F: Fn(State, Out) -> StreamResult<State> + Send + Sync + 'static,
    {
        self.via(Flow::identity().scan_result_with_supervision(seed, f, decider))
    }

    #[must_use]
    pub fn sliding(self, size: usize, step: usize) -> Source<Vec<Out>, Mat>
    where
        Out: Clone,
    {
        self.via(Flow::identity().sliding(size, step))
    }

    #[must_use]
    pub fn fold<Acc, F>(self, zero: Acc, f: F) -> Source<Acc, Mat>
    where
        Acc: Clone + Send + Sync + 'static,
        F: Fn(Acc, Out) -> Acc + Send + Sync + 'static,
    {
        self.via(Flow::identity().fold(zero, f))
    }

    #[must_use]
    pub fn fold_async<Acc, F, Fut>(self, zero: Acc, f: F) -> Source<Acc, Mat>
    where
        Acc: Clone + Send + Sync + 'static,
        F: Fn(Acc, Out) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = StreamResult<Acc>> + Send + 'static,
    {
        self.via(Flow::identity().fold_async(zero, f))
    }

    #[must_use]
    pub fn map_with_resource<Resource, Next, Create, F, Close>(
        self,
        create: Create,
        f: F,
        close: Close,
    ) -> Source<Next, Mat>
    where
        Resource: Send + 'static,
        Next: Send + 'static,
        Create: Fn() -> StreamResult<Resource> + Send + Sync + 'static,
        F: Fn(&mut Resource, Out) -> StreamResult<Next> + Send + Sync + 'static,
        Close: Fn(Resource) -> StreamResult<Option<Next>> + Send + Sync + 'static,
    {
        self.via(Flow::identity().map_with_resource(create, f, close))
    }

    #[must_use]
    pub fn fold_result<Acc, F>(self, zero: Acc, f: F) -> Source<Acc, Mat>
    where
        Acc: Clone + Send + Sync + 'static,
        F: Fn(Acc, Out) -> StreamResult<Acc> + Send + Sync + 'static,
    {
        self.via(Flow::identity().fold_result(zero, f))
    }

    #[must_use]
    pub fn fold_result_with_supervision<Acc, F>(
        self,
        zero: Acc,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Acc, Mat>
    where
        Acc: Clone + Send + Sync + 'static,
        F: Fn(Acc, Out) -> StreamResult<Acc> + Send + Sync + 'static,
    {
        self.via(Flow::identity().fold_result_with_supervision(zero, f, decider))
    }

    #[must_use]
    pub fn reduce<F>(self, f: F) -> Source<Out, Mat>
    where
        F: Fn(Out, Out) -> Out + Send + Sync + 'static,
    {
        self.via(Flow::identity().reduce(f))
    }

    #[must_use]
    pub fn reduce_result<F>(self, f: F) -> Source<Out, Mat>
    where
        Out: Clone,
        F: Fn(Out, Out) -> StreamResult<Out> + Send + Sync + 'static,
    {
        self.via(Flow::identity().reduce_result(f))
    }

    #[must_use]
    pub fn reduce_result_with_supervision<F>(
        self,
        f: F,
        decider: SupervisionDecider,
    ) -> Source<Out, Mat>
    where
        Out: Clone,
        F: Fn(Out, Out) -> StreamResult<Out> + Send + Sync + 'static,
    {
        self.via(Flow::identity().reduce_result_with_supervision(f, decider))
    }

    #[must_use]
    pub fn map_error<F>(self, f: F) -> Source<Out, Mat>
    where
        F: Fn(StreamError) -> StreamError + Send + Sync + 'static,
    {
        self.via(Flow::identity().map_error(f))
    }

    #[must_use]
    pub fn recover<F>(self, f: F) -> Source<Out, Mat>
    where
        F: Fn(StreamError) -> Option<Out> + Send + Sync + 'static,
    {
        self.via(Flow::identity().recover(f))
    }

    #[must_use]
    pub fn recover_with<F>(self, f: F) -> Source<Out, Mat>
    where
        F: Fn(StreamError) -> Option<Source<Out>> + Send + Sync + 'static,
    {
        self.via(Flow::identity().recover_with(f))
    }

    #[must_use]
    pub fn recover_with_retries<F>(self, retries: usize, f: F) -> Source<Out, Mat>
    where
        F: Fn(StreamError) -> Option<Source<Out>> + Send + Sync + 'static,
    {
        self.via(Flow::identity().recover_with_retries(retries, f))
    }

    #[must_use]
    pub fn on_error_complete(self) -> Source<Out, Mat> {
        self.via(Flow::identity().on_error_complete())
    }

    #[must_use]
    pub fn concat<Mat2>(self, that: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                let secondary = match Arc::clone(&that_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                };
                Ok((concat_source_streams(vec![primary, secondary]), mat))
            },
            hints,
        )
    }

