Use ahash with IndexSet in simple_cycles() (#688)

We recently merged a new simple_cycles() function in #633 that added an
implementation of johnson's algorithm for finding all the simple cycles
in a directed graph. In that function we used IndexSet to have a set
with O(1) lookup with a deterministic ordering. As part of that we used
the default hashing algorithm from the Rust stdlib. This leaves some
performance on the table as the stdlib hashing algorithm is slower in
order to have proven resistance to HashDoS attacks. Since this is
relevant for the internal usage in Johnson's algorithm this commit
switches the hasing algorithm to ahash (which is the default hashbrown
uses) which offers better performance.

src/connectivity/johnson_simple_cycles.rs

@@ -61,7 +61,7 @@ pub struct SimpleCycleIter {
     blocked: HashSet<NodeIndex>,
     closed: HashSet<NodeIndex>,
     block: HashMap<NodeIndex, HashSet<NodeIndex>>,
-    stack: Vec<(NodeIndex, IndexSet<NodeIndex>)>,
+    stack: Vec<(NodeIndex, IndexSet<NodeIndex, ahash::RandomState>)>,
     start_node: NodeIndex,
     node_map: HashMap<NodeIndex, NodeIndex>,
     reverse_node_map: HashMap<NodeIndex, NodeIndex>,
@@ -115,7 +115,8 @@ fn unblock(
     blocked: &mut HashSet<NodeIndex>,
     block: &mut HashMap<NodeIndex, HashSet<NodeIndex>>,
 ) {
-    let mut stack: IndexSet<NodeIndex> = IndexSet::new();
+    let mut stack: IndexSet<NodeIndex, ahash::RandomState> =
+        IndexSet::with_hasher(ahash::RandomState::new());
     stack.insert(node);
     while let Some(stack_node) = stack.pop() {
         if blocked.remove(&stack_node) {
@@ -141,7 +142,7 @@ fn unblock(
 #[allow(clippy::too_many_arguments)]
 fn process_stack(
     start_node: NodeIndex,
-    stack: &mut Vec<(NodeIndex, IndexSet<NodeIndex>)>,
+    stack: &mut Vec<(NodeIndex, IndexSet<NodeIndex, ahash::RandomState>)>,
     path: &mut Vec<NodeIndex>,
     closed: &mut HashSet<NodeIndex>,
     blocked: &mut HashSet<NodeIndex>,
@@ -165,7 +166,7 @@ fn process_stack(
                     next_node,
                     subgraph
                         .neighbors(next_node)
-                        .collect::<IndexSet<NodeIndex>>(),
+                        .collect::<IndexSet<NodeIndex, ahash::RandomState>>(),
                 ));
                 closed.remove(&next_node);
                 blocked.insert(next_node);
@@ -206,7 +207,8 @@ impl SimpleCycleIter {
             }));
         }
         // Restore previous state if it exists
-        let mut stack: Vec<(NodeIndex, IndexSet<NodeIndex>)> = std::mem::take(&mut slf.stack);
+        let mut stack: Vec<(NodeIndex, IndexSet<NodeIndex, ahash::RandomState>)> =
+            std::mem::take(&mut slf.stack);
         let mut path: Vec<NodeIndex> = std::mem::take(&mut slf.path);
         let mut closed: HashSet<NodeIndex> = std::mem::take(&mut slf.closed);
         let mut blocked: HashSet<NodeIndex> = std::mem::take(&mut slf.blocked);
@@ -267,7 +269,7 @@ impl SimpleCycleIter {
                 slf.start_node,
                 subgraph
                     .neighbors(slf.start_node)
-                    .collect::<IndexSet<NodeIndex>>(),
+                    .collect::<IndexSet<NodeIndex, ahash::RandomState>>(),
             )];
             if let Some(res) = process_stack(
                 slf.start_node,