fix(rules): apply cover by attacker's best corner for multi-square attackers

README.md

@@ -495,7 +495,12 @@ Règles modélisées :
    (+2 partiel, +4 standard, +8 amélioré via `cover_bonus`) sur la touche et la
    confirmation de critique ; couvert mou : une créature active entre
    l'attaquant et la cible octroie aussi +4 CA aux attaques à distance (les
-  attaques de mêlée ignorent les créatures).
+   attaques de mêlée ignorent les créatures). Conformance B20 (CRB « Cover »,
+   règle des coins) : c'est **l'attaquant qui choisit son meilleur coin** —
+   une créature multi-cases (Large +) ne subit que le couvert de SA meilleure
+   case occupée (minimum sur ses cases), pas le pire. Le côté cible reste
+   évalué par la règle RAW : toute ligne vers tout coin des cases de la cible
+   compte (maximum sur les coins de la cible).
 - Flanquement : +2 au jet d'attaque de mêlée (toucher de mêlée inclus, voir
   « Touch AC ») si un allié actif menace réellement la cible depuis la
   bordure ou le coin opposé — conformance P2 : l'allié doit **menacer** une
@@ -703,7 +708,10 @@ Règles modélisées :
   Les attaques de mêlée ne sont pas concernées.
 - Couvert : les trois niveaux PF1e sont modélisés via `cover_bonus` sur le
   terrain — partiel +2, standard +4 (ou `cover: true`), amélioré +8 ; le
-  couvert mou des créatures reste +4 (couvert standard).
+  couvert mou des créatures reste +4 (couvert standard). B20 (conformance) :
+  pour un attaquant multi-cases, le bonus effectif est le **meilleur** (plus
+  faible) couvert parmi ses cases occupées — l'attaquant choisit son meilleur
+  coin/case selon le CRB ; les créatures 1×1 sont inchangées.
 - Invocation : les créatures invoquées agissent (calées sur la case d'initiative de
   leur conjurateur via `_build_round_order`) puis expirent après leur `summon_duration`
   (`_expire_summons` décrémente puis retire).

src/pf1e_simulator/los.py

@@ -254,9 +254,12 @@ def cover_bonus_cells(
     """Effective cover bonus (0/2/4/8) for an attack between two sets of cells.
 
     0 when there is no cover (see has_cover_cells for the corner rule);
-    otherwise the highest cover level among the cells blocking the
+    otherwise the cover level of the attacker's BEST occupied cell: each
-    attacker-corner to target-corner lines. Interposed creatures grant soft
+    attacker cell is scored by the per-cell corner rule against all of the
-    cover (+4) on ranged attacks only, exactly as in has_cover_cells.
+    target's cells, and the attacker picks the lowest-scoring (clearest)
+    cell. Interposed creatures grant soft cover (+4) on ranged attacks only,
+    exactly as in has_cover_cells. For a 1x1 attacker this is exactly the
+    single-cell value.
     """
     if not attacker_cells or not target_cells:
         return 0
@@ -271,10 +274,18 @@ def cover_bonus_cells(
         occupied if ranged else frozenset(),
         bonus_of=terrain_cover_bonus,
     )
-    return max(
+    # PF1e CRB "Cover", rule of corners: the attacker chooses the corner (and,
+    # for a multi-square creature, the occupied square) that gives it the best
+    # shot, so it suffers only the MINIMUM cover over its own cells. The
+    # target side is unchanged: RAW checks ANY line to ANY target corner, so
+    # each attacker cell is scored by the MAXIMUM over its corner lines.
+    per_cell_best = (
+        max(
             lines.bonus(corner_a, corner_t)
-        for cell_a in attacker_cells
             for corner_a in _corners(cell_a)
             for cell_t in target_cells
             for corner_t in _corners(cell_t)
         )
+        for cell_a in attacker_cells
+    )
+    return min(per_cell_best)

