Get the action prescribed by a strategy at an information set

ChangeLog

@@ -10,8 +10,10 @@
   been removed as planned. (#357)
 
 ### Added
-- Implement `GetPlays()` (C++) and `get_plays` (Python) to compute the set of terminal nodes consistent
-  with a node, information set, or action (#517)
+- Implement `GetPlays()` (C++) and `get_plays` (Python) to compute the set of terminal nodes
+  consistent with a node, information set, or action (#517)
+- Implement `GameStrategyRep::GetAction` (C++) and `Strategy.action` (Python) retrieving the action
+  prescribed by a strategy at an information set
 
 
 ## [16.3.1] - unreleased

doc/pygambit.api.rst

@@ -178,6 +178,7 @@ Information about the game
    Strategy.game
    Strategy.player
    Strategy.number
+   Strategy.action
 
 
 Player behavior

src/games/game.cc

@@ -46,6 +46,19 @@ GameOutcomeRep::GameOutcomeRep(GameRep *p_game, int p_number) : m_game(p_game),
   }
 }
 
+//========================================================================
+//                      class GameStrategyRep
+//========================================================================
+
+GameAction GameStrategyRep::GetAction(const GameInfoset &p_infoset) const
+{
+  if (p_infoset->GetPlayer() != m_player) {
+    throw MismatchException();
+  }
+  const int action = m_behav[p_infoset->GetNumber()];
+  return (action) ? *std::next(p_infoset->GetActions().cbegin(), action - 1) : nullptr;
+}
+
 //========================================================================
 //                       class GamePlayerRep
 //========================================================================

src/games/game.h

@@ -358,6 +358,9 @@ class GameStrategyRep : public GameObject {
   GamePlayer GetPlayer() const;
   /// Returns the index of the strategy for its player
   int GetNumber() const { return m_number; }
+
+  /// Returns the action specified by the strategy at the information set
+  GameAction GetAction(const GameInfoset &) const;
   //@}
 };
 

src/pygambit/gambit.pxd

@@ -73,6 +73,7 @@ cdef extern from "games/game.h":
         c_GameNodeRep *deref "operator->"() except +RuntimeError
 
     cdef cppclass c_GameAction "GameObjectPtr<GameActionRep>":
+        bool operator !() except +
         bool operator !=(c_GameAction) except +
         c_GameActionRep *deref "operator->"() except +RuntimeError
 
@@ -97,6 +98,7 @@ cdef extern from "games/game.h":
         c_GamePlayer GetPlayer() except +
         string GetLabel() except +
         void SetLabel(string) except +
+        c_GameAction GetAction(c_GameInfoset) except +
 
     cdef cppclass c_GameActionRep "GameActionRep":
         int GetNumber() except +

src/pygambit/strategy.pxi

@@ -73,3 +73,46 @@ class Strategy:
     def number(self) -> int:
         """The number of the strategy."""
         return self.strategy.deref().GetNumber() - 1
+
+    def action(self, infoset: typing.Union[Infoset, str]) -> typing.Optional[Action]:
+        """Get the action prescribed by a strategy for a given information set.
+
+        .. versionadded:: 16.4.0
+
+        Parameters
+        ----------
+        infoset
+            The information set for which to find the prescribed action.
+            Can be an Infoset object or its string label.
+
+        Returns
+        -------
+        Action or None
+            The prescribed action or None if the strategy is not defined for this
+            information set, that is, the information set is unreachable under this strategy.
+
+        Raises
+        ------
+        UndefinedOperationError
+            If the game is not an extensive-form (tree) game.
+        ValueError
+            If the information set belongs to a different player than the strategy.
+        """
+        if not self.game.is_tree:
+            raise UndefinedOperationError(
+                "Strategy.action is only defined for strategies in extensive-form games."
+            )
+
+        resolved_infoset: Infoset = self.game._resolve_infoset(infoset, "Strategy.action")
+
+        if resolved_infoset.player != self.player:
+            raise ValueError(
+                f"Information set {resolved_infoset} belongs to player "
+                f"'{resolved_infoset.player.label}', but this strategy "
+                f"belongs to player '{self.player.label}'."
+            )
+
+        action: c_GameAction = self.strategy.deref().GetAction(resolved_infoset.infoset)
+        if not action:
+            return None
+        return Action.wrap(action)

tests/test_actions.py

@@ -139,7 +139,103 @@ def test_action_plays():
     test_action = list_infosets[2].actions[0]  # members' paths=[0, 1, 0], [0, 1]
 
     expected_set_of_plays = {
-        list_nodes[4], list_nodes[7]
-    }  # paths=[0, 1, 0], [0, 1]
+        list_nodes[4], list_nodes[7]           # paths=[0, 1, 0], [0, 1]
+    }
 
