feat(observation): expose attack failure reasons

README.md

@@ -660,7 +660,14 @@ Règles modélisées :
     cover_bonus (int — bonus de couvert que la cible gagne contre une attaque
     de l'acteur, calculé comme dans `resolve_attack` : terrain +2/+4/+8 via la
     règle des coins, couvert mou +4 des créatures interposées pour les seules
-    attaques à distance ; 0 si `weapon_for` est None).
+    attaques à distance ; 0 si `weapon_for` est None), weapon_reason (str |
+    None — None si `weapon_for` a choisi une arme ; sinon la raison normalisée
+    de l'indisponibilité d'attaque, dans le vocabulaire `ACTION_RESULT_REASONS` :
+    `no_weapon` (aucune arme utilisable), `no_line_of_effect` (ligne d'effet
+    bloquée), `out_of_range` (arme existante mais cible hors de portée)),
+    charge_reason (str | None — None si `can_charge` est vrai ; sinon la
+    raison de `ChargeResult` verbatim (`no_line_of_effect` / `no_charge_path`)
+    ou `no_weapon` si aucune arme de mêlée utilisable).
   - Top-level : round, acting_combatant, combatants, pairs, terrain, elevation,
     threatened (grille bool des cases menacées par les ennemis),
     initiative_order (U2 : tuple d'ids dans l'ordre d'initiative du round

src/pf1e_simulator/combat.py

@@ -576,8 +576,9 @@ class Action:
 
 # Normalized action-result reason vocabulary. U1 emits ok, unsupported_kind,
 # missing_target and the downgraded:* reasons; no_weapon, out_of_range,
-# no_line_of_effect, no_slot and insufficient_resource are declared here for
-# the deeper execution-feedback package (U6b). U3 adds delayed /
+# no_line_of_effect are reused by U6b to explain, per pair, why an attack or
+# a charge is unavailable. no_slot and insufficient_resource are declared
+# here for the deeper execution-feedback package. U3 adds delayed /
 # already_delayed for the delay action. U4 adds ready for the ready action.
 # U5b adds not_prone / already_prone for the prone actions. U5c adds
 # unreachable_destination for move / 5foot_step with an explicit destination.
@@ -604,6 +605,9 @@ _REASON_NOT_PRONE = "not_prone"
 _REASON_ALREADY_PRONE = "already_prone"
 _REASON_UNREACHABLE_DESTINATION = "unreachable_destination"
 _REASON_WEAPON_UNAVAILABLE = "weapon_unavailable"
+_REASON_NO_WEAPON = "no_weapon"
+_REASON_OUT_OF_RANGE = "out_of_range"
+_REASON_NO_LINE_OF_EFFECT = "no_line_of_effect"
 
 # Action kinds that are declared but have no engine dispatch (U1 surfaces
 # them as explicit rejections instead of silent no-ops).
@@ -1016,6 +1020,16 @@ class CombatEngine:
         charge = self.can_charge(state, target)
         can_charge = charge is not None and charge.weapon is not None
         occupied = state.occupied_cells()
+        line_of_effect = has_line_of_effect_cells(
+            self._grid, occupied, target.occupied_cells()
+        )
+        weapon_reason = self._weapon_failure_reason(
+            state, weapon, line_of_effect=line_of_effect
+        )
+        charge_reason: str | None = None
+        if not can_charge:
+            # can_charge() returns None only when no melee weapon is usable.
+            charge_reason = charge.reason if charge is not None else _REASON_NO_WEAPON
         target_threatens_me = bool(
             target.active
             and any(w.kind == "melee" for w in target.usable_attacks())
@@ -1044,12 +1058,12 @@ class CombatEngine:
             "target_index": self._states.index(target),
             "distance_ft": distance_ft,
             "weapon_for": weapon_name,
+            "weapon_reason": weapon_reason,
             "range_penalty": range_penalty,
             "can_charge": can_charge,
+            "charge_reason": charge_reason,
             "can_step5_to_attack": self.can_step5_to_attack(state, target),
-            "has_line_of_effect": has_line_of_effect_cells(
-                self._grid, state.occupied_cells(), target.occupied_cells()
-            ),
+            "has_line_of_effect": line_of_effect,
             "flanking": self._flanking_bonus(state, target) > 0,
             "movement_cost_ft": self.movement_cost_ft(state, target),
             "target_threatens_me": target_threatens_me,
@@ -1057,6 +1071,27 @@ class CombatEngine:
             "cover_bonus": cover_bonus,
         }
 
