palimpsest-dataflow 0.1.1

//! Breadth-first distance labeling.

use std::hash::Hash;

use timely::dataflow::*;

use crate::lattice::Lattice;
use crate::operators::*;
use crate::{ExchangeData, VecCollection};

/// Returns pairs (node, dist) indicating distance of each node from a root.
pub fn bfs<G, N>(
    edges: &VecCollection<G, (N, N)>,
    roots: &VecCollection<G, N>,
) -> VecCollection<G, (N, u32)>
where
    G: Scope<Timestamp: Lattice + Ord>,
    N: ExchangeData + Hash,
{
    use crate::operators::arrange::arrangement::ArrangeByKey;
    let edges = edges.arrange_by_key();
    bfs_arranged(&edges, roots)
}

use crate::operators::arrange::Arranged;
use crate::trace::TraceReader;

/// Returns pairs (node, dist) indicating distance of each node from a root.
pub fn bfs_arranged<G, N, Tr>(
    edges: &Arranged<G, Tr>,
    roots: &VecCollection<G, N>,
) -> VecCollection<G, (N, u32)>
where
    G: Scope<Timestamp = Tr::Time>,
    N: ExchangeData + Hash,
    Tr: for<'a> TraceReader<Key<'a> = &'a N, Val<'a> = &'a N, Diff = isize> + Clone + 'static,
{
    // initialize roots as reaching themselves at distance 0
    let nodes = roots.map(|x| (x, 0));

    // repeatedly update minimal distances each node can be reached from each root
    nodes.iterate(|inner| {
        let edges = edges.enter(&inner.scope());
        let nodes = nodes.enter(&inner.scope());

        inner
            .join_core(&edges, |_k, l, d| Some((d.clone(), l + 1)))
            .concat(&nodes)
            .reduce(|_, s, t| t.push((*s[0].0, 1)))
    })
}