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use visit::IntoNeighbors;
use visit::{VisitMap, Visitable};
/// Strictly monotonically increasing event time for a depth first search.
#[derive(Copy, Clone, Debug, PartialEq, PartialOrd, Eq, Ord, Default, Hash)]
pub struct Time(pub usize);
/// A depth first search (DFS) visitor event.
#[derive(Copy, Clone, Debug)]
pub enum DfsEvent<N> {
Discover(N, Time),
/// An edge of the tree formed by the traversal.
TreeEdge(N, N),
/// An edge to an already visited node.
BackEdge(N, N),
/// A cross or forward edge.
///
/// For an edge *(u, v)*, if the discover time of *v* is greater than *u*,
/// then it is a forward edge, else a cross edge.
CrossForwardEdge(N, N),
Finish(N, Time),
}
/// Return if the expression is a break value.
macro_rules! try_control {
($e:expr) => {
match $e {
x => if x.should_break() {
return x;
}
}
}
}
/// Control flow for callbacks.
///
/// `Break` can carry a value.
#[derive(Copy, Clone, Debug)]
pub enum Control<B> {
Continue,
Break(B),
}
impl<B> Control<B> {
pub fn breaking() -> Control<()> { Control::Break(()) }
/// Get the value in `Control::Break(_)`, if present.
pub fn break_value(self) -> Option<B> {
match self {
Control::Continue => None,
Control::Break(b) => Some(b),
}
}
}
/// Control flow for callbacks.
///
/// The empty return value `()` is equivalent to continue.
pub trait ControlFlow {
fn continuing() -> Self;
fn should_break(&self) -> bool;
}
impl ControlFlow for () {
fn continuing() { }
#[inline]
fn should_break(&self) -> bool { false }
}
impl<B> ControlFlow for Control<B> {
fn continuing() -> Self { Control::Continue }
fn should_break(&self) -> bool {
if let Control::Break(_) = *self { true } else { false }
}
}
impl<E> ControlFlow for Result<(), E> {
fn continuing() -> Self { Ok(()) }
fn should_break(&self) -> bool {
self.is_err()
}
}
/// The default is `Continue`.
impl<B> Default for Control<B> {
fn default() -> Self { Control::Continue }
}
/// A recursive depth first search.
///
/// Starting points are the nodes in the iterator `starts` (specify just one
/// start vertex *x* by using `Some(x)`).
///
/// The traversal emits discovery and finish events for each reachable vertex,
/// and edge classification of each reachable edge. `visitor` is called for each
/// event, see [`DfsEvent`][de] for possible values.
///
/// If the return value of the visitor is simply `()`, the visit runs until it
/// is finished. If the return value is a `Control<B>`, it can be used to
/// break the visit early, and the last control value is returned by the
/// function.
///
/// [de]: enum.DfsEvent.html
///
/// # Example
///
/// Find a path from vertex 0 to 5, and exit the visit as soon as we reach
/// the goal vertex.
///
/// ```
/// use petgraph::prelude::*;
/// use petgraph::graph::node_index as n;
/// use petgraph::visit::depth_first_search;
/// use petgraph::visit::{DfsEvent, Control};
///
/// let gr: Graph<(), ()> = Graph::from_edges(&[
/// (0, 1), (0, 2), (0, 3),
/// (1, 3),
/// (2, 3), (2, 4),
/// (4, 0), (4, 5),
/// ]);
///
/// // record each predecessor, mapping node → node
/// let mut predecessor = vec![NodeIndex::end(); gr.node_count()];
/// let start = n(0);
/// let goal = n(5);
/// depth_first_search(&gr, Some(start), |event| {
/// if let DfsEvent::TreeEdge(u, v) = event {
/// predecessor[v.index()] = u;
/// if v == goal {
/// return Control::Break(v);
/// }
/// }
/// Control::Continue
/// });
///
/// let mut next = goal;
/// let mut path = vec![next];
/// while next != start {
/// let pred = predecessor[next.index()];
/// path.push(pred);
/// next = pred;
/// }
/// path.reverse();
/// assert_eq!(&path, &[n(0), n(2), n(4), n(5)]);
/// ```
pub fn depth_first_search<G, I, F, C>(graph: G, starts: I, mut visitor: F) -> C
where G: IntoNeighbors + Visitable,
I: IntoIterator<Item=G::NodeId>,
F: FnMut(DfsEvent<G::NodeId>) -> C,
C: ControlFlow,
{
let time = &mut Time(0);
let discovered = &mut graph.visit_map();
let finished = &mut graph.visit_map();
for start in starts {
try_control!(dfs_visitor(graph, start, &mut visitor, discovered, finished, time));
}
C::continuing()
}
fn dfs_visitor<G, F, C>(graph: G, u: G::NodeId, visitor: &mut F,
discovered: &mut G::Map, finished: &mut G::Map,
time: &mut Time) -> C
where G: IntoNeighbors + Visitable,
F: FnMut(DfsEvent<G::NodeId>) -> C,
C: ControlFlow,
{
if !discovered.visit(u) {
return C::continuing();
}
try_control!(visitor(DfsEvent::Discover(u, time_post_inc(time))));
for v in graph.neighbors(u) {
if !discovered.is_visited(&v) {
try_control!(visitor(DfsEvent::TreeEdge(u, v)));
try_control!(dfs_visitor(graph, v, visitor, discovered, finished, time));
} else if !finished.is_visited(&v) {
try_control!(visitor(DfsEvent::BackEdge(u, v)));
} else {
try_control!(visitor(DfsEvent::CrossForwardEdge(u, v)));
}
}
let first_finish = finished.visit(u);
debug_assert!(first_finish);
try_control!(visitor(DfsEvent::Finish(u, time_post_inc(time))));
C::continuing()
}
fn time_post_inc(x: &mut Time) -> Time {
let v = *x;
x.0 += 1;
v
}