+    def _weapon_failure_reason(
+        self,
+        attacker: CombatantState,
+        weapon: AttackSpec | None,
+        *,
+        line_of_effect: bool,
+    ) -> str | None:
+        """Why ``weapon_for`` found no weapon against a target, or None if it did.
+
+        U6b: the cause is drawn from the ``ACTION_RESULT_REASONS`` vocabulary
+        — no usable weapon at all, no line of effect, or every usable weapon
+        out of range — so a policy learns which obstacle to remove.
+        """
+        if weapon is not None:
+            return None
+        if not attacker.usable_attacks():
+            return _REASON_NO_WEAPON
+        if not line_of_effect:
+            return _REASON_NO_LINE_OF_EFFECT
+        return _REASON_OUT_OF_RANGE
+
     def _take_turn(self, state: CombatantState) -> bool:
         """Execute one combatant's full turn; return True if the battle is over."""
         self._decrement_durations(state)

tests/test_observation_enriched.py

@@ -676,8 +676,10 @@ def test_observation_enriched_pair_existing_keys_preserved() -> None:
         "target_index",
         "distance_ft",
         "weapon_for",
+        "weapon_reason",
         "range_penalty",
         "can_charge",
+        "charge_reason",
         "can_step5_to_attack",
         "has_line_of_effect",
         "flanking",

