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```
```// Copyright 2018-2019 Joe Neeman.
//
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// of this distribution.

use std::collections::{HashMap, HashSet};

use crate::Graph;

/// A partition of a the nodes of a graph.
///
/// Tarjan's algorithm decomposes a directed graph into strongly connected components.  Moreover,
/// those components are ordered topologically.
pub struct Partition<G: Graph + ?Sized> {
pub(crate) sets: Vec<HashSet<G::Node>>,
node_map: HashMap<G::Node, usize>,
edges: HashMap<usize, Vec<usize>>,
back_edges: HashMap<usize, Vec<usize>>,
}

impl<G: Graph + ?Sized> Partition<G> {
pub(crate) fn new(g: &G, sets: Vec<HashSet<G::Node>>) -> Partition<G> {
let mut node_map = HashMap::new();
for (i, component) in sets.iter().enumerate() {
for u in component {
node_map.insert(*u, i);
}
}

let mut edges = (0..sets.len())
.map(|u| (u, Vec::new()))
.collect::<HashMap<_, _>>();
let mut back_edges = (0..sets.len())
.map(|u| (u, Vec::new()))
.collect::<HashMap<_, _>>();
for u in g.nodes() {
let u_idx = node_map[&u];
for v in g.out_neighbors(&u) {
let v_idx = node_map[&v];

if u_idx != v_idx {
edges.get_mut(&u_idx).unwrap().push(v_idx);
back_edges.get_mut(&v_idx).unwrap().push(u_idx);
}
}
}
Partition {
sets,
node_map,
edges,
back_edges,
}
}

pub fn num_components(&self) -> usize {
self.sets.len()
}

pub fn parts<'b>(&'b self) -> impl Iterator<Item = &'b HashSet<G::Node>> {
self.sets.iter()
}

pub fn part(&self, i: usize) -> &HashSet<G::Node> {
&self.sets[i]
}

pub fn index_of(&self, u: &G::Node) -> usize {
self.node_map[&u]
}

pub fn into_parts(self) -> Vec<HashSet<G::Node>> {
self.sets
}
}

impl<G: Graph + ?Sized> Graph for Partition<G> {
type Node = usize;
type Edge = usize;

fn nodes<'a>(&'a self) -> Box<dyn Iterator<Item = usize>> {
Box::new(0..self.num_components())
}

fn out_edges<'a>(&'a self, u: &usize) -> Box<dyn Iterator<Item = usize> + 'a> {
Box::new(self.edges[&*u].iter().cloned())
}

fn in_edges<'a>(&'a self, u: &usize) -> Box<dyn Iterator<Item = usize> + 'a> {
Box::new(self.back_edges[&*u].iter().cloned())
}
}
```