petgraph/algo/
tred.rs

1//! Compute the transitive reduction and closure of a directed acyclic graph
2//!
3//! ## Transitive reduction and closure
4//! The *transitive closure* of a graph **G = (V, E)** is the graph **Gc = (V, Ec)**
5//! such that **(i, j)** belongs to **Ec** if and only if there is a path connecting
6//! **i** to **j** in **G**. The *transitive reduction* of **G** is the graph **Gr
7//! = (V, Er)** such that **Er** is minimal wrt. inclusion in **E** and the transitive
8//! closure of **Gr** is the same as that of **G**.
9//! The transitive reduction is well-defined for acyclic graphs only.
10
11use crate::adj::{List, UnweightedList};
12use crate::graph::IndexType;
13use crate::visit::{
14    GraphBase, IntoNeighbors, IntoNeighborsDirected, NodeCompactIndexable, NodeCount,
15};
16use crate::Direction;
17use fixedbitset::FixedBitSet;
18
19/// Creates a representation of the same graph respecting topological order for use in `tred::dag_transitive_reduction_closure`.
20///
21/// `toposort` must be a topological order on the node indices of `g` (for example obtained
22/// from [`toposort`]).
23///
24/// [`toposort`]: ../fn.toposort.html
25///
26/// Returns a pair of a graph `res` and the reciprocal of the topological sort `revmap`.
27///
28/// `res` is the same graph as `g` with the following differences:
29/// * Node and edge weights are stripped,
30/// * Node indices are replaced by the corresponding rank in `toposort`,
31/// * Iterating on the neighbors of a node respects topological order.
32///
33/// `revmap` is handy to get back to map indices in `g` to indices in `res`.
34/// ```
35/// use petgraph::prelude::*;
36/// use petgraph::graph::DefaultIx;
37/// use petgraph::visit::IntoNeighbors;
38/// use petgraph::algo::tred::dag_to_toposorted_adjacency_list;
39///
40/// let mut g = Graph::<&str, (), Directed, DefaultIx>::new();
41/// let second = g.add_node("second child");
42/// let top = g.add_node("top");
43/// let first = g.add_node("first child");
44/// g.extend_with_edges(&[(top, second), (top, first), (first, second)]);
45///
46/// let toposort = vec![top, first, second];
47///
48/// let (res, revmap) = dag_to_toposorted_adjacency_list(&g, &toposort);
49///
50/// // let's compute the children of top in topological order
51/// let children: Vec<NodeIndex> = res
52///     .neighbors(revmap[top.index()])
53///     .map(|ix: NodeIndex| toposort[ix.index()])
54///     .collect();
55/// assert_eq!(children, vec![first, second])
56/// ```
57///
58/// Runtime: **O(|V| + |E|)**.
59///
60/// Space complexity: **O(|V| + |E|)**.
61pub fn dag_to_toposorted_adjacency_list<G, Ix: IndexType>(
62    g: G,
63    toposort: &[G::NodeId],
64) -> (UnweightedList<Ix>, Vec<Ix>)
65where
66    G: GraphBase + IntoNeighborsDirected + NodeCompactIndexable + NodeCount,
67    G::NodeId: IndexType,
68{
69    let mut res = List::with_capacity(g.node_count());
70    // map from old node index to rank in toposort
71    let mut revmap = vec![Ix::default(); g.node_bound()];
72    for (ix, &old_ix) in toposort.iter().enumerate() {
73        let ix = Ix::new(ix);
74        revmap[old_ix.index()] = ix;
75        let iter = g.neighbors_directed(old_ix, Direction::Incoming);
76        let new_ix: Ix = res.add_node_with_capacity(iter.size_hint().0);
77        debug_assert_eq!(new_ix.index(), ix.index());
78        for old_pre in iter {
79            let pre: Ix = revmap[old_pre.index()];
80            res.add_edge(pre, ix, ());
81        }
82    }
83    (res, revmap)
84}
85
86/// Computes the transitive reduction and closure of a DAG.
87///
88/// The algorithm implemented here comes from [On the calculation of
89/// transitive reduction-closure of
90/// orders](https://www.sciencedirect.com/science/article/pii/0012365X9390164O) by Habib, Morvan
91/// and Rampon.
92///
93/// The input graph must be in a very specific format: an adjacency
94/// list such that:
95/// * Node indices are a toposort, and
96/// * The neighbors of all nodes are stored in topological order.
97/// To get such a representation, use the function [`dag_to_toposorted_adjacency_list`].
98///
99/// [`dag_to_toposorted_adjacency_list`]: ./fn.dag_to_toposorted_adjacency_list.html
100///
101/// The output is the pair of the transitive reduction and the transitive closure.
102///
103/// Runtime complexity: **O(|V| + \sum_{(x, y) \in Er} d(y))** where **d(y)**
104/// denotes the outgoing degree of **y** in the transitive closure of **G**.
105/// This is still **O(|V|³)** in the worst case like the naive algorithm but
106/// should perform better for some classes of graphs.
107///
108/// Space complexity: **O(|E|)**.
109pub fn dag_transitive_reduction_closure<E, Ix: IndexType>(
110    g: &List<E, Ix>,
111) -> (UnweightedList<Ix>, UnweightedList<Ix>) {
112    let mut tred = List::with_capacity(g.node_count());
113    let mut tclos = List::with_capacity(g.node_count());
114    let mut mark = FixedBitSet::with_capacity(g.node_count());
115    for i in g.node_indices() {
116        tred.add_node();
117        tclos.add_node_with_capacity(g.neighbors(i).len());
118    }
119    // the algorithm relies on this iterator being toposorted
120    for i in g.node_indices().rev() {
121        // the algorighm relies on this iterator being toposorted
122        for x in g.neighbors(i) {
123            if !mark[x.index()] {
124                tred.add_edge(i, x, ());
125                tclos.add_edge(i, x, ());
126                for e in tclos.edge_indices_from(x) {
127                    let y = tclos.edge_endpoints(e).unwrap().1;
128                    if !mark[y.index()] {
129                        mark.insert(y.index());
130                        tclos.add_edge(i, y, ());
131                    }
132                }
133            }
134        }
135        for y in tclos.neighbors(i) {
136            mark.set(y.index(), false);
137        }
138    }
139    (tred, tclos)
140}
141
142#[cfg(test)]
143#[test]
144fn test_easy_tred() {
145    let mut input = List::new();
146    let a: u8 = input.add_node();
147    let b = input.add_node();
148    let c = input.add_node();
149    input.add_edge(a, b, ());
150    input.add_edge(a, c, ());
151    input.add_edge(b, c, ());
152    let (tred, tclos) = dag_transitive_reduction_closure(&input);
153    assert_eq!(tred.node_count(), 3);
154    assert_eq!(tclos.node_count(), 3);
155    assert!(tred.find_edge(a, b).is_some());
156    assert!(tred.find_edge(b, c).is_some());
157    assert!(tred.find_edge(a, c).is_none());
158    assert!(tclos.find_edge(a, b).is_some());
159    assert!(tclos.find_edge(b, c).is_some());
160    assert!(tclos.find_edge(a, c).is_some());
161}