hydro_lang/location/
tick.rs

1//! Clock domains for batching streaming data into discrete time steps.
2//!
3//! In Hydro, a [`Tick`] represents a logical clock that can be used to batch
4//! unbounded streaming data into discrete, bounded time steps. This is essential
5//! for implementing iterative algorithms, synchronizing data across multiple
6//! streams, and performing aggregations over windows of data.
7//!
8//! A tick is created from a top-level location (such as [`Process`] or [`Cluster`])
9//! using [`Location::tick`]. Once inside a tick, bounded live collections can be
10//! manipulated with operations like fold, reduce, and cross-product, and the
11//! results can be emitted back to the unbounded stream using methods like
12//! `all_ticks()`.
13//!
14//! The [`Atomic`] wrapper provides atomicity guarantees within a tick, ensuring
15//! that reads and writes within a tick are serialized.
16//!
17//! The [`NoTick`] marker trait is used to constrain APIs that should only be
18//! called on top-level locations (not inside a tick), while [`NoAtomic`] constrains
19//! APIs that should not be called inside an atomic context.
20
21use sealed::sealed;
22use stageleft::{QuotedWithContext, q};
23
24#[cfg(stageleft_runtime)]
25use super::dynamic::DynLocation;
26use super::{Cluster, Location, LocationId, Process};
27use crate::compile::builder::{ClockId, FlowState};
28use crate::compile::ir::{HydroNode, HydroSource};
29#[cfg(stageleft_runtime)]
30use crate::forward_handle::{CycleCollection, CycleCollectionWithInitial};
31use crate::forward_handle::{TickCycle, TickCycleHandle};
32use crate::live_collections::boundedness::Bounded;
33use crate::live_collections::optional::Optional;
34use crate::live_collections::singleton::Singleton;
35use crate::live_collections::stream::{ExactlyOnce, Stream, TotalOrder};
36use crate::nondet::nondet;
37
38/// Marker trait for locations that are **not** inside a [`Tick`] clock domain.
39///
40/// This trait is implemented by top-level locations such as [`Process`] and [`Cluster`],
41/// as well as [`Atomic`]. It is used to constrain APIs that should only be called
42/// outside of a tick context (e.g., creating a new tick or sourcing external data).
43#[sealed]
44pub trait NoTick {}
45#[sealed]
46impl<T> NoTick for Process<'_, T> {}
47#[sealed]
48impl<T> NoTick for Cluster<'_, T> {}
49
50/// Marker trait for locations that are **not** inside an [`Atomic`] context.
51///
52/// This trait is implemented by top-level locations ([`Process`], [`Cluster`]) and
53/// by [`Tick`]. It is used to constrain APIs that should not be called from within
54/// an atomic block.
55#[sealed]
56pub trait NoAtomic {}
57#[sealed]
58impl<T> NoAtomic for Process<'_, T> {}
59#[sealed]
60impl<T> NoAtomic for Cluster<'_, T> {}
61#[sealed]
62impl<'a, L> NoAtomic for Tick<L> where L: Location<'a> {}
63
64/// A location wrapper that provides atomicity guarantees within a [`Tick`].
65///
66/// An `Atomic` context establishes a happens-before relationship between operations:
67/// - Downstream computations from `atomic()` are associated with an internal tick
68/// - Outputs from `end_atomic()` are held until all computations in the tick complete
69/// - Snapshots via `use::atomic` are guaranteed to reflect all updates from associated `end_atomic()`
70///
71/// This ensures read-after-write consistency: if a client receives an acknowledgement
72/// from `end_atomic()`, any subsequent `use::atomic` snapshot will include the effects
73/// of that acknowledged operation.
74#[derive(Clone)]
75pub struct Atomic<Loc> {
76    pub(crate) tick: Tick<Loc>,
77}
78
79impl<L: DynLocation> DynLocation for Atomic<L> {
80    fn id(&self) -> LocationId {
81        LocationId::Atomic(Box::new(self.tick.id()))
82    }
83
84    fn flow_state(&self) -> &FlowState {
85        self.tick.flow_state()
86    }
87
88    fn is_top_level() -> bool {
89        L::is_top_level()
90    }
91
92    fn multiversioned(&self) -> bool {
93        self.tick.multiversioned()
94    }
95}
96
97impl<'a, L> Location<'a> for Atomic<L>
98where
99    L: Location<'a>,
100{
101    type Root = L::Root;
102
103    fn root(&self) -> Self::Root {
104        self.tick.root()
105    }
106}
107
108#[sealed]
109impl<L> NoTick for Atomic<L> {}
110
111/// Trait for live collections that can be deferred by one tick.