    #[must_use]
    pub fn concat_lazy<Mat2>(self, that: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                Ok((
                    concat_source_streams_lazy(
                        primary,
                        vec![Arc::clone(&that_factory)],
                        materializer,
                    ),
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn concat_all_lazy<Mat2, I>(self, those: I) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
        I: IntoIterator<Item = Source<Out, Mat2>>,
    {
        let factory = self.factory;
        let hints = self.hints;
        let other_factories: Vec<_> = those.into_iter().map(|source| source.factory).collect();
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                Ok((
                    concat_source_streams_lazy(primary, other_factories.clone(), materializer),
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn prepend<Mat2>(self, that: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                let secondary = match Arc::clone(&that_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                };
                Ok((concat_source_streams(vec![secondary, primary]), mat))
            },
            hints,
        )
    }

    #[must_use]
    pub fn prepend_lazy<Mat2>(self, that: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        self.prepend(that)
    }

    #[must_use]
    pub fn or_else<Mat2>(self, secondary: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let secondary_factory = secondary.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                let secondary = match Arc::clone(&secondary_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                };
                Ok((or_else_source_stream(primary, secondary), mat))
            },
            hints,
        )
    }

    #[must_use]
    pub fn interleave<Mat2>(self, that: Source<Out, Mat2>, segment_size: usize) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
    {
        self.interleave_all([that], segment_size, false)
    }

    #[must_use]
    pub fn interleave_all<Mat2, I>(
        self,
        those: I,
        segment_size: usize,
        eager_close: bool,
    ) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
        I: IntoIterator<Item = Source<Out, Mat2>>,
    {
        let factory = self.factory;
        let hints = self.hints;
        let other_factories: Vec<_> = those.into_iter().map(|source| source.factory).collect();
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                let mut streams = Vec::with_capacity(other_factories.len() + 1);
                streams.push(primary);
                for other in &other_factories {
                    let stream = match Arc::clone(other).create(materializer) {
                        Ok((stream, _)) => stream,
                        Err(error) => Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                    };
                    streams.push(stream);
                }
                Ok((
                    interleave_source_streams(streams, segment_size, eager_close),
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn merge_sorted<Mat2>(self, that: Source<Out, Mat2>) -> Source<Out, Mat>
    where
        Out: Ord,
        Mat2: Send + 'static,
    {
        self.via(Flow::identity().merge_sorted(that))
    }

    #[must_use]
    pub fn merge_latest<Mat2>(
        self,
        that: Source<Out, Mat2>,
        eager_complete: bool,
    ) -> Source<Vec<Out>, Mat>
    where
        Out: Clone,
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (left, mat) = Arc::clone(&factory).create(materializer)?;
                let right = match Arc::clone(&that_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<Vec<Out>>,
                            mat,
                        ));
                    }
                };
                Ok((merge_latest_streams(vec![left, right], eager_complete), mat))
            },
            hints,
        )
    }

    #[must_use]
    pub fn merge_all<Mat2, I>(self, those: I, eager_complete: bool) -> Source<Out, Mat>
    where
        Mat2: Send + 'static,
        I: IntoIterator<Item = Source<Out, Mat2>>,
    {
        let factory = self.factory;
        let hints = self.hints;
        let other_factories: Vec<_> = those.into_iter().map(|source| source.factory).collect();
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (primary, mat) = Arc::clone(&factory).create(materializer)?;
                let mut streams = Vec::with_capacity(other_factories.len() + 1);
                streams.push(primary);
                for other in &other_factories {
                    let stream = match Arc::clone(other).create(materializer) {
                        Ok((stream, _)) => stream,
                        Err(error) => {
                            return Ok((
                                Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                                mat,
                            ));
                        }
                    };
                    streams.push(stream);
                }
                Ok((merge_streams(streams, eager_complete), mat))
            },
            hints,
        )
    }

    #[must_use]
    pub fn zip_with<Mat2, Out2, Next, F>(
        self,
        that: Source<Out2, Mat2>,
        combine: F,
    ) -> Source<Next, Mat>
    where
        Out2: Send + 'static,
        Next: Send + 'static,
        Mat2: Send + 'static,
        F: Fn(Out, Out2) -> Next + Send + Sync + 'static,
    {
        self.via(Flow::identity().zip_with(that, combine))
    }

    #[must_use]
    pub fn zip_latest<Mat2, Out2>(self, that: Source<Out2, Mat2>) -> Source<(Out, Out2), Mat>
    where
        Out: Clone,
        Out2: Clone + Send + 'static,
        Mat2: Send + 'static,
    {
        self.zip_latest_with(that, true, |left, right| (left, right))
    }

