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//! Assign unique identifiers to records.
use timely::dataflow::Scope;
use crate::{Collection, ExchangeData, Hashable};
use crate::lattice::Lattice;
use crate::operators::*;
use crate::difference::Abelian;
/// Assign unique identifiers to elements of a collection.
pub trait Identifiers<G: Scope, D: ExchangeData, R: ExchangeData+Abelian> {
/// Assign unique identifiers to elements of a collection.
///
/// # Example
/// ```
/// use differential_dataflow::input::Input;
/// use differential_dataflow::algorithms::identifiers::Identifiers;
/// use differential_dataflow::operators::Threshold;
///
/// ::timely::example(|scope| {
///
/// let identifiers =
/// scope.new_collection_from(1 .. 10).1
/// .identifiers()
/// // assert no conflicts
/// .map(|(data, id)| id)
/// .threshold(|_id,cnt| if cnt > &1 { *cnt } else { 0 })
/// .assert_empty();
/// });
/// ```
fn identifiers(&self) -> Collection<G, (D, u64), R>;
}
impl<G, D, R> Identifiers<G, D, R> for Collection<G, D, R>
where
G: Scope<Timestamp: Lattice>,
D: ExchangeData + ::std::hash::Hash,
R: ExchangeData + Abelian,
{
fn identifiers(&self) -> Collection<G, (D, u64), R> {
// The design here is that we iteratively develop a collection
// of pairs (round, record), where each pair is a proposal that
// the hash for record should be (round, record).hashed().
//
// Iteratively, any colliding pairs establish a winner (the one
// with the lower round, breaking ties by record), and indicate
// that the losers should increment their round and try again.
//
// Non-obviously, this happens via a `reduce` operator that yields
// additions and subtractions of losers, rather than reproducing
// the winners. This is done under the premise that losers are
// very rare, and maintaining winners in both the input and output
// of `reduce` is an unnecessary duplication.
use crate::collection::AsCollection;
let init = self.map(|record| (0, record));
timely::dataflow::operators::generic::operator::empty(&init.scope())
.as_collection()
.iterate(|diff|
init.enter(&diff.scope())
.concat(diff)
.map(|pair| (pair.hashed(), pair))
.reduce(|_hash, input, output| {
// keep round-positive records as changes.
let ((round, record), count) = &input[0];
if *round > 0 {
let mut neg_count = count.clone();
neg_count.negate();
output.push(((0, record.clone()), neg_count));
output.push(((*round, record.clone()), count.clone()));
}
// if any losers, increment their rounds.
for ((round, record), count) in input[1..].iter() {
let mut neg_count = count.clone();
neg_count.negate();
output.push(((0, record.clone()), neg_count));
output.push(((*round+1, record.clone()), count.clone()));
}
})
.map(|(_hash, pair)| pair)
)
.concat(&init)
.map(|pair| { let hash = pair.hashed(); (pair.1, hash) })
}
}
#[cfg(test)]
mod tests {
#[test]
fn are_unique() {
// It is hard to test the above method, because we would want
// to exercise the case with hash collisions. Instead, we test
// a version with a crippled hash function to see that even if
// there are collisions, everyone gets a unique identifier.
use crate::input::Input;
use crate::operators::{Threshold, Reduce};
use crate::operators::iterate::Iterate;
::timely::example(|scope| {
let input = scope.new_collection_from(1 .. 4).1;
use crate::collection::AsCollection;
let init = input.map(|record| (0, record));
timely::dataflow::operators::generic::operator::empty(&init.scope())
.as_collection()
.iterate(|diff|
init.enter(&diff.scope())
.concat(&diff)
.map(|(round, num)| ((round + num) / 10, (round, num)))
.reduce(|_hash, input, output| {
println!("Input: {:?}", input);
// keep round-positive records as changes.
let ((round, record), count) = &input[0];
if *round > 0 {
output.push(((0, record.clone()), -*count));
output.push(((*round, record.clone()), *count));
}
// if any losers, increment their rounds.
for ((round, record), count) in input[1..].iter() {
output.push(((0, record.clone()), -*count));
output.push(((*round+1, record.clone()), *count));
}
})
.inspect(|x| println!("{:?}", x))
.map(|(_hash, pair)| pair)
)
.concat(&init)
.map(|(round, num)| { (num, (round + num) / 10) })
.map(|(_data, id)| id)
.threshold(|_id,cnt| if cnt > &1 { *cnt } else { 0 })
.assert_empty();
});
}
}