Reconsider settled pathfinder goals after adding roots

CHANGELOG.md

@@ -6,6 +6,10 @@ This project adheres to [Semantic Versioning](https://semver.org/) since version
 
 ## [Unreleased]
 
+### Fixed
+
+- Lazy pathfinder queries reconsider previously visited goals after adding a root or lowering a root's distance.
+
 ## [21.2.1] - 2026-06-04
 
 ### Deployment

tcod/path.py

@@ -1326,7 +1326,13 @@ def resolve(self, goal: tuple[int, ...] | None = None) -> None:
                 goal = goal[::-1]
             if self._distance[goal] != np.iinfo(self._distance.dtype).max:  # noqa: SIM102
                 if not lib.frontier_has_index(self._frontier_p, goal):
-                    return
+                    if not lib.TCOD_frontier_size(self._frontier_p):
+                        return
+                    # A new root may improve an already visited goal.  Queue its
+                    # current distance so the search still stops at this goal
+                    # when the remaining frontier cannot improve it.
+                    distance = int(self._distance[goal])
+                    lib.TCOD_frontier_push(self._frontier_p, goal, distance, distance)
         self._update_heuristic(goal)
         self._graph._resolve(self)
 

tests/test_path_lazy.py

@@ -0,0 +1,59 @@
+"""Regression tests for resolving an existing distance after adding roots."""
+
+from typing import Literal
+
+import numpy as np
+import pytest
+
+import tcod.path
+
+
+@pytest.mark.parametrize("order", ["C", "F"])
+@pytest.mark.parametrize("heuristic", [False, True])
+@pytest.mark.parametrize("full_resolve", [False, True])
+def test_resolve_after_lowering_root(order: Literal["C", "F"], heuristic: bool, full_resolve: bool) -> None:
+    cost = np.ones((9, 13), dtype=np.int8, order=order)
+    graph = tcod.path.SimpleGraph(cost=cost, cardinal=1, diagonal=0)
+    if not heuristic:
+        graph.set_heuristic(cardinal=0, diagonal=0)
+    pathfinder = tcod.path.Pathfinder(graph)
+    pathfinder.add_root((0, 0), value=7)
+    pathfinder.resolve(None if full_resolve else (8, 12))
+    pathfinder.add_root((0, 0), value=-3)
+    for goal in ((8, 12), (0, 1), (4, 6)):
+        path = pathfinder.path_from(goal)
+        assert pathfinder.distance[goal] == sum(goal) - 3
+        assert len(path) == sum(goal) + 1
+        np.testing.assert_array_equal(path[-1], (0, 0))
+
+
+@pytest.mark.parametrize("order", ["C", "F"])
+@pytest.mark.parametrize("heuristic", [False, True])
+def test_resolve_after_adding_root(order: Literal["C", "F"], heuristic: bool) -> None:
+    cost = np.ones((9, 13), dtype=np.int8, order=order)
+    graph = tcod.path.SimpleGraph(cost=cost, cardinal=1, diagonal=0)
+    if not heuristic:
+        graph.set_heuristic(cardinal=0, diagonal=0)
+    pathfinder = tcod.path.Pathfinder(graph)
+    pathfinder.add_root((0, 0))
+    pathfinder.resolve()
+    pathfinder.add_root((8, 12))
+    for goal in ((8, 11), (4, 6), (7, 12)):
+        pathfinder.resolve(goal)
+        assert pathfinder.distance[goal] == min(sum(goal), 20 - sum(goal))
+
+
+@pytest.mark.parametrize("heuristic", [False, True])
+def test_cached_goal_does_not_resolve_entire_frontier(heuristic: bool) -> None:
+    graph = tcod.path.SimpleGraph(cost=np.ones((101, 101), dtype=np.int8), cardinal=1, diagonal=0)
+    if not heuristic:
+        graph.set_heuristic(cardinal=0, diagonal=0)
+    pathfinder = tcod.path.Pathfinder(graph)
+    pathfinder.add_root((0, 0))
+    pathfinder.resolve()
+    pathfinder.add_root((100, 100))
+    before = pathfinder.distance.copy()
+    pathfinder.resolve((0, 1))
+    assert pathfinder.distance[0, 1] == 1
+    assert np.count_nonzero(pathfinder.distance != before) <= 4
+    assert pathfinder.distance[100, 98] == before[100, 98]