    #[must_use]
    pub fn zip_latest_with<Mat2, Out2, Next, F>(
        self,
        that: Source<Out2, Mat2>,
        eager_complete: bool,
        combine: F,
    ) -> Source<Next, Mat>
    where
        Out: Clone,
        Out2: Clone + Send + 'static,
        Next: Send + 'static,
        Mat2: Send + 'static,
        F: Fn(Out, Out2) -> Next + Send + Sync + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        let combine = Arc::new(combine);
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (left, mat) = Arc::clone(&factory).create(materializer)?;
                let right = match Arc::clone(&that_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<Next>,
                            mat,
                        ));
                    }
                };
                Ok((
                    zip_latest_with_stream(left, right, eager_complete, Arc::clone(&combine)),
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn zip_with_index(self) -> Source<(Out, u64), Mat> {
        let factory = self.factory;
        let hints = self.hints;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (mut stream, mat) = Arc::clone(&factory).create(materializer)?;
                let mut index = 0_u64;
                Ok((
                    Box::new(std::iter::from_fn(move || {
                        stream.next().map(|item| {
                            item.map(|value| {
                                let pair = (value, index);
                                index = index.wrapping_add(1);
                                pair
                            })
                        })
                    })) as BoxStream<(Out, u64)>,
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn zip_all<Mat2, Out2>(
        self,
        that: Source<Out2, Mat2>,
        this_elem: Out,
        that_elem: Out2,
    ) -> Source<(Out, Out2), Mat>
    where
        Out: Clone + Sync,
        Out2: Clone + Send + Sync + 'static,
        Mat2: Send + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let that_factory = that.factory;
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (left, mat) = Arc::clone(&factory).create(materializer)?;
                let right = match Arc::clone(&that_factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<(Out, Out2)>,
                            mat,
                        ));
                    }
                };
                Ok((
                    zip_all_stream(left, right, this_elem.clone(), that_elem.clone()),
                    mat,
                ))
            },
            hints,
        )
    }

    #[must_use]
    pub fn also_to<SinkMat>(self, sink: Sink<Out, SinkMat>) -> Source<Out, Mat>
    where
        Out: Clone,
        SinkMat: Send + 'static,
    {
        self.via(Flow::identity().also_to(sink))
    }

    #[must_use]
    pub fn also_to_all<SinkMat, I>(self, sinks: I) -> Source<Out, Mat>
    where
        Out: Clone,
        SinkMat: Send + 'static,
        I: IntoIterator<Item = Sink<Out, SinkMat>>,
    {
        self.via(Flow::identity().also_to_all(sinks))
    }

    #[must_use]
    pub fn divert_to<SinkMat, F>(self, sink: Sink<Out, SinkMat>, predicate: F) -> Source<Out, Mat>
    where
        SinkMat: Send + 'static,
        F: Fn(&Out) -> bool + Send + Sync + 'static,
    {
        self.via(Flow::identity().divert_to(sink, predicate))
    }

    #[must_use]
    pub fn wire_tap<SinkMat>(self, sink: Sink<Out, SinkMat>) -> Source<Out, Mat>
    where
        Out: Clone,
        SinkMat: Send + 'static,
    {
        self.via(Flow::identity().wire_tap(sink))
    }

    pub fn run_with<SinkMat: Send + 'static>(
        self,
        sink: Sink<Out, SinkMat>,
    ) -> StreamResult<SinkMat> {
        // Split-segment fast path: if source has a segment hook and sink has a
        // fold fast-path descriptor, drive the fold inline without spawning a task.
        let fast_result = self
            .split_hook
            .as_ref()
            .zip(sink.fold_fp.as_deref())
            .and_then(|(hook, fp)| fp.try_register(Arc::clone(hook)));
        if let Some(result) = fast_result {
            return result?.downcast::<SinkMat>().map(|b| *b).map_err(|_| {
                StreamError::Failed("split fast path: unexpected mat type (internal error)".into())
            });
        }
        self.to_mat(sink, Keep::right).run()
    }

    pub fn run_with_materializer<SinkMat: Send + 'static>(
        self,
        sink: Sink<Out, SinkMat>,
        materializer: &Materializer,
    ) -> StreamResult<SinkMat> {
        self.to_mat(sink, Keep::right)
            .run_with_materializer(materializer)
    }

    #[must_use]
    pub fn to<SinkMat>(self, sink: Sink<Out, SinkMat>) -> RunnableGraph<Mat>
    where
        SinkMat: Send + 'static,
    {
        self.to_mat(sink, Keep::left)
    }

    #[must_use]
    pub fn to_mat<SinkMat, Combined, F>(
        self,
        sink: Sink<Out, SinkMat>,
        combine: F,
    ) -> RunnableGraph<Combined>
    where
        SinkMat: Send + 'static,
        Combined: Send + 'static,
        F: Fn(Mat, SinkMat) -> Combined + Send + Sync + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let combine = Arc::new(combine);
        RunnableGraph::from_runner(move |materializer| {
            let (stream, source_mat) = Arc::clone(&factory).create(materializer)?;
            let stream =
                runtime_checked_stream(stream, Arc::clone(&materializer.inner.state), None);
            let sink_mat = if hints.inline_head_terminal && sink.can_inline() {
                sink.run_inline(stream, materializer)?
            } else {
                sink.run(stream, materializer)?
            };
            Ok(combine(source_mat, sink_mat))
        })
    }

    pub fn run_collect(self) -> StreamResult<Vec<Out>> {
        self.run_with(Sink::collect())?.wait()
    }

