timely 0.2.0

A low-latency data-parallel dataflow system in Rust
Documentation 
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//! General purpose intra-timestamp aggregation
use std::hash::Hash;
use std::collections::HashMap;

use ::{Data, ExchangeData};
use dataflow::{Stream, Scope};
use dataflow::operators::unary::Unary;
use dataflow::channels::pact::Exchange;

/// Generic intra-timestamp aggregation
/// 
/// Extension method supporting aggregation of keyed data within timestamp. 
/// For inter-timestamp aggregation, consider `StateMachine`.
pub trait Aggregate<S: Scope, K: ExchangeData+Hash, V: ExchangeData> {
	/// Aggregates data of the form `(key, val)`, using user-supplied logic.
	///
	/// The `aggregate` method is implemented for streams of `(K, V)` data, 
	/// and takes functions `fold`, `emit`, and `hash`; used to combine new `V` 
	/// data with existing `D` state, to produce `R` output from `D` state, and 
	/// to route `K` keys, respectively.
	///
	/// Aggregation happens within each time, and results are produced once the 
	/// time is complete.
	///
    /// #Examples
    /// ```
    /// use timely::dataflow::operators::{ToStream, Map, Inspect};
    /// use timely::dataflow::operators::aggregation::Aggregate;
    ///
    ///	timely::example(|scope| {
	///
    ///		(0..10).to_stream(scope)
    ///			   .map(|x| (x % 2, x))
    ///			   .aggregate(
    ///			   	   |_key, val, agg| { *agg += val; }, 
    ///                |key, agg: i32| (key, agg), 
    ///                |key| *key as u64
    ///            )
    ///			   .inspect(|x| assert!(*x == (0, 20) || *x == (1, 25)));
    /// });
    /// ```
    ///
    /// By changing the type of the aggregate value, one can accumulate into different types.
    /// Here we accumulate the data into a `Vec<i32>` and report its length (which we could 
    /// obviously do more efficiently; imagine we were doing a hash instead).
    /// 	
    /// ```
    /// use timely::dataflow::operators::{ToStream, Map, Inspect};
    /// use timely::dataflow::operators::aggregation::Aggregate;
    ///
    ///	timely::example(|scope| {
	///
    ///		(0..10).to_stream(scope)
    ///			   .map(|x| (x % 2, x))
    ///            .aggregate::<_,Vec<i32>,_,_,_>(
    ///                |_key, val, agg| { agg.push(val); }, 
    ///                |key, agg| (key, agg.len()), 
    ///                |key| *key as u64
    ///            )
    ///            .inspect(|x| assert!(*x == (0, 5) || *x == (1, 5)));
    /// });
    /// ```	
    fn aggregate<
		R: Data, 
		D: Default+'static, 
		F: Fn(&K, V, &mut D)+'static, 
		G: Fn(K, D)->R+'static,
		H: Fn(&K)->u64+'static,
	>(&self, fold: F, emit: G, hash: H) -> Stream<S, R> where S::Timestamp : Hash+Eq;
} 

impl<S: Scope, K: ExchangeData+Hash+Eq, V: ExchangeData> Aggregate<S, K, V> for Stream<S, (K, V)> {

		fn aggregate<
			R: Data, 
			D: Default+'static, 
			F: Fn(&K, V, &mut D)+'static, 
			G: Fn(K, D)->R+'static,
			H: Fn(&K)->u64+'static,
		>(&self, fold: F, emit: G, hash: H) -> Stream<S, R> where S::Timestamp : Hash+Eq {

		let mut aggregates = HashMap::new(); 

		self.unary_notify(Exchange::new(move |&(ref k, _)| hash(k)), "Aggregate", vec![], move |input, output, notificator| {

			// read each input, fold into aggregates
			input.for_each(|time, data| {
				let agg_time = aggregates.entry(time.time().clone()).or_insert_with(HashMap::new);
				for (key, val) in data.drain(..) {
					let agg = agg_time.entry(key.clone()).or_insert_with(Default::default);
					fold(&key, val, agg);
				}
				notificator.notify_at(time);
			});

			// pop completed aggregates, send along whatever
			notificator.for_each(|time,_,_| {
				if let Some(aggs) = aggregates.remove(&time.time()) {
					let mut session = output.session(&time);
					for (key, agg) in aggs {
						session.give(emit(key, agg));
					}
				}
			});
		})

	}
}