Fix: reject spurious subgame roots on single-action chains in GameTreeRep::BuildSubgameRoots

src/games/gametree.cc

@@ -1185,7 +1185,7 @@ void GameTreeRep::BuildSubgameRoots() const
 
   // Phase 2: Reachability and detection
   struct BridgeVisitor {
-    const std::unordered_map<GameNodeRep *, Range> &m_disc;
+    std::unordered_map<GameNodeRep *, Range> &m_disc;
     const std::unordered_map<GameInfosetRep *, Range> &m_hull;
     std::vector<GameNodeRep *> &m_subgames;
     std::unordered_map<GameNodeRep *, Range> m_low;
@@ -1210,10 +1210,27 @@ void GameTreeRep::BuildSubgameRoots() const
 
       for (const auto &child : p_node->GetChildren()) {
         low.Merge(m_low.at(child.get()));
+        m_low.erase(child.get());
       }
 
       if (low == m_disc.at(node)) {
-        m_subgames.push_back(node);
+        // The `low == disc` test is exact only with distinct terminal spans. A single-action
+        // chain above a candidate node collapses the spans and can create false positives.
+        // Reject a node if some single-action ancestor's infoset (possibly the node's own)
+        // has a member in the node's subtree (possibly the node itself).
+        // Note that such an infoset is necessarily absent-minded.
+        bool spurious = false;
+        for (auto *anc = node->m_parent; anc && anc->m_children.size() == 1 && !spurious;
+             anc = anc->m_parent) {
+          const auto &members = anc->m_infoset->m_members;
+          spurious = members.size() >= 2 &&
+                     std::any_of(members.begin(), members.end(), [&](const auto &member) {
+                       return member.get() != anc && member->IsSuccessorOf(p_node);
+                     });
+        }
+        if (!spurious) {
+          m_subgames.push_back(node);
+        }
       }
 
       return DFSCallbackResult::Continue;

tests/test_games/AM-unary-branches.efg

@@ -0,0 +1,17 @@
+EFG 2 R "Unary chain with AM-infosets: Example 2" { "Player 1" "Player 2" }
+"A tailored edge case for the interval Tarjan subgame detector:
+low == disc is an exact bridge test only when nodes have distinct terminal spans.
+On this single-action chain an absent-minded infoset collapses the spans, so the
+Player 1 branching node passes the low == disc test but is not a subgame root."
+
+p "" 2 1 "" { "T" "B" } 0
+p "" 1 1 "" { "1" } 0
+p "" 1 1 "" { "1" } 0
+p "" 1 2 "" { "1" } 0
+p "" 2 2 "" { "1" } 0
+p "" 1 3 "" { "t" "b" } 0
+p "" 2 2 "" { "1" } 0
+t "" 1 "Outcome 1" { 1, -1 }
+t "" 2 "Outcome 2" { 2, -2 }
+p "" 1 2 "" { "1" } 0
+t "" 3 "Outcome 3" { 3, -3 }

tests/test_games/AM-unary-hops.efg

@@ -0,0 +1,19 @@
+EFG 2 R "Unary chain with AM-infosets: Example 1" { "Player 1" "Player 2" }
+"A tailored edge case for the interval Tarjan subgame detector:
+low == disc is an exact bridge test only when nodes have distinct terminal spans.
+This single-action chain carries several absent-minded infosets that collapse the spans,
+so several nodes pass the low == disc test without being subgame roots: among them an
+interior chain node straddled by one absent-minded set while it is the upper member of
+another, and the Player 1 branching node straddled by a member nested below it.
+The only true subgame roots are the game root and the two Player 2 nodes."
+
+p "" 1 1 "" { "1" } 0
+p "" 1 1 "" { "1" } 0
+p "" 2 1 "" { "1" } 0
+p "" 1 2 "" { "1" } 0
+p "" 2 1 "" { "1" } 0
+p "" 1 3 "" { "T" "B" } 0
+p "" 2 2 "" { "1" } 0
+t "" 1 "Outcome 1" { 1, -1 }
+p "" 1 2 "" { "1" } 0
+t "" 2 "Outcome 2" { 2, -2 }

tests/test_node.py

@@ -198,6 +198,20 @@ class SubgameRootsTestCase:
         ),
         id="Absent-minded-game-with-paths-intersecting-infoset-three-times"
     ),
+    pytest.param(
+        SubgameRootsTestCase(
+            factory=functools.partial(games.read_from_file, "AM-unary-hops.efg"),
+            expected_paths=[[], ["1", "1"], ["T", "1", "1", "1", "1", "1"]]
+        ),
+        id="Absent-minded-game-with-paths-intersecting-infoset-two-times"
+    ),
+    pytest.param(
+        SubgameRootsTestCase(
+            factory=functools.partial(games.read_from_file, "AM-unary-branches.efg"),
+            expected_paths=[[], ["1", "1", "1", "T"]]
+        ),
+        id="Absent-minded-game-with-paths-intersecting-infoset-two-times"
+    ),
 ]