    #[must_use]
    pub fn map_materialized_value<NextMat, F>(self, f: F) -> Source<Out, NextMat>
    where
        NextMat: Send + 'static,
        F: Fn(Mat) -> NextMat + Send + Sync + 'static,
    {
        let factory = self.factory;
        let hints = self.hints;
        let f = Arc::new(f);
        Source::from_materialized_factory_with_hints(
            move |materializer| {
                let (stream, mat) = Arc::clone(&factory).create(materializer)?;
                Ok((stream, f(mat)))
            },
            hints,
        )
    }
}

impl<Out: Clone + Send + Sync + 'static> Source<Out, NotUsed> {
    #[must_use]
    pub fn combine<Mat1, Mat2, MatRest, I>(
        first: Source<Out, Mat1>,
        second: Source<Out, Mat2>,
        rest: I,
        strategy: SourceCombineStrategy,
    ) -> Source<Out, NotUsed>
    where
        Mat1: Send + 'static,
        Mat2: Send + 'static,
        MatRest: Send + 'static,
        I: IntoIterator<Item = Source<Out, MatRest>>,
    {
        let mut factories: Vec<Arc<CombinedSourceFactory<Out>>> = vec![
            Arc::new(move |materializer| {
                Arc::clone(&first.factory)
                    .create(materializer)
                    .map(|(stream, _)| stream)
            }),
            Arc::new(move |materializer| {
                Arc::clone(&second.factory)
                    .create(materializer)
                    .map(|(stream, _)| stream)
            }),
        ];
        factories.extend(rest.into_iter().map(|source| {
            Arc::new(move |materializer: &Materializer| {
                Arc::clone(&source.factory)
                    .create(materializer)
                    .map(|(stream, _)| stream)
            }) as Arc<CombinedSourceFactory<Out>>
        }));
        Source::from_materialized_factory(move |materializer| {
            let mut streams = Vec::with_capacity(factories.len());
            for factory in &factories {
                let stream = match factory(materializer) {
                    Ok(stream) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                            NotUsed,
                        ));
                    }
                };
                streams.push(stream);
            }
            let stream = match &strategy {
                SourceCombineStrategy::Merge { eager_complete } => {
                    merge_streams(streams, *eager_complete)
                }
                SourceCombineStrategy::Concat => concat_source_streams(streams),
                SourceCombineStrategy::Prioritized {
                    priorities,
                    eager_complete,
                } => {
                    if priorities.len() != streams.len() {
                        return Err(StreamError::GraphValidation(format!(
                            "combine priorities length {} did not match source count {}",
                            priorities.len(),
                            streams.len()
                        )));
                    }
                    merge_prioritized_streams(streams, priorities.clone(), *eager_complete)
                }
            };
            Ok((stream, NotUsed))
        })
    }

    #[must_use]
    pub fn zip_n<Mat2, I>(sources: I) -> Source<Vec<Out>, NotUsed>
    where
        I: IntoIterator<Item = Source<Out, Mat2>>,
        Mat2: Send + 'static,
        Out: Clone,
    {
        Self::zip_with_n(sources, |values| values)
    }

    #[must_use]
    pub fn zip_with_n<Mat2, I, Next, F>(sources: I, zipper: F) -> Source<Next, NotUsed>
    where
        I: IntoIterator<Item = Source<Out, Mat2>>,
        Mat2: Send + 'static,
        Next: Send + 'static,
        F: Fn(Vec<Out>) -> Next + Send + Sync + 'static,
    {
        let factories: Vec<_> = sources.into_iter().map(|source| source.factory).collect();
        let zipper = Arc::new(zipper);
        Source::from_materialized_factory(move |materializer| {
            let mut streams = Vec::with_capacity(factories.len());
            for factory in &factories {
                let stream = match Arc::clone(factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<Next>,
                            NotUsed,
                        ));
                    }
                };
                streams.push(stream);
            }
            Ok((zip_n_streams(streams, Arc::clone(&zipper)), NotUsed))
        })
    }

    #[must_use]
    pub fn merge_prioritized_n<Mat2, I>(
        sources_and_priorities: I,
        eager_complete: bool,
    ) -> Source<Out, NotUsed>
    where
        I: IntoIterator<Item = (Source<Out, Mat2>, usize)>,
        Mat2: Send + 'static,
    {
        let sources_and_priorities: Vec<_> = sources_and_priorities.into_iter().collect();
        if sources_and_priorities.is_empty() {
            return Source::empty();
        }
        let (factories, priorities): (Vec<_>, Vec<_>) = sources_and_priorities
            .into_iter()
            .map(|(source, priority)| (source.factory, priority))
            .unzip();
        Source::from_materialized_factory(move |materializer| {
            let mut streams = Vec::with_capacity(factories.len());
            for factory in &factories {
                let stream = match Arc::clone(factory).create(materializer) {
                    Ok((stream, _)) => stream,
                    Err(error) => {
                        return Ok((
                            Box::new(std::iter::once(Err(error))) as BoxStream<Out>,
                            NotUsed,
                        ));
                    }
                };
                streams.push(stream);
            }
            Ok((
                merge_prioritized_streams(streams, priorities.clone(), eager_complete),
                NotUsed,
            ))
        })
    }