tests/test_conformance_cover.py

@@ -0,0 +1,152 @@
+"""Conformance B20: cover uses the ATTACKER's best corner/square.
+
+PF1e CRB "Cover" (rule of corners): the attacker chooses the corner of its
+square (and, for a multi-square creature, the occupied square) that gives it
+the best shot, so a multi-cell attacker suffers the MINIMUM cover over its
+own cells, not the maximum. The target side is unchanged: RAW checks ANY
+line to ANY target corner, so each attacker cell is scored by the maximum
+over its corner lines. 1x1 attackers are bit-identical to the old behavior.
+"""
+
+from __future__ import annotations
+
+from pf1e_simulator.combat import CombatEngine
+from pf1e_simulator.grid import Grid
+from pf1e_simulator.los import cover_bonus_cells
+from pf1e_simulator.map import MapSpec, TerrainType
+from pf1e_simulator.rng import ScriptedRng
+from tests.test_combat import make_combatant, make_state
+
+_FLOOR = TerrainType(type="floor", move_cost=1)
+_WALL = TerrainType(type="wall", move_cost=None, blocks_los=True)
+_PARTIAL = TerrainType(type="ruin", move_cost=None, cover_bonus=2)
+
+
+def _grid(rows: tuple[str, ...]) -> Grid:
+    legend = {".": _FLOOR, "#": _WALL, "W": _PARTIAL}
+    return Grid.from_spec(MapSpec(name="b20", terrain=rows, legend=legend))
+
+
+# Two-cell attacker {(0,0),(1,0)} shooting right at target (1,4):
+# the +2 ruins at (0,2)/(1,2) sit on the top cell's lines, the +4 wall at
+# (2,1) additionally sits on the bottom cell's low lines.
+_CORNER_GRID = ("..W..", "..W..", ".#...", ".....")
+
+
+# ---------------------------------------------------------------------------
+# (a) multi-cell attacker picks its best square: minimum, not maximum
+# ---------------------------------------------------------------------------
+
+
+def test_two_cell_attacker_uses_best_cell_cover() -> None:
+    grid = _grid(_CORNER_GRID)
+    top = frozenset({(0, 0)})
+    bottom = frozenset({(1, 0)})
+    target = frozenset({(1, 4)})
+    # Per-cell scores pin the geometry: the top cell only sees +2 ruins,
+    # the bottom cell's low lines also graze the +4 wall.
+    assert cover_bonus_cells(grid, top, target, ranged=True) == 2
+    assert cover_bonus_cells(grid, bottom, target, ranged=True) == 4
+    # B20 fix: the attacker picks its best square -> min(2, 4) = 2.
+    assert (
+        cover_bonus_cells(grid, top | bottom, target, ranged=True) == 2
+    )
+
+
+def test_two_cell_attacker_best_cell_alone_is_unchanged() -> None:
+    grid = _grid(_CORNER_GRID)
+    target = frozenset({(1, 4)})
+    # A single-square attacker is the golden path: exactly its own score.
+    assert cover_bonus_cells(grid, frozenset({(0, 0)}), target, ranged=True) == 2
+    assert cover_bonus_cells(grid, frozenset({(1, 0)}), target, ranged=True) == 4
+
+
+# ---------------------------------------------------------------------------
+# (b) 1x1 attacker vs 2x2 target: target side keeps the max over corners
+# ---------------------------------------------------------------------------
+
+
+def test_single_attacker_vs_large_target_pins_cover_level() -> None:
+    grid = _grid(("......", "..W...", "......"))
+    attacker = frozenset({(1, 0)})
+    large_target = frozenset({(0, 4), (0, 5), (1, 4), (1, 5)})
+    # Cover is evaluated over ALL target corners: the big target keeps the
+    # +2 it gains from the ruin, exactly as before the B20 fix.
+    assert cover_bonus_cells(grid, attacker, large_target, ranged=True) == 2
+    assert cover_bonus_cells(grid, attacker, frozenset({(1, 4)}), ranged=True) == 2
+
+
+# ---------------------------------------------------------------------------
+# (c) regression: single-square attacker vs wall still gets exactly +4
+# ---------------------------------------------------------------------------
+
+
+def test_single_square_attacker_wall_cover_unchanged() -> None:
+    grid = _grid((".....", "..#..", "....."))
+    assert (
+        cover_bonus_cells(
+            grid, frozenset({(1, 0)}), frozenset({(1, 4)}), ranged=True
+        )
+        == 4
+    )
+
+
+# ---------------------------------------------------------------------------
+# (d) engine: Large attacker resolves with its best cell; snapshot agrees
+# ---------------------------------------------------------------------------
+
+
+def _large_archer_engine() -> CombatEngine:
+    grid = _grid(_CORNER_GRID)
+    archer = make_combatant(
+        "ogre-archer", attack_bonus=4, kind="ranged", range_increment_ft=60
+    )
+    large = archer.model_copy(update={"size": "Large"})
+    target = make_combatant("target", ac=13)
+    states = [
+        make_state(large, "players", (0, 0)),
+        make_state(target, "monsters", (1, 4)),
+    ]
+    return CombatEngine(ScriptedRng([12, 3, 2, 2]), grid, states)
+
+
+def test_large_attacker_ac_uses_best_cell_cover() -> None:
+    engine = _large_archer_engine()
+    archer, target = engine._states[0], engine._states[1]
+    assert len(archer.occupied_cells()) == 4
+    result = engine.resolve_attack(archer, target, archer.combatant.attacks[0])
+    # 13 base + 2 (best cell of the footprint), NOT 13 + 4 (worst cell).
+    assert result.ac == 15
+    assert result.hit is True  # 12 + 4 = 16 >= 15
+
+
+def test_observation_pair_cover_bonus_matches_attack() -> None:
+    engine = _large_archer_engine()
+    archer, target = engine._states[0], engine._states[1]
+    obs = engine._build_observation(archer).to_dict()
+    pair = next(p for p in obs["pairs"] if p["target_index"] == 1)
+    result = engine.resolve_attack(archer, target, archer.combatant.attacks[0])
+    assert pair["cover_bonus"] == 2
+    assert result.ac - target.combatant.ac.total == pair["cover_bonus"]
+
+
+# ---------------------------------------------------------------------------
+# (e) false-positive lock: cover still applies to touch attacks (Oracle)
+# ---------------------------------------------------------------------------
+
+
+def test_ranged_touch_attack_still_gets_cover() -> None:
+    grid = _grid((".....", "..#..", "....."))
+    caster = make_combatant(
+        "sorcerer", attack_bonus=4, kind="touch", range_increment_ft=30
+    )
+    target = make_combatant("target", ac=13)
+    states = [
+        make_state(caster, "players", (1, 0)),
+        make_state(target, "monsters", (1, 4)),
+    ]
+    engine = CombatEngine(ScriptedRng([12, 3]), grid, states)
+    result = engine.resolve_attack(states[0], states[1], caster.attacks[0])
+    # Touch AC 13 + 4 standard cover: cover is NOT removed for touch.
+    assert result.ac == 17
+    assert result.hit is False  # 12 + 4 = 16 < 17