112///
113/// When a collection implements `DeferTick`, calling `defer_tick` delays its
114/// values by one clock cycle. This is primarily used internally to implement
115/// tick-based cycles ([`Tick::cycle`]), ensuring that feedback loops advance
116/// by one tick to avoid infinite recursion within a single tick.
117pub trait DeferTick {
118    /// Returns a new collection whose values are delayed by one tick.
119    fn defer_tick(self) -> Self;
120}
121
122/// Marks the stream as being inside the single global clock domain.
123#[derive(Clone)]
124pub struct Tick<L> {
125    pub(crate) id: ClockId,
126    /// Location.
127    pub(crate) l: L,
128}
129
130impl<L: DynLocation> DynLocation for Tick<L> {
131    fn id(&self) -> LocationId {
132        LocationId::Tick(self.id, Box::new(self.l.id()))
133    }
134
135    fn flow_state(&self) -> &FlowState {
136        self.l.flow_state()
137    }
138
139    fn is_top_level() -> bool {
140        false
141    }
142
143    fn multiversioned(&self) -> bool {
144        self.l.multiversioned()
145    }
146}
147
148impl<'a, L> Location<'a> for Tick<L>
149where
150    L: Location<'a>,
151{
152    type Root = L::Root;
153
154    fn root(&self) -> Self::Root {
155        self.l.root()
156    }
157}
158
159impl<'a, L> Tick<L>
160where
161    L: Location<'a>,
162{
163    /// Returns a reference to the outer (parent) location that this tick is nested within.
164    ///
165    /// For example, if a `Tick` was created from a `Process`, this returns a reference
166    /// to that `Process`.
167    pub fn outer(&self) -> &L {
168        &self.l
169    }
170
171    /// Creates a bounded stream of `()` values inside this tick, with a fixed batch size.
172    ///
173    /// This is useful for driving computations inside a tick that need to process
174    /// a specific number of elements per tick. Each tick will produce exactly
175    /// `batch_size` unit values.
176    pub fn spin_batch(
177        &self,
178        batch_size: impl QuotedWithContext<'a, usize, L> + Copy + 'a,
179    ) -> Stream<(), Self, Bounded, TotalOrder, ExactlyOnce>
180    where
181        L: NoTick,
182    {
183        let out = self
184            .l
185            .spin()
186            .flat_map_ordered(q!(move |_| 0..batch_size))
187            .map(q!(|_| ()));
188
189        out.batch(self, nondet!(/** at runtime, `spin` produces a single value per tick, so each batch is guaranteed to be the same size. */))
190    }
191
192    /// Constructs a [`Singleton`] materialized inside this tick with the given static value.
193    ///
194    /// The singleton will have the provided value on every tick. This is useful
195    /// for providing constant values to computations inside a tick.
196    ///
197    /// See also: [`Location::singleton`], for creating a singleton _not_ inside a tick.
198    pub fn singleton<T>(
199        &self,
200        e: impl QuotedWithContext<'a, T, Tick<L>>,
201    ) -> Singleton<T, Self, Bounded>
202    where
203        T: Clone,
204    {
205        let e = e.splice_untyped_ctx(self);
206
207        Singleton::new(
208            self.clone(),
209            HydroNode::SingletonSource {
210                value: e.into(),
211                first_tick_only: false,
212                metadata: self.new_node_metadata(Singleton::<T, Self, Bounded>::collection_kind()),
213            },
214        )
215    }
216
217    /// Creates an [`Optional`] which has a null value on every tick.