    #[must_use]
    pub fn maybe() -> (MaybeHandle<Out>, Self) {
        let value = Arc::new(Mutex::new(None));
        let handle = MaybeHandle {
            value: Arc::clone(&value),
        };
        let source = Self::from_factory(move || {
            let result = value
                .lock()
                .expect("maybe source poisoned")
                .clone()
                .unwrap_or(Err(StreamError::MaybeIncomplete));
            Box::new(std::iter::once(result))
        });
        (handle, source)
    }

    #[must_use]
    pub fn single(item: Out) -> Self {
        Self::from_factory_with_hints(
            move || Box::new(std::iter::once(Ok(item.clone()))),
            SourceHints::with_inline_micro(1),
        )
    }

    #[must_use]
    pub fn repeat(item: Out) -> Self {
        Self::from_factory(move || {
            let item = item.clone();
            Box::new(std::iter::repeat_with(move || Ok(item.clone())))
        })
    }

    #[must_use]
    pub fn from_iterable<I>(items: I) -> Self
    where
        I: IntoIterator<Item = Out>,
    {
        items.into_iter().collect()
    }
}

impl<Out: Clone + Send + Sync + 'static> FromIterator<Out> for Source<Out, NotUsed> {
    fn from_iter<T: IntoIterator<Item = Out>>(iter: T) -> Self {
        // Share the backing storage across materializations and clone elements
        // lazily on demand, instead of cloning the whole container each run.
        let items: Arc<[Out]> = iter.into_iter().collect();
        let len = items.len();
        Self::from_factory_with_hints(
            move || {
                let items = Arc::clone(&items);
                let mut index = 0;
                Box::new(std::iter::from_fn(move || {
                    let item = items.get(index)?.clone();
                    index += 1;
                    Some(Ok(item))
                }))
            },
            // Exact success item count: enables the coordinator inline drain path.
            SourceHints::with_inline_micro(len),
        )
    }
}

/// Test-only helper: create a Source whose factory calls the given closure but
/// marks it as inline-micro eligible. Used to test the inline drain path with
/// custom (possibly blocking) iterators without broadening production
/// eligibility.
#[cfg(test)]
pub(in crate::stream) fn test_source_with_inline_micro_hint<Out: Send + 'static>(
    factory: impl Fn() -> BoxStream<Out> + Send + Sync + 'static,
    max_success_items: usize,
) -> Source<Out, NotUsed> {
    Source::from_factory_with_hints(factory, SourceHints::with_inline_micro(max_success_items))
}

struct UnfoldResourceStream<Resource, Out, Create, Read, Close>
where
    Create: Fn() -> StreamResult<Resource>,
    Read: Fn(&mut Resource) -> StreamResult<Option<Out>>,
    Close: Fn(Resource) -> StreamResult<()>,
{
    create: Arc<Create>,
    read: Arc<Read>,
    close: Arc<Close>,
    resource: Option<Resource>,
    created: bool,
    terminated: bool,
    _marker: PhantomData<fn() -> Out>,
}

impl<Resource, Out, Create, Read, Close> UnfoldResourceStream<Resource, Out, Create, Read, Close>
where
    Create: Fn() -> StreamResult<Resource>,
    Read: Fn(&mut Resource) -> StreamResult<Option<Out>>,
    Close: Fn(Resource) -> StreamResult<()>,
{
    fn ensure_created(&mut self) -> StreamResult<()> {
        if self.created {
            return Ok(());
        }
        self.created = true;
        let resource = catch_unwind_failed("unfold_resource create", || (self.create)())
            .and_then(|result| result)?;
        self.resource = Some(resource);
        Ok(())
    }

    fn close_resource(&mut self) -> StreamResult<()> {
        match self.resource.take() {
            Some(resource) => {
                catch_unwind_failed("unfold_resource close", || (self.close)(resource))
                    .and_then(|result| result)
            }
            None => Ok(()),
        }
    }
}

impl<Resource, Out, Create, Read, Close> Iterator
    for UnfoldResourceStream<Resource, Out, Create, Read, Close>
where
    Create: Fn() -> StreamResult<Resource>,
    Read: Fn(&mut Resource) -> StreamResult<Option<Out>>,
    Close: Fn(Resource) -> StreamResult<()>,
{
    type Item = StreamResult<Out>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.terminated {
            return None;
        }
        if let Err(error) = self.ensure_created() {
            self.terminated = true;
            return Some(Err(error));
        }

