nit: improve SCC comments
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@ -177,9 +177,6 @@ where
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/// D' (i.e., D' < D), we know that N, N', and all nodes in
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/// D' (i.e., D' < D), we know that N, N', and all nodes in
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/// between them on the stack are part of an SCC.
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/// between them on the stack are part of an SCC.
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///
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///
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/// For each node, we track the lowest depth of any successor we
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/// have found, along with that
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///
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/// [wikipedia]: https://bit.ly/2EZIx84
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/// [wikipedia]: https://bit.ly/2EZIx84
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fn construct(graph: &'c G) -> Sccs<G::Node, S> {
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fn construct(graph: &'c G) -> Sccs<G::Node, S> {
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let num_nodes = graph.num_nodes();
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let num_nodes = graph.num_nodes();
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@ -213,6 +210,17 @@ where
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}
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}
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}
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}
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/// Visit a node during the DFS. We first examine its current
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/// state -- if it is not yet visited (`NotVisited`), we can push
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/// it onto the stack and start walking its successors.
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///
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/// If it is already on the DFS stack it will be in the state
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/// `BeingVisited`. In that case, we have found a cycle and we
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/// return the depth from the stack.
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///
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/// Otherwise, we are looking at a node that has already been
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/// completely visited. We therefore return `WalkReturn::Complete`
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/// with its associated SCC index.
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fn walk_node(&mut self, depth: usize, node: G::Node) -> WalkReturn<S> {
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fn walk_node(&mut self, depth: usize, node: G::Node) -> WalkReturn<S> {
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debug!("walk_node(depth = {:?}, node = {:?})", depth, node);
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debug!("walk_node(depth = {:?}, node = {:?})", depth, node);
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match self.find_state(node) {
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match self.find_state(node) {
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@ -276,6 +284,7 @@ where
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_ => false,
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_ => false,
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});
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});
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// Push `node` onto the stack.
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self.node_states[node] = NodeState::BeingVisited { depth };
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self.node_states[node] = NodeState::BeingVisited { depth };
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self.node_stack.push(node);
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self.node_stack.push(node);
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@ -294,6 +303,7 @@ where
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WalkReturn::Cycle {
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WalkReturn::Cycle {
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min_depth: successor_min_depth,
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min_depth: successor_min_depth,
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} => {
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} => {
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// Track the minimum depth we can reach.
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assert!(successor_min_depth <= depth);
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assert!(successor_min_depth <= depth);
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if successor_min_depth < min_depth {
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if successor_min_depth < min_depth {
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debug!(
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debug!(
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@ -308,6 +318,8 @@ where
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WalkReturn::Complete {
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WalkReturn::Complete {
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scc_index: successor_scc_index,
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scc_index: successor_scc_index,
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} => {
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} => {
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// Push the completed SCC indices onto
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// the `successors_stack` for later.
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debug!(
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debug!(
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"walk_unvisited_node: node = {:?} successor_scc_index = {:?}",
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"walk_unvisited_node: node = {:?} successor_scc_index = {:?}",
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node, successor_scc_index
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node, successor_scc_index
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@ -317,9 +329,12 @@ where
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}
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}
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}
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}
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// Completed walk, remove `node` from the stack.
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let r = self.node_stack.pop();
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let r = self.node_stack.pop();
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debug_assert_eq!(r, Some(node));
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debug_assert_eq!(r, Some(node));
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// If `min_depth == depth`, then we are the root of the
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// cycle: we can't reach anyone further down the stack.
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if min_depth == depth {
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if min_depth == depth {
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// Note that successor stack may have duplicates, so we
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// Note that successor stack may have duplicates, so we
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// want to remove those:
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// want to remove those:
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