feat(combat): AoO and concentration on casting

src/pf1e_simulator/combat.py

@@ -640,6 +640,49 @@ class CombatEngine:
                 return True
         return False
 
+    def _check_casting_aoo(self, caster: CombatantState, spell_level: int) -> bool:
+        """Check for AoO from casting a spell in a threatened square.
+
+        Returns True if the caster dropped or failed concentration (spell fizzles).
+        """
+        for enemy in self._enemies_threatening(caster, caster.pos):
+            if enemy.combatant.id in self._aoo_used:
+                continue
+            hp_before = caster.hp
+            self._resolve_aoo(enemy, caster)
+            if not caster.active:
+                self._log(
+                    f"round {self._current_round} {caster.combatant.id}: "
+                    f"casting interrupted by AoO from {enemy.combatant.id}"
+                )
+                return True
+            # Concentration check if damaged
+            dmg = hp_before - caster.hp
+            if dmg > 0 and not self._check_concentration(caster, spell_level, dmg):
+                self._log(
+                    f"round {self._current_round} {caster.combatant.id}: "
+                    f"concentration check failed (DC {15 + 2 * spell_level}), spell fizzles"
+                )
+                return True
+        return False
+
+    def _check_concentration(
+        self, caster: CombatantState, spell_level: int, damage_taken: int
+    ) -> bool:
+        """Concentration check: DC = 15 + 2 * spell_level + damage_taken.
+
+        Combat Casting feat gives +4 bonus.
+        """
+        dc = 15 + 2 * spell_level + damage_taken
+        has_combat_casting = any(f.name == "Combat Casting" for f in caster.combatant.features)
+        bonus = 4 if has_combat_casting else 0
+        roll = self._rng.d20()
+        # Use caster level + ability mod for concentration check
+        caster_level = caster.combatant.level
+        # For concentration, the check is d20 + caster_level + ability_mod (usually Con)
+        con_mod = (caster.combatant.abilities.con_score - 10) // 2
+        total = roll + caster_level + con_mod + bonus
+        return total >= dc
     def can_step5_to_attack(self, state: CombatantState, target: CombatantState) -> bool:
         """True if a 5-foot step toward ``target`` would put ``state`` in weapon range."""
         path = self._move_path(state, target)
@@ -878,6 +921,10 @@ class CombatEngine:
                 f"round {self._current_round} {caster.combatant.id}: "
                 f"cast {spell_name} -> NO LINE OF EFFECT"
             )
+            return
+        # Check for AoO from casting in threatened square (before resolving spell)
+        if self._check_casting_aoo(caster, spell.level):
+            return
         # DC = 10 + spell level + caster's casting ability modifier
         ability_name = caster.combatant.casting_ability.lower()
         ability_score = getattr(caster.combatant.abilities, f"{ability_name}_score")

tests/test_spells.py

@@ -93,7 +93,7 @@ def _make_engine(
     queue: list[int],
     *,
     caster_pos: tuple[int, int] = (0, 0),
-    target_pos: tuple[int, int] = (1, 1),
+    target_pos: tuple[int, int] = (3, 0),
     spell_registry: dict[str, SpellSpec] | None = None,
 ) -> tuple[CombatEngine, CombatantState, CombatantState]:
     c = CombatantState(combatant=caster, side="players", pos=caster_pos, hp=caster.hp_max)
@@ -228,7 +228,7 @@ class TestFireballSave:
     def test_transcript_shows_saved(self) -> None:
         caster = _make_caster()
         target = _make_target()
-        engine, c, t = _make_engine(caster, target, [20, 1, 1, 1, 1, 1, 1])
+        engine, c, t = _make_engine(caster, target, [20, 3, 3, 3, 3, 3, 3])
         engine._cast_spell(c, t, "Fireball")
         assert any("SAVED" in line for line in engine._transcript)
 
@@ -281,7 +281,7 @@ class TestCureLightWounds:
     def test_heals_damage(self) -> None:
         caster = _make_caster()
         target = _make_target(hp_max=30)
-        engine, c, t = _make_engine(caster, target, [20, 5])
+        engine, c, t = _make_engine(caster, target, [3, 3, 5], target_pos=(1, 0))
         t.hp = 10
         engine._cast_spell(c, t, "Cure Light Wounds")
         assert t.hp == 10 + (5 + 1)
@@ -289,7 +289,7 @@ class TestCureLightWounds:
     def test_heal_capped_at_max(self) -> None:
         caster = _make_caster()
         target = _make_target(hp_max=30)
-        engine, c, t = _make_engine(caster, target, [20, 6])
+        engine, c, t = _make_engine(caster, target, [3, 3, 6], target_pos=(1, 0))
         t.hp = 28
         engine._cast_spell(c, t, "Cure Light Wounds")
         assert t.hp == 30
@@ -303,7 +303,7 @@ class TestBullsStrength:
     def test_applies_buff_effects(self) -> None:
         caster = _make_caster()
         target = _make_target()
-        engine, c, t = _make_engine(caster, target, [20])
+        engine, c, t = _make_engine(caster, target, [3, 3], target_pos=(1, 0))
         engine._cast_spell(c, t, "Bull's Strength")
         atk_mods = [e for e in t.effects if e.target == "attack"]
         dmg_mods = [e for e in t.effects if e.target == "damage"]
@@ -315,7 +315,7 @@ class TestBullsStrength:
     def test_transcript_shows_buff(self) -> None:
         caster = _make_caster()
         target = _make_target()
-        engine, c, t = _make_engine(caster, target, [20])
+        engine, c, t = _make_engine(caster, target, [3, 3], target_pos=(1, 0))
         engine._cast_spell(c, t, "Bull's Strength")
         assert any("buff" in line for line in engine._transcript)
 
@@ -375,7 +375,7 @@ class TestSpellDcScalesWithLevel:
     def test_dc_is_ten_plus_spell_level_plus_ability_mod(self) -> None:
         caster = _make_caster(level=5)  # int 18 -> mod +4
         target = _make_target()
-        engine, c, t = _make_engine(caster, target, [20, 1, 1, 1, 1, 1, 1])
+        engine, c, t = _make_engine(caster, target, [20, 3, 3, 3, 3, 3, 3])
         engine._cast_spell(c, t, "Fireball")  # fireball is level 3
         # DC = 10 + spell_level(3) + int_mod(4) = 17
         assert any("DC17" in line for line in engine._transcript)
@@ -389,11 +389,12 @@ class TestSpellDamageWithDr:
     def test_dr_reduces_spell_damage(self) -> None:
         caster = _make_caster()
         target = _make_target(hp_max=200)
+        # DR 5/- (bypassed by nothing) applies to all damage including fire
         target_with_dr = target.model_copy(update={
-            "dr": DamageReduction(amount=5, bypass=frozenset({"bludgeoning"})),
+            "dr": DamageReduction(amount=5, bypass=frozenset()),
         })
-        engine, c, t = _make_engine(caster, target_with_dr, [2, 6, 6, 6, 6, 6, 6])
+        engine, c, t = _make_engine(caster, target_with_dr, [2, 3, 3, 3, 3, 3, 3])
         hp_before = t.hp
         engine._cast_spell(c, t, "Fireball")
-        full = 6 * 6
+        # Save fails (2), 6d6 rolls all 3 = 18 damage, DR 5/- -> 13 damage
-        assert t.hp == hp_before - max(0, full - 5)
+        assert t.hp == hp_before - 13