        let result = {
            let resource = self
                .resource
                .as_mut()
                .expect("unfold_resource resource is open");
            catch_unwind_failed("unfold_resource read", || (self.read)(resource))
                .and_then(|result| result)
        };

        match result {
            Ok(Some(item)) => Some(Ok(item)),
            Ok(None) => {
                self.terminated = true;
                match self.close_resource() {
                    Ok(()) => None,
                    Err(error) => Some(Err(error)),
                }
            }
            Err(read_error) => {
                self.terminated = true;
                let _ = self.close_resource();
                Some(Err(read_error))
            }
        }
    }
}

impl<Resource, Out, Create, Read, Close> Drop
    for UnfoldResourceStream<Resource, Out, Create, Read, Close>
where
    Create: Fn() -> StreamResult<Resource>,
    Read: Fn(&mut Resource) -> StreamResult<Option<Out>>,
    Close: Fn(Resource) -> StreamResult<()>,
{
    fn drop(&mut self) {
        let _ = self.close_resource();
    }
}

type UnfoldResourceAsyncMarker<Out, CreateFut, ReadFut, CloseFut> =
    fn() -> (Out, CreateFut, ReadFut, CloseFut);

struct UnfoldResourceAsyncStream<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut>
where
    Resource: Send + 'static,
    Create: Fn() -> CreateFut,
    CreateFut: Future<Output = StreamResult<Resource>> + Send + 'static,
    Read: Fn(&mut Resource) -> ReadFut,
    ReadFut: Future<Output = StreamResult<Option<Out>>> + Send + 'static,
    Close: Fn(Resource) -> CloseFut,
    CloseFut: Future<Output = StreamResult<()>> + Send + 'static,
{
    create: Arc<Create>,
    read: Arc<Read>,
    close: Arc<Close>,
    resource: Option<Resource>,
    created: bool,
    terminated: bool,
    _marker: PhantomData<UnfoldResourceAsyncMarker<Out, CreateFut, ReadFut, CloseFut>>,
}

impl<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut>
    UnfoldResourceAsyncStream<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut>
where
    Resource: Send + 'static,
    Create: Fn() -> CreateFut,
    CreateFut: Future<Output = StreamResult<Resource>> + Send + 'static,
    Read: Fn(&mut Resource) -> ReadFut,
    ReadFut: Future<Output = StreamResult<Option<Out>>> + Send + 'static,
    Close: Fn(Resource) -> CloseFut,
    CloseFut: Future<Output = StreamResult<()>> + Send + 'static,
{
    fn ensure_created(&mut self) -> StreamResult<()> {
        if self.created {
            return Ok(());
        }
        self.created = true;
        let resource = catch_unwind_failed("unfold_resource_async create", || (self.create)())
            .and_then(flow::run_future_inline_or_spawn)?;
        self.resource = Some(resource);
        Ok(())
    }

    fn close_resource(&mut self) -> StreamResult<()> {
        match self.resource.take() {
            Some(resource) => {
                catch_unwind_failed("unfold_resource_async close", || (self.close)(resource))
                    .and_then(flow::run_future_inline_or_spawn)
            }
            None => Ok(()),
        }
    }
}

impl<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut> Iterator
    for UnfoldResourceAsyncStream<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut>
where
    Resource: Send + 'static,
    Out: Send + 'static,
    Create: Fn() -> CreateFut,
    CreateFut: Future<Output = StreamResult<Resource>> + Send + 'static,
    Read: Fn(&mut Resource) -> ReadFut,
    ReadFut: Future<Output = StreamResult<Option<Out>>> + Send + 'static,
    Close: Fn(Resource) -> CloseFut,
    CloseFut: Future<Output = StreamResult<()>> + Send + 'static,
{
    type Item = StreamResult<Out>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.terminated {
            return None;
        }
        if let Err(error) = self.ensure_created() {
            self.terminated = true;
            return Some(Err(error));
        }

        let result = {
            let resource = self
                .resource
                .as_mut()
                .expect("unfold_resource_async resource is open");
            catch_unwind_failed("unfold_resource_async read", || (self.read)(resource))
                .and_then(flow::run_future_inline_or_spawn)
        };

        match result {
            Ok(Some(item)) => Some(Ok(item)),
            Ok(None) => {
                self.terminated = true;
                match self.close_resource() {
                    Ok(()) => None,
                    Err(error) => Some(Err(error)),
                }
            }
            Err(read_error) => {
                self.terminated = true;
                let _ = self.close_resource();
                Some(Err(read_error))
            }
        }
    }
}

impl<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut> Drop
    for UnfoldResourceAsyncStream<Resource, Out, Create, CreateFut, Read, ReadFut, Close, CloseFut>
where
    Resource: Send + 'static,
    Create: Fn() -> CreateFut,
    CreateFut: Future<Output = StreamResult<Resource>> + Send + 'static,
    Read: Fn(&mut Resource) -> ReadFut,
    ReadFut: Future<Output = StreamResult<Option<Out>>> + Send + 'static,
    Close: Fn(Resource) -> CloseFut,
    CloseFut: Future<Output = StreamResult<()>> + Send + 'static,
{
    fn drop(&mut self) {
        let _ = self.close_resource();
    }
}