tests/test_pair_failure_reasons.py

@@ -0,0 +1,253 @@
+"""U6b: per-pair attack/charge failure reasons in the Observation snapshot.
+
+The pair dict now exposes WHY an attack or a charge is unavailable, not just
+that it is: ``weapon_reason`` is None when ``weapon_for`` selected a weapon,
+otherwise the blocking cause (``no_weapon`` / ``no_line_of_effect`` /
+``out_of_range``, drawn from the U1 ``ACTION_RESULT_REASONS`` vocabulary);
+``charge_reason`` is None when ``can_charge`` is True, otherwise the
+``ChargeResult.reason`` verbatim (``no_line_of_effect`` / ``no_charge_path``)
+or ``no_weapon`` when the actor has no usable melee weapon at all.
+
+Covers:
+- (a) unarmed actor -> ``weapon_reason`` / ``charge_reason`` ``no_weapon``;
+- (b) wall column blocking the line -> ``no_line_of_effect``;
+- (c) melee-only actor facing a distant target -> ``out_of_range``;
+- (d) valid adjacent target -> ``weapon_reason`` None;
+- (e) creatures interposed on every straight charge line -> the engine's
+  ``no_charge_path``;
+- (f) valid charge line -> ``charge_reason`` None; ranged-only actor ->
+  ``no_weapon``;
+- (g) ``to_dict()`` stays JSON-serializable.
+"""
+
+from __future__ import annotations
+
+import json
+
+from pf1e_simulator.combat import CombatantState, CombatEngine
+from pf1e_simulator.dice import parse_dice
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.models import (
+    AbilityScores,
+    ACProfile,
+    AttackSpec,
+    Combatant,
+    DamageComponent,
+    Saves,
+)
+from pf1e_simulator.rng import ScriptedRng
+from tests.test_observation_enriched import _pair_for, _snapshot, make_state
+
+_LEGEND = {
+    ".": TerrainType(type="floor", move_cost=1),
+    "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+}
+
+
+def _grid(rows: list[str]) -> Grid:
+    spec = MapSpec(name="reasons-test", terrain=tuple(rows), legend=_LEGEND)
+    return Grid.from_spec(spec)
+
+
+def _open_rows() -> list[str]:
+    return ["........"] * 8
+
+
+def _bow(cid: str) -> AttackSpec:
+    return AttackSpec(
+        id=f"{cid}-bow",
+        name="short bow",
+        kind="ranged",
+        attack_bonus=5,
+        damage=[DamageComponent(formula=parse_dice("1d6"), types=["piercing"])],
+        range_increment_ft=60,
+    )
+
+
+def _sword(cid: str) -> AttackSpec:
+    return AttackSpec(
+        id=f"{cid}-sword",
+        name="short sword",
+        kind="melee",
+        attack_bonus=5,
+        damage=[DamageComponent(formula=parse_dice("1d6"), types=["slashing"])],
+    )
+
+
+def _combatant(cid: str, attacks: list[AttackSpec]) -> Combatant:
+    return Combatant(
+        id=cid,
+        name=cid,
+        level=1,
+        abilities=AbilityScores(
+            str_score=10,
+            dex_score=10,
+            con_score=10,
+            int_score=10,
+            wis_score=10,
+            cha_score=10,
+        ),
+        ac=ACProfile(total=10, touch=10, flat_footed=10),
+        bab=1,
+        initiative_mod=0,
+        speed_land_ft=30,
+        hp_max=100,
+        saves=Saves(fort=0, ref=0, will=0),
+        attacks=attacks,
+    )
+
+
+def _engine(states: list[CombatantState], rows: list[str]) -> CombatEngine:
+    return CombatEngine(ScriptedRng([]), _grid(rows), states)
+
+
+# (a) no usable weapon at all.
+
+
+def test_weapon_reason_no_weapon_when_actor_unarmed() -> None:
+    """Given: an actor with no attacks at all facing an adjacent foe
+    When: the Observation is built for the actor
+    Then: weapon_for is None with weapon_reason no_weapon, and the charge
+    helper's no-melee-weapon None maps to charge_reason no_weapon too."""
+    actor = make_state(_combatant("unarmed", []), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 1))
+    engine = _engine([actor, foe], _open_rows())
+
+    obs = _snapshot(engine, actor)
+    pair = _pair_for(obs, 1)
+    assert pair["weapon_for"] is None
+    assert pair["weapon_reason"] == "no_weapon"
+    assert pair["can_charge"] is False
+    assert pair["charge_reason"] == "no_weapon"
+
+
+# (b) line of effect blocked by a wall column.
+
+
+def test_weapon_reason_no_line_of_effect_behind_wall_column() -> None:
+    """Given: an archer at (4, 0) and a foe at (4, 4) behind a full wall column
+    When: the Observation is built for the archer
+    Then: weapon_reason and charge_reason are both no_line_of_effect."""
+    archer = make_state(_combatant("archer", [_bow("archer")]), "players", (4, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (4, 4))
+    rows = ["..#....."] * 8
+    engine = _engine([archer, foe], rows)
+
+    obs = _snapshot(engine, archer)
+    pair = _pair_for(obs, 1)
+    assert pair["has_line_of_effect"] is False
+    assert pair["weapon_for"] is None