218    ///
219    /// # Example
220    /// ```rust
221    /// # #[cfg(feature = "deploy")] {
222    /// # use hydro_lang::prelude::*;
223    /// # use futures::StreamExt;
224    /// # tokio_test::block_on(hydro_lang::test_util::stream_transform_test(|process| {
225    /// let tick = process.tick();
226    /// let optional = tick.none::<i32>();
227    /// optional.unwrap_or(tick.singleton(q!(123)))
228    /// # .all_ticks()
229    /// # }, |mut stream| async move {
230    /// // 123
231    /// # assert_eq!(stream.next().await.unwrap(), 123);
232    /// # }));
233    /// # }
234    /// ```
235    pub fn none<T>(&self) -> Optional<T, Self, Bounded> {
236        let e = q!([]);
237        let e = QuotedWithContext::<'a, [(); 0], Self>::splice_typed_ctx(e, self);
238
239        let unit_optional: Optional<(), Self, Bounded> = Optional::new(
240            self.clone(),
241            HydroNode::Source {
242                source: HydroSource::Iter(e.into()),
243                metadata: self.new_node_metadata(Optional::<(), Self, Bounded>::collection_kind()),
244            },
245        );
246
247        unit_optional.map(q!(|_| unreachable!())) // always empty
248    }
249
250    /// Creates an [`Optional`] which will have the provided static value on the first tick, and be
251    /// null on all subsequent ticks.
252    ///
253    /// This is useful for bootstrapping stateful computations which need an initial value.
254    ///
255    /// # Example
256    /// ```rust
257    /// # #[cfg(feature = "deploy")] {
258    /// # use hydro_lang::prelude::*;
259    /// # use futures::StreamExt;
260    /// # tokio_test::block_on(hydro_lang::test_util::stream_transform_test(|process| {
261    /// let tick = process.tick();
262    /// // ticks are lazy by default, forces the second tick to run
263    /// tick.spin_batch(q!(1)).all_ticks().for_each(q!(|_| {}));
264    /// let optional = tick.optional_first_tick(q!(5));
265    /// optional.unwrap_or(tick.singleton(q!(123))).all_ticks()
266    /// # }, |mut stream| async move {
267    /// // 5, 123, 123, 123, ...
268    /// # assert_eq!(stream.next().await.unwrap(), 5);
269    /// # assert_eq!(stream.next().await.unwrap(), 123);
270    /// # assert_eq!(stream.next().await.unwrap(), 123);
271    /// # assert_eq!(stream.next().await.unwrap(), 123);
272    /// # }));
273    /// # }
274    /// ```
275    pub fn optional_first_tick<T: Clone>(
276        &self,
277        e: impl QuotedWithContext<'a, T, Tick<L>>,
278    ) -> Optional<T, Self, Bounded> {
279        let e = e.splice_untyped_ctx(self);
280
281        Optional::new(
282            self.clone(),
283            HydroNode::SingletonSource {
284                value: e.into(),
285                first_tick_only: true,
286                metadata: self.new_node_metadata(Optional::<T, Self, Bounded>::collection_kind()),
287            },
288        )
289    }
290
291    /// Creates a feedback cycle within this tick for implementing iterative computations.
292    ///
293    /// Returns a handle that must be completed with the actual collection, and a placeholder
294    /// collection that represents the output of the previous tick (deferred by one tick).
295    /// This is useful for implementing fixed-point computations where the output of one
296    /// tick feeds into the input of the next.
297    ///
298    /// The cycle automatically defers values by one tick to prevent infinite recursion.
299    #[expect(
300        private_bounds,
301        reason = "only Hydro collections can implement ReceiverComplete"
302    )]
303    pub fn cycle<S>(&self) -> (TickCycleHandle<'a, S>, S)
304    where
305        S: CycleCollection<'a, TickCycle, Location = Self> + DeferTick,
306        L: NoTick,
307    {
308        let cycle_id = self.flow_state().borrow_mut().next_cycle_id();
309        (
310            TickCycleHandle::new(cycle_id, Location::id(self)),
311            S::create_source(cycle_id, self.clone()).defer_tick(),
312        )
313    }
314
315    /// Creates a feedback cycle with an initial value for the first tick.
316    ///
317    /// Similar to [`Tick::cycle`], but allows providing an initial collection
318    /// that will be used as the value on the first tick before any feedback
319    /// is available. This is useful for bootstrapping iterative computations
320    /// that need a starting state.
