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//! Aggregates the weights of equal records into at most one record.
//!
//! As differential dataflow streams are unordered and taken to be the accumulation of all records,
//! no semantic change happens via `consolidate`. However, there is a practical difference between
//! a collection that aggregates down to zero records, and one that actually has no records. The
//! underlying system can more clearly see that no work must be done in the later case, and we can
//! drop out of, e.g. iterative computations.

use timely::dataflow::Scope;

use ::{Collection, ExchangeData, Hashable};
use ::difference::Semigroup;
use operators::arrange::arrangement::Arrange;

/// An extension method for consolidating weighted streams.
pub trait Consolidate<D: ExchangeData+Hashable> : Sized {
    /// Aggregates the weights of equal records into at most one record.
    ///
    /// This method uses the type `D`'s `hashed()` method to partition the data. The data are
    /// accumulated in place, each held back until their timestamp has completed.
    ///
    /// # Examples
    ///
    /// ```
    /// extern crate timely;
    /// extern crate differential_dataflow;
    ///
    /// use differential_dataflow::input::Input;
    /// use differential_dataflow::operators::Consolidate;
    ///
    /// fn main() {
    ///     ::timely::example(|scope| {
    ///
    ///         let x = scope.new_collection_from(1 .. 10u32).1;
    ///
    ///         x.negate()
    ///          .concat(&x)
    ///          .consolidate() // <-- ensures cancellation occurs
    ///          .assert_empty();
    ///     });
    /// }
    /// ```
    fn consolidate(&self) -> Self {
        self.consolidate_named("Consolidate")
    }

    /// As `consolidate` but with the ability to name the operator.
    fn consolidate_named(&self, name: &str) -> Self;
}

impl<G: Scope, D, R> Consolidate<D> for Collection<G, D, R>
where
    D: ExchangeData+Hashable,
    R: ExchangeData+Semigroup,
    G::Timestamp: ::lattice::Lattice+Ord,
 {
    fn consolidate_named(&self, name: &str) -> Self {
        use trace::implementations::ord::OrdKeySpine as DefaultKeyTrace;
        self.map(|k| (k, ()))
            .arrange_named::<DefaultKeyTrace<_,_,_>>(name)
            .as_collection(|d: &D, _| d.clone())
    }
}

/// An extension method for consolidating weighted streams.
pub trait ConsolidateStream<D: ExchangeData+Hashable> {
    /// Aggregates the weights of equal records.
    ///
    /// Unlike `consolidate`, this method does not exchange data and does not
    /// ensure that at most one copy of each `(data, time)` pair exists in the
    /// results. Instead, it acts on each batch of data and collapses equivalent
    /// `(data, time)` pairs found therein, suppressing any that accumulate to
    /// zero.
    ///
    /// # Examples
    ///
    /// ```
    /// extern crate timely;
    /// extern crate differential_dataflow;
    ///
    /// use differential_dataflow::input::Input;
    /// use differential_dataflow::operators::consolidate::ConsolidateStream;
    ///
    /// fn main() {
    ///     ::timely::example(|scope| {
    ///
    ///         let x = scope.new_collection_from(1 .. 10u32).1;
    ///
    ///         // nothing to assert, as no particular guarantees.
    ///         x.negate()
    ///          .concat(&x)
    ///          .consolidate_stream();
    ///     });
    /// }
    /// ```
    fn consolidate_stream(&self) -> Self;
}

impl<G: Scope, D, R> ConsolidateStream<D> for Collection<G, D, R>
where
    D: ExchangeData+Hashable,
    R: ExchangeData+Semigroup,
    G::Timestamp: ::lattice::Lattice+Ord,
 {
    fn consolidate_stream(&self) -> Self {

        use timely::dataflow::channels::pact::Pipeline;
        use timely::dataflow::operators::Operator;
        use collection::AsCollection;

        self.inner
            .unary(Pipeline, "ConsolidateStream", |_cap, _info| {

                let mut vector = Vec::new();
                move |input, output| {
                    input.for_each(|time, data| {
                        data.swap(&mut vector);
                        crate::consolidation::consolidate_updates(&mut vector);
                        output.session(&time).give_vec(&mut vector);
                    })
                }
            })
            .as_collection()
    }
}