     assert set(test_action.plays) == expected_set_of_plays
+
+
+@pytest.mark.parametrize(
+    "game, player_ind, str_ind, infoset_ind, expected_action_ind",
+    [
+        (games.read_from_file("e01.efg"), 0, 0, 0, 0),
+        (games.read_from_file("e01.efg"), 0, 1, 0, 1),
+        (games.read_from_file("e01.efg"), 1, 0, 1, 0),
+        (games.read_from_file("e01.efg"), 1, 1, 1, 1),
+        (games.read_from_file("e01.efg"), 2, 0, 2, 0),
+        (games.read_from_file("e01.efg"), 2, 1, 2, 1),
+        (games.read_from_file("e02.efg"), 0, 0, 0, 0),
+        (games.read_from_file("e02.efg"), 0, 1, 0, 1),
+        (games.read_from_file("e02.efg"), 1, 0, 2, 0),
+        (games.read_from_file("e02.efg"), 1, 1, 2, 1),
+        (games.read_from_file("basic_extensive_game.efg"), 0, 0, 0, 0),
+        (games.read_from_file("basic_extensive_game.efg"), 0, 1, 0, 1),
+        (games.read_from_file("basic_extensive_game.efg"), 1, 0, 1, 0),
+        (games.read_from_file("basic_extensive_game.efg"), 1, 1, 1, 1),
+        (games.read_from_file("basic_extensive_game.efg"), 2, 0, 2, 0),
+        (games.read_from_file("basic_extensive_game.efg"), 2, 1, 2, 1),
+    ],
+)
+def test_strategy_action_defined(game, player_ind, str_ind, infoset_ind, expected_action_ind):
+    """Verify `Strategy.action` retrieves the correct action for defined actions.
+    """
+    player = game.players[player_ind]
+    strategy = player.strategies[str_ind]
+    infoset = game.infosets[infoset_ind]
+    expected_action = infoset.actions[expected_action_ind]
+
+    prescribed_action = strategy.action(infoset)
+
+    assert prescribed_action == expected_action
+
+
+@pytest.mark.parametrize(
+    "game, player_ind, str_ind, infoset_ind",
+    [
+        (games.read_from_file("e02.efg"), 0, 0, 1),
+        (games.read_from_file("cent3.efg"), 0, 0, 1),
+        (games.read_from_file("cent3.efg"), 0, 0, 2),
+        (games.read_from_file("cent3.efg"), 0, 1, 2),
+        (games.read_from_file("cent3.efg"), 1, 0, 7),
+        (games.read_from_file("cent3.efg"), 1, 0, 7),
+        (games.read_from_file("cent3.efg"), 1, 1, 8),
+    ],
+)
+def test_strategy_action_undefined_returns_none(game, player_ind, str_ind, infoset_ind):
+    """Verify `Strategy.action` returns None when called on an unreached player's infoset
+    """
+    player = game.players[player_ind]
+    strategy = player.strategies[str_ind]
+    infoset = game.infosets[infoset_ind]
+
+    prescribed_action = strategy.action(infoset)
+
+    assert prescribed_action is None
+
+
+@pytest.mark.parametrize(
+    "game, player_ind, infoset_ind",
+    [
+        (games.read_from_file("e01.efg"), 0, 1),
+        (games.read_from_file("e01.efg"), 1, 0),
+        (games.read_from_file("e02.efg"), 0, 2),
+        (games.read_from_file("e02.efg"), 1, 0),
+        (games.read_from_file("basic_extensive_game.efg"), 0, 1),
+        (games.read_from_file("basic_extensive_game.efg"), 1, 2),
+        (games.read_from_file("basic_extensive_game.efg"), 2, 0),
+    ],
+)
+def test_strategy_action_raises_value_error_for_wrong_player(game, player_ind, infoset_ind):
+    """
+    Verify `Strategy.action` raises ValueError when the infoset belongs
+    to a different player than the strategy.
+    """
+    player = game.players[player_ind]
+    strategy = player.strategies[0]
+    other_players_infoset = game.infosets[infoset_ind]
+
+    with pytest.raises(ValueError):
+        strategy.action(other_players_infoset)
+
+
+def test_strategy_action_raises_error_for_strategic_game():
+    """Verify `Strategy.action` retrieves the action prescribed by the strategy
+    """
+    game_efg = games.read_from_file("e02.efg")
+    game_nfg = game_efg.from_arrays(game_efg.to_arrays()[0], game_efg.to_arrays()[1])
+    alice = game_nfg.players[0]
+    strategy = alice.strategies[0]
+    test_infoset = game_efg.infosets[0]
+
+    with pytest.raises(gbt.UndefinedOperationError):
+        strategy.action(test_infoset)