321    #[expect(
322        private_bounds,
323        reason = "only Hydro collections can implement ReceiverComplete"
324    )]
325    pub fn cycle_with_initial<S>(&self, initial: S) -> (TickCycleHandle<'a, S>, S)
326    where
327        S: CycleCollectionWithInitial<'a, TickCycle, Location = Self>,
328    {
329        let cycle_id = self.flow_state().borrow_mut().next_cycle_id();
330        (
331            TickCycleHandle::new(cycle_id, Location::id(self)),
332            // no need to defer_tick, create_source_with_initial does it for us
333            S::create_source_with_initial(cycle_id, initial, self.clone()),
334        )
335    }
336}
337
338#[cfg(test)]
339mod tests {
340    #[cfg(feature = "sim")]
341    use stageleft::q;
342
343    #[cfg(feature = "sim")]
344    use crate::live_collections::sliced::sliced;
345    #[cfg(feature = "sim")]
346    use crate::location::Location;
347    #[cfg(feature = "sim")]
348    use crate::nondet::nondet;
349    #[cfg(feature = "sim")]
350    use crate::prelude::FlowBuilder;
351
352    #[cfg(feature = "sim")]
353    #[test]
354    fn sim_atomic_stream() {
355        let mut flow = FlowBuilder::new();
356        let node = flow.process::<()>();
357
358        let (write_send, write_req) = node.sim_input();
359        let (read_send, read_req) = node.sim_input::<(), _, _>();
360
361        let atomic_write = write_req.atomic();
362        let current_state = atomic_write.clone().fold(
363            q!(|| 0),
364            q!(|state: &mut i32, v: i32| {
365                *state += v;
366            }),
367        );
368
369        let write_ack_recv = atomic_write.end_atomic().sim_output();
370        let read_response_recv = sliced! {
371            let batch_of_req = use(read_req, nondet!(/** test */));
372            let latest_singleton = use::atomic(current_state, nondet!(/** test */));
373            batch_of_req.cross_singleton(latest_singleton)
374        }
375        .sim_output();
376
377        let sim_compiled = flow.sim().compiled();
378        let instances = sim_compiled.exhaustive(async || {
379            write_send.send(1);
380            write_ack_recv.assert_yields([1]).await;
381            read_send.send(());
382            assert!(read_response_recv.next().await.is_some_and(|(_, v)| v >= 1));
383        });
384
385        assert_eq!(instances, 1);
386
387        let instances_read_before_write = sim_compiled.exhaustive(async || {
388            write_send.send(1);
389            read_send.send(());
390            write_ack_recv.assert_yields([1]).await;
391            let _ = read_response_recv.next().await;
392        });
393
394        assert_eq!(instances_read_before_write, 3); // read before write, write before read, both in same tick
395    }
396
397    #[cfg(feature = "sim")]
398    #[test]
399    #[should_panic]
400    fn sim_non_atomic_stream() {
401        // shows that atomic is necessary
402        let mut flow = FlowBuilder::new();
403        let node = flow.process::<()>();
404
405        let (write_send, write_req) = node.sim_input();
406        let (read_send, read_req) = node.sim_input::<(), _, _>();
407
408        let current_state = write_req.clone().fold(
409            q!(|| 0),
410            q!(|state: &mut i32, v: i32| {
411                *state += v;
412            }),
413        );
414
415        let write_ack_recv = write_req.sim_output();
416
417        let read_response_recv = sliced! {
418            let batch_of_req = use(read_req, nondet!(/** test */));
419            let latest_singleton = use(current_state, nondet!(/** test */));
420            batch_of_req.cross_singleton(latest_singleton)
421        }
422        .sim_output();
423
424        flow.sim().exhaustive(async || {
425            write_send.send(1);
426            write_ack_recv.assert_yields([1]).await;
427            read_send.send(());
428
429            if let Some((_, v)) = read_response_recv.next().await {
430                assert_eq!(v, 1);
431            }
432        });
433    }
434}
hydro_lang/location/tick.rs

hydro_lang/location/
tick.rs
