fix: Exporting converging control flow edges (#1890)

Fixes #1889.

hugr-core/src/export.rs

@@ -16,8 +16,9 @@ use crate::{
     Direction, Hugr, HugrView, IncomingPort, Node, Port,
 };
 use bumpalo::{collections::String as BumpString, collections::Vec as BumpVec, Bump};
-use fxhash::FxHashMap;
+use fxhash::{FxBuildHasher, FxHashMap};
 use hugr_model::v0::{self as model};
+use petgraph::unionfind::UnionFind;
 use std::fmt::Write;
 
 pub(crate) const OP_FUNC_CALL_INDIRECT: &str = "func.call-indirect";
@@ -58,13 +59,8 @@ struct Context<'a> {
     /// Mapping from extension operations to their declarations.
     decl_operations: FxHashMap<(ExtensionId, OpName), model::NodeId>,
 
-    /// Table that is used to track which ports are connected.
-    ///
-    /// Each group of ports that is connected together is represented by a
-    /// single link. When traversing the [`Hugr`] graph we assign a link to each
-    /// port by finding the smallest node/port pair among all the linked ports
-    /// and looking up the link for that pair in this table.
-    links: model::scope::LinkTable<(Node, Port)>,
+    /// Auxiliary structure for tracking the links between ports.
+    links: Links,
 
     /// The symbol table tracking symbols that are currently in scope.
     symbols: model::scope::SymbolTable<'a>,
@@ -85,11 +81,13 @@ impl<'a> Context<'a> {
     pub fn new(hugr: &'a Hugr, bump: &'a Bump) -> Self {
         let mut module = model::Module::default();
         module.nodes.reserve(hugr.node_count());
+        let links = Links::new(hugr);
 
         Self {
             hugr,
             module,
             bump,
+            links,
             term_map: FxHashMap::default(),
             local_scope: None,
             decl_operations: FxHashMap::default(),
@@ -98,7 +96,6 @@ impl<'a> Context<'a> {
             implicit_imports: FxHashMap::default(),
             node_to_id: FxHashMap::default(),
             id_to_node: FxHashMap::default(),
-            links: model::scope::LinkTable::default(),
         }
     }
 
@@ -144,19 +141,6 @@ impl<'a> Context<'a> {
         };
     }
 
-    /// Returns the edge id for a given port, creating a new edge if necessary.
-    ///
-    /// Any two ports that are linked will be represented by the same link.
-    fn get_link_index(&mut self, node: Node, port: impl Into<Port>) -> model::LinkIndex {
-        // To ensure that linked ports are represented by the same edge, we take the minimum port
-        // among all the linked ports, including the one we started with.
-        let port = port.into();
-        let linked_ports = self.hugr.linked_ports(node, port);
-        let all_ports = std::iter::once((node, port)).chain(linked_ports);
-        let repr = all_ports.min().unwrap();
-        self.links.use_link(repr)
-    }
-
     pub fn make_ports(
         &mut self,
         node: Node,
@@ -167,7 +151,7 @@ impl<'a> Context<'a> {
         let mut links = BumpVec::with_capacity_in(ports.size_hint().0, self.bump);
 
         for port in ports.take(num_ports) {
-            links.push(self.get_link_index(node, port));
+            links.push(self.links.use_link(node, port));
         }
 
         links.into_bump_slice()
@@ -715,7 +699,7 @@ impl<'a> Context<'a> {
                     }
 
                     if source.is_none() {
-                        source = Some(self.get_link_index(child, IncomingPort::from(0)));
+                        source = Some(self.links.use_link(child, IncomingPort::from(0)));
                     }
                 }
             }
@@ -1151,6 +1135,81 @@ impl<'a> Context<'a> {
     }
 }
 
+type FxIndexSet<T> = indexmap::IndexSet<T, FxBuildHasher>;
+
+/// Data structure for translating the edges between ports in the `Hugr` graph
+/// into the hypergraph representation used by `hugr_model`.
+struct Links {
+    /// Scoping helper that keeps track of the current nesting of regions
+    /// and translates the group of connected ports into a link index.
+    scope: model::scope::LinkTable<u32>,
+
+    /// A mapping from each port to the group of connected ports it belongs to.
+    groups: FxHashMap<(Node, Port), u32>,
+}
+
+impl Links {
+    /// Create the `Links` data structure from a `Hugr` graph by recording the
+    /// connectivity of the ports.
+    pub fn new(hugr: &Hugr) -> Self {
+        let scope = model::scope::LinkTable::new();
+
+        // We collect all ports that are in the hugr into an index set so that
+        // we have an association between the port and a numeric index.
+        let node_ports: FxIndexSet<(Node, Port)> = hugr
+            .nodes()
+            .flat_map(|node| hugr.all_node_ports(node).map(move |port| (node, port)))
+            .collect();
+
+        // We then use a union-find data structure to group together all ports that are connected.
+        let mut uf = UnionFind::<u32>::new(node_ports.len());
+
+        for (i, (node, port)) in node_ports.iter().enumerate() {
+            if let Ok(port) = port.as_incoming() {
+                for (other_node, other_port) in hugr.linked_outputs(*node, port) {
+                    let other_port = Port::from(other_port);
+                    let j = node_ports.get_index_of(&(other_node, other_port)).unwrap();
+                    uf.union(i as u32, j as u32);
+                }
+            }
+        }
+
+        // We then collect the association between the port and the group of connected ports it belongs to.
+        let groups = node_ports
+            .into_iter()
+            .enumerate()
+            .map(|(i, node_port)| (node_port, uf.find(i as u32)))
+            .collect();
+
+        Self { scope, groups }
+    }
+
+    /// Enter an isolated region.
+    pub fn enter(&mut self, region: model::RegionId) {
+        self.scope.enter(region);
+    }
+
+    /// Leave an isolated region, returning the number of links and ports in the region.
+    ///
+    /// # Panics
+    ///
+    /// Panics if there is no remaining open scope to exit.
+    pub fn exit(&mut self) -> (u32, u32) {
+        self.scope.exit()
+    }
+
+    /// Obtain the link index for a node and port.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the port does not exist in the [`Hugr`] that was passed to `[Self::new]`.
+    pub fn use_link(&mut self, node: Node, port: impl Into<Port>) -> model::LinkIndex {
+        let port = port.into();
+        let group = self.groups[&(node, port)];
+        self.scope.use_link(group)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use rstest::{fixture, rstest};

hugr-core/tests/snapshots/model__roundtrip_cfg.snap

@@ -21,7 +21,7 @@ expression: "roundtrip(include_str!(\"../../hugr-model/tests/fixtures/model-cfg.
             [%4] [%5]
             (signature (-> [?0] [(adt [[?0]])] (ext)))
             (tag 0 [%4] [%5] (signature (-> [?0] [(adt [[?0]])] (ext))))))
-        (block [%6] [%3 %9]
+        (block [%6] [%3 %6]
           (signature (-> [(ctrl [?0])] [(ctrl [?0]) (ctrl [?0])] (ext)))
           (dfg
             [%7] [%8]