fix(rules): give Tiny-or-smaller creatures zero reach and square sharing

tests/test_tiny_cooccupancy.py

@@ -0,0 +1,99 @@
+"""Tests for Tiny-or-smaller reach zero and square sharing (PF1e S4c).
+
+SRD "Big and Little Creatures in Combat": Tiny, Diminutive and Fine creatures
+"take up less than 1 square of space", have "a natural reach of 0 feet", do not
+threaten the squares around them, and "must enter an opponent's square to
+attack" — so a Tiny-or-smaller creature and a strictly larger one may share a
+square (move through and end on it), while two creatures of the same (Small+)
+size never share.
+"""
+
+from __future__ import annotations
+
+from pf1e_simulator.combat import _effective_reach_ft
+from tests.test_creature_sizes import make_combatant, make_engine, make_state
+
+
+class TestTinyReachZero:
+    """A Tiny creature whose melee attack has reach_ft 0 reaches only its cell."""
+
+    def test_threatens_no_squares(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 4))
+        eng = make_engine([s_tiny, s_foe])
+        assert eng.threatened_squares(s_tiny) == frozenset()
+
+    def test_attacks_a_creature_in_its_own_cell(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 3))  # sharing the square
+        eng = make_engine([s_tiny, s_foe])
+        assert eng.weapon_for(s_tiny, s_foe) is not None
+
+    def test_cannot_reach_an_adjacent_creature(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 4))
+        eng = make_engine([s_tiny, s_foe])
+        assert eng.weapon_for(s_tiny, s_foe) is None
+
+    def test_authored_reach_is_kept(self) -> None:
+        # A natural attack authored with reach_ft 5 (e.g. ierlieth's bite)
+        # overrides the size default: the creature reaches and threatens 5 ft.
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=5)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 4))
+        eng = make_engine([s_tiny, s_foe])
+        assert _effective_reach_ft(s_tiny, tiny.attacks[0]) == 5
+        assert (3, 4) in eng.threatened_squares(s_tiny)
+        assert eng.weapon_for(s_tiny, s_foe) is not None
+
+
+class TestCoOccupancy:
+    """A Tiny-or-smaller creature shares a square with a strictly larger one."""
+
+    def test_tiny_may_end_on_a_larger_creature_cell(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 4))
+        eng = make_engine([s_tiny, s_foe])
+        assert eng._destination_reachable(s_tiny, (3, 4)) is True
+
+    def test_larger_may_end_on_a_tiny_cell(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 4))
+        s_foe = make_state(foe, "b", (3, 3))
+        eng = make_engine([s_tiny, s_foe])
+        assert eng._destination_reachable(s_foe, (3, 4)) is True
+
+    def test_two_medium_never_share(self) -> None:
+        m1 = make_combatant("m1", size="Medium")
+        m2 = make_combatant("m2", size="Medium")
+        s1 = make_state(m1, "a", (3, 3))
+        s2 = make_state(m2, "b", (3, 4))
+        eng = make_engine([s1, s2])
+        assert eng._destination_reachable(s1, (3, 4)) is False
+        assert eng._cells_free(s2.occupied_cells(), s1) is False
+
+    def test_tiny_may_cross_a_larger_hostile_cell(self) -> None:
+        tiny = make_combatant("tiny", size="Tiny", reach_ft=0)
+        foe = make_combatant("foe", size="Medium")
+        s_tiny = make_state(tiny, "a", (3, 3))
+        s_foe = make_state(foe, "b", (3, 4))
+        eng = make_engine([s_tiny, s_foe])
+        assert eng._cells_walkable(s_foe.occupied_cells(), s_tiny) is True
+
+    def test_medium_cannot_cross_a_medium_hostile_cell(self) -> None:
+        m1 = make_combatant("m1", size="Medium")
+        m2 = make_combatant("m2", size="Medium")
+        s1 = make_state(m1, "a", (3, 3))
+        s2 = make_state(m2, "b", (3, 4))
+        eng = make_engine([s1, s2])
+        assert eng._cells_walkable(s2.occupied_cells(), s1) is False