struct LazySourceStream<Out, InnerMat, F> {
    create: Arc<F>,
    materializer: Materializer,
    current: Option<BoxStream<Out>>,
    mat_sender: Option<oneshot::Sender<StreamResult<InnerMat>>>,
    initialized: bool,
    terminated: bool,
}

impl<Out, InnerMat, F> LazySourceStream<Out, InnerMat, F>
where
    Out: Send + 'static,
    InnerMat: Send + 'static,
    F: Fn() -> Source<Out, InnerMat>,
{
    fn complete_mat(&mut self, result: StreamResult<InnerMat>) {
        if let Some(sender) = self.mat_sender.take() {
            let _ = sender.send(result);
        }
    }

    fn initialize(&mut self) -> StreamResult<()> {
        if self.initialized {
            return Ok(());
        }
        self.initialized = true;
        let source = match catch_unwind_failed("lazy_source factory", || (self.create)()) {
            Ok(source) => source,
            Err(error) => {
                self.complete_mat(Err(error.clone()));
                return Err(error);
            }
        };
        match Arc::clone(&source.factory).create(&self.materializer) {
            Ok((stream, mat)) => {
                self.current = Some(stream);
                self.complete_mat(Ok(mat));
                Ok(())
            }
            Err(error) => {
                self.complete_mat(Err(error.clone()));
                Err(error)
            }
        }
    }
}

impl<Out, InnerMat, F> Iterator for LazySourceStream<Out, InnerMat, F>
where
    Out: Send + 'static,
    InnerMat: Send + 'static,
    F: Fn() -> Source<Out, InnerMat>,
{
    type Item = StreamResult<Out>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.terminated {
            return None;
        }
        if let Err(error) = self.initialize() {
            self.terminated = true;
            return Some(Err(error));
        }
        match self
            .current
            .as_mut()
            .expect("lazy_source current stream initialized")
            .next()
        {
            Some(Ok(item)) => Some(Ok(item)),
            Some(Err(error)) => {
                self.terminated = true;
                Some(Err(error))
            }
            None => {
                self.terminated = true;
                None
            }
        }
    }
}

impl<Out, InnerMat, F> Drop for LazySourceStream<Out, InnerMat, F> {
    fn drop(&mut self) {
        if !self.initialized
            && let Some(sender) = self.mat_sender.take()
        {
            let _ = sender.send(Err(StreamError::Failed(
                "lazy source was never materialized".into(),
            )));
        }
    }
}

struct LazyFutureSourceStream<Out, InnerMat, F, Fut> {
    create: Arc<F>,
    materializer: Materializer,
    current: Option<BoxStream<Out>>,
    mat_sender: Option<oneshot::Sender<StreamResult<InnerMat>>>,
    initialized: bool,
    terminated: bool,
    _marker: PhantomData<fn() -> Fut>,
}

impl<Out, InnerMat, F, Fut> LazyFutureSourceStream<Out, InnerMat, F, Fut>
where
    Out: Send + 'static,
    InnerMat: Send + 'static,
    F: Fn() -> Fut,
    Fut: Future<Output = StreamResult<Source<Out, InnerMat>>> + Send + 'static,
{
    fn complete_mat(&mut self, result: StreamResult<InnerMat>) {
        if let Some(sender) = self.mat_sender.take() {
            let _ = sender.send(result);
        }
    }

    fn initialize(&mut self) -> StreamResult<()> {
        if self.initialized {
            return Ok(());
        }
        self.initialized = true;
        let source = match catch_unwind_failed("lazy_future_source factory", || (self.create)())
            .and_then(flow::run_future_inline_or_spawn)
        {
            Ok(source) => source,
            Err(error) => {
                self.complete_mat(Err(error.clone()));
                return Err(error);
            }
        };
        match Arc::clone(&source.factory).create(&self.materializer) {
            Ok((stream, mat)) => {
                self.current = Some(stream);
                self.complete_mat(Ok(mat));
                Ok(())
            }
            Err(error) => {
                self.complete_mat(Err(error.clone()));
                Err(error)
            }
        }
    }
}

impl<Out, InnerMat, F, Fut> Iterator for LazyFutureSourceStream<Out, InnerMat, F, Fut>
where
    Out: Send + 'static,
    InnerMat: Send + 'static,
    F: Fn() -> Fut,
    Fut: Future<Output = StreamResult<Source<Out, InnerMat>>> + Send + 'static,
{
    type Item = StreamResult<Out>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.terminated {
            return None;
        }
        if let Err(error) = self.initialize() {
            self.terminated = true;
            return Some(Err(error));
        }
        match self
            .current
            .as_mut()
            .expect("lazy_future_source current stream initialized")
            .next()
        {
            Some(Ok(item)) => Some(Ok(item)),
            Some(Err(error)) => {
                self.terminated = true;
                Some(Err(error))
            }
            None => {
                self.terminated = true;
                None
            }
        }
    }
}