+    assert pair["weapon_reason"] == "no_line_of_effect"
+    assert pair["can_charge"] is False
+    assert pair["charge_reason"] == "no_line_of_effect"
+
+
+# (c) weapon usable but the target out of reach.
+
+
+def test_weapon_reason_out_of_range_with_clear_line() -> None:
+    """Given: a melee-only actor at (1, 0) and a foe at (1, 4) on open ground
+    When: the Observation is built for the actor
+    Then: weapon_reason is out_of_range (weapon exists, LoE is clear)."""
+    striker = make_state(_combatant("swordsman", [_sword("swordsman")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 4))
+    engine = _engine([striker, foe], _open_rows())
+
+    obs = _snapshot(engine, striker)
+    pair = _pair_for(obs, 1)
+    assert pair["has_line_of_effect"] is True
+    assert pair["weapon_for"] is None
+    assert pair["weapon_reason"] == "out_of_range"
+
+
+# (d) a valid attack carries no reason.
+
+
+def test_weapon_reason_none_for_valid_adjacent_attack() -> None:
+    """Given: an armed actor adjacent to a foe on open ground
+    When: the Observation is built
+    Then: weapon_for names the weapon and weapon_reason is None."""
+    actor = make_state(_combatant("hero", [_sword("hero")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 1))
+    engine = _engine([actor, foe], _open_rows())
+
+    obs = _snapshot(engine, actor)
+    pair = _pair_for(obs, 1)
+    assert pair["weapon_for"] == "short sword"
+    assert pair["weapon_reason"] is None
+
+
+# (e) charge blocked by interposed creatures despite a clear line of effect.
+
+
+def test_charge_reason_no_charge_path_when_creatures_block_the_line() -> None:
+    """Given: an actor at (1, 0), a foe at (1, 3), creatures at (1, 1), (0, 2)
+    and (2, 2) — every straight Bresenham line to any attackable anchor passes
+    through one of them, while terrain LoE stays clear
+    When: the Observation is built for the actor
+    Then: can_charge is False and charge_reason is the engine's no_charge_path."""
+    actor = make_state(_combatant("hero", [_sword("hero")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 3))
+    guard_a = make_state(_combatant("guard-a", [_sword("guard-a")]), "monsters", (1, 1))
+    guard_b = make_state(_combatant("guard-b", [_sword("guard-b")]), "monsters", (0, 2))
+    guard_c = make_state(_combatant("guard-c", [_sword("guard-c")]), "monsters", (2, 2))
+    engine = _engine([actor, foe, guard_a, guard_b, guard_c], _open_rows())
+
+    obs = _snapshot(engine, actor)
+    pair = _pair_for(obs, 1)
+    assert pair["has_line_of_effect"] is True
+    assert pair["can_charge"] is False
+    assert pair["charge_reason"] == "no_charge_path"
+
+
+# (f) valid charge, and the ranged-only charge refusal.
+
+
+def test_charge_reason_none_for_valid_charge() -> None:
+    """Given: an armed actor at (1, 0) and a foe at (1, 4) on an open straight line
+    When: the Observation is built
+    Then: can_charge is True and charge_reason is None."""
+    actor = make_state(_combatant("hero", [_sword("hero")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 4))
+    engine = _engine([actor, foe], _open_rows())
+
+    obs = _snapshot(engine, actor)
+    pair = _pair_for(obs, 1)
+    assert pair["can_charge"] is True
+    assert pair["charge_reason"] is None
+
+
+def test_charge_reason_no_weapon_for_ranged_only_actor() -> None:
+    """Given: a bow-only actor facing a foe 20 ft away on open ground
+    When: the Observation is built
+    Then: the attack is valid (weapon_reason None) but the charge helper
+    returns None (no melee weapon) -> charge_reason no_weapon."""
+    archer = make_state(_combatant("archer", [_bow("archer")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 4))
+    engine = _engine([archer, foe], _open_rows())
+
+    obs = _snapshot(engine, archer)
+    pair = _pair_for(obs, 1)
+    assert pair["weapon_for"] == "short bow"
+    assert pair["weapon_reason"] is None
+    assert pair["can_charge"] is False
+    assert pair["charge_reason"] == "no_weapon"
+
+
+# (g) JSON serializability of the extended snapshot.
+
+
+def test_pair_failure_reasons_snapshot_is_json_serializable() -> None:
+    """Given: a snapshot whose pairs carry weapon_reason and charge_reason
+    When: to_dict() is dumped to JSON
+    Then: serialization round-trips and keeps both new keys."""
+    actor = make_state(_combatant("swordsman", [_sword("swordsman")]), "players", (1, 0))
+    foe = make_state(_combatant("foe", [_sword("foe")]), "monsters", (1, 4))
+    engine = _engine([actor, foe], _open_rows())
+
+    obs = _snapshot(engine, actor)
+    data = json.loads(json.dumps(obs))
+    pair = _pair_for(data, 1)
+    assert pair["weapon_reason"] == "out_of_range"
+    assert pair["charge_reason"] is None