impl<Out, InnerMat, F, Fut> Drop for LazyFutureSourceStream<Out, InnerMat, F, Fut> {
    fn drop(&mut self) {
        if !self.initialized
            && let Some(sender) = self.mat_sender.take()
        {
            let _ = sender.send(Err(StreamError::Failed(
                "lazy future source was never materialized".into(),
            )));
        }
    }
}

fn concat_source_streams<Out>(streams: Vec<BoxStream<Out>>) -> BoxStream<Out>
where
    Out: Send + 'static,
{
    let mut streams: VecDeque<_> = streams.into();
    let mut current = streams.pop_front();
    Box::new(std::iter::from_fn(move || {
        loop {
            match current.as_mut() {
                Some(stream) => match stream.next() {
                    Some(item) => return Some(item),
                    None => current = streams.pop_front(),
                },
                None => return None,
            }
        }
    }))
}

fn concat_source_streams_lazy<Out, Mat>(
    initial: BoxStream<Out>,
    factories: Vec<Arc<dyn SourceFactory<Out, Mat>>>,
    materializer: &Materializer,
) -> BoxStream<Out>
where
    Out: Send + 'static,
    Mat: Send + 'static,
{
    let mut current = Some(initial);
    let mut remaining: VecDeque<_> = factories.into();
    let materializer = materializer.with_name_prefix(materializer.name_prefix().to_owned());
    Box::new(std::iter::from_fn(move || {
        loop {
            match current.as_mut() {
                Some(stream) => match stream.next() {
                    Some(item) => return Some(item),
                    None => {
                        current = remaining.pop_front().map(|factory| {
                            match factory.create(&materializer) {
                                Ok((stream, _)) => stream,
                                Err(error) => {
                                    Box::new(std::iter::once(Err(error))) as BoxStream<Out>
                                }
                            }
                        });
                    }
                },
                None => return None,
            }
        }
    }))
}

fn or_else_source_stream<Out>(
    mut primary: BoxStream<Out>,
    mut secondary: BoxStream<Out>,
) -> BoxStream<Out>
where
    Out: Send + 'static,
{
    let mut primary_emitted = false;
    let mut using_secondary = false;
    Box::new(std::iter::from_fn(move || {
        loop {
            if using_secondary {
                return secondary.next();
            }

            match primary.next() {
                Some(Ok(item)) => {
                    primary_emitted = true;
                    return Some(Ok(item));
                }
                Some(Err(error)) => return Some(Err(error)),
                None if primary_emitted => return None,
                None => using_secondary = true,
            }
        }
    }))
}

fn interleave_source_streams<Out>(
    streams: Vec<BoxStream<Out>>,
    segment_size: usize,
    eager_close: bool,
) -> BoxStream<Out>
where
    Out: Send + 'static,
{
    if segment_size == 0 {
        return Box::new(std::iter::once(Err(StreamError::GraphValidation(
            "interleave segment size must be greater than zero".into(),
        ))));
    }

    let mut streams: Vec<Option<BoxStream<Out>>> = streams.into_iter().map(Some).collect();
    let mut pending: Vec<Option<StreamResult<Out>>> = (0..streams.len()).map(|_| None).collect();
    let mut current = 0usize;
    let mut emitted = 0usize;
    Box::new(std::iter::from_fn(move || {
        loop {
            if streams.iter().all(Option::is_none) {
                return None;
            }
            if streams[current].is_none() {
                match next_active_source_stream(&streams, current) {
                    Some(next) => {
                        current = next;
                        emitted = 0;
                    }
                    None => return None,
                }
            }

            let Some(stream) = streams[current].as_mut() else {
                continue;
            };
            let next_item = pending[current].take().or_else(|| stream.next());
            match next_item {
                Some(Ok(item)) => {
                    emitted += 1;
                    if emitted == segment_size {
                        emitted = 0;
                        if let Some(next) = next_active_source_stream(&streams, current) {
                            current = next;
                        }
                    }
                    return Some(Ok(item));
                }
                Some(Err(error)) => return Some(Err(error)),
                None => {
                    streams[current] = None;
                    emitted = 0;
                    if eager_close {
                        return None;
                    }
                    match next_active_source_stream(&streams, current) {
                        Some(next) => current = next,
                        None => return None,
                    }
                }
            }
        }
    }))
}

fn next_active_source_stream<Out>(
    streams: &[Option<BoxStream<Out>>],
    current: usize,
) -> Option<usize>
where
    Out: Send + 'static,
{
    if streams.is_empty() {
        return None;
    }
    for offset in 1..=streams.len() {
        let index = (current + offset) % streams.len();
        if streams[index].is_some() {
            return Some(index);
        }
    }
    None
}