fix(rules): line spells stop at a line-of-effect barrier (SRD aiming-a-spell.md: a line 'extends to the limit of its range or until it strikes a barrier that blocks line of effect') - line_cells truncates at the first blocks_los cell, creatures behind a solid wall are never caught

src/pf1e_simulator/areas.py

@@ -14,8 +14,12 @@ PF1e (CRB, Magic — Aiming a Spell):
   line's length. The line "starts from any corner of your square", so the
   CASTER's own square is NOT one of the affected squares: the affected
   squares are the ones beyond the origin along the ray (SRD Lightning Bolt:
-  "The bolt begins at your fingertips" — it leaves your square, it does not
-  fill it).
+  "The bolt begins at your fingertips" - it leaves your square, it does not
+  fill it). The line also stops at the first square that blocks line of
+  effect (CRB, Magic - Aiming a Spell, Line: it "extends to the limit of its
+  range or until it strikes a barrier that blocks line of effect"): the
+  barrier square itself and everything beyond it are outside the template,
+  so a bolt never reaches creatures behind a solid wall.
 
 Bursts need no template here: they are circles around a center cell and are
 resolved by ``CombatEngine._creatures_in_range`` (unchanged).
@@ -78,10 +82,13 @@ def line_cells(
     The line runs along the Bresenham ray from the origin toward ``aim``,
     extended to ``length_cells`` squares (CRB: it "extends to the limit of
     its range"). The line "starts from any corner of your square" (CRB, Magic
-    — Aiming a Spell), so the CASTER's own square is NOT affected: the
+    - Aiming a Spell), so the CASTER's own square is NOT affected: the
     ``length_cells`` affected squares are the ones *beyond* the origin along
     the ray. Out-of-bounds cells simply drop out of the template (creatures
-    only exist in bounds). A degenerate aim (origin == aim) affects nothing.
+    only exist in bounds). The ray stops at the first cell that blocks line
+    of effect (CRB, Line: "or until it strikes a barrier that blocks line of
+    effect") - that barrier cell and everything beyond it are excluded. A
+    degenerate aim (origin == aim) affects nothing.
     """
     if length_cells <= 0:
         return frozenset()
@@ -98,7 +105,14 @@ def line_cells(
     # 120 ft / 5 ft = 24 squares, none of them the caster's own (SRD Lightning
     # Bolt: "The bolt begins at your fingertips" — it leaves your square).
     ray = _bresenham(origin, end)
-    return frozenset(cell for cell in ray[1 : length_cells + 1] if grid.in_bounds(cell))
+    cells: list[Pos] = []
+    for cell in ray[1 : length_cells + 1]:
+        if not grid.in_bounds(cell):
+            continue
+        if grid.terrain_at(cell).blocks_los:
+            break
+        cells.append(cell)
+    return frozenset(cells)
 
 
 def _bresenham(start: Pos, end: Pos) -> list[Pos]:

tests/test_area_shapes.py

@@ -18,10 +18,11 @@ from typing import TYPE_CHECKING
 
 import pytest
 
+from pf1e_simulator.areas import line_cells
 from pf1e_simulator.combat import Action, 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.map import MapSpec, Pos, TerrainType
 from pf1e_simulator.rng import ScriptedRng
 from pf1e_simulator.spells import (
     AreaEffect,
@@ -103,6 +104,20 @@ def _open_grid(side: int = 12) -> Grid:
     return Grid.from_spec(spec)
 
 
+def _walled_grid(walls: frozenset[Pos], side: int = 12) -> Grid:
+    """Open ``side``x``side`` floor with impassable walls at ``walls``."""
+    legend = {
+        ".": TerrainType(type="floor", move_cost=1),
+        "#": TerrainType(type="wall", move_cost=None, blocks_los=True),
+    }
+    rows = tuple(
+        "".join("#" if (r, c) in walls else "." for c in range(side))
+        for r in range(side)
+    )
+    spec = MapSpec(name="walled", terrain=rows, legend=legend)
+    return Grid.from_spec(spec)
+
+
 def _engine(states: list[CombatantState], rolls: list[int]) -> CombatEngine:
     return CombatEngine(
         ScriptedRng(rolls), _open_grid(), states, spell_registry=_REGISTRY
@@ -232,6 +247,43 @@ def test_line_follows_a_diagonal_bresenham_ray() -> None:
     assert foe_off.hp == 100
 
 
+def test_line_cells_stop_at_a_line_of_effect_barrier() -> None:
+    """Given: a wall cell (6,6) on the east ray from origin (6,2)
+    When: line_cells builds a 10-cell template
+    Then: cells before the barrier are in the template; the barrier cell and
+    everything beyond it are not (CRB: the line "extends to the limit of its
+    range or until it strikes a barrier that blocks line of effect")."""
+    grid = _walled_grid(frozenset({(6, 6)}))
+    cells = line_cells(grid, (6, 2), (6, 8), 10)
+    assert (6, 5) in cells
+    assert (6, 6) not in cells
+    assert (6, 7) not in cells
+    assert (6, 8) not in cells
+
+
+def test_line_behind_a_wall_does_not_hit_sheltered_creatures() -> None:
+    """Given: caster (6,2) aiming east; a wall column at col 6 (rows 4-8);
+    foe_front (6,4) on the near side, foe_behind (6,8) fully behind the wall
+    When: cast the 120-ft line (24 cells - long enough to cross the map)
+    Then: the bolt stops at the barrier: only foe_front is caught (one 1d6
+    consumed); foe_behind is untouched (SRD Lightning Bolt cannot reach
+    around or through a solid wall)."""
+    caster = _caster()
+    walls = frozenset({(4, 6), (5, 6), (6, 6), (7, 6), (8, 6)})
+    foe_front = make_state(make_combatant("foe_front", hp=100), "monsters", (6, 4))
+    foe_behind = make_state(make_combatant("foe_behind", hp=100), "monsters", (6, 8))
+    eng = CombatEngine(
+        ScriptedRng([4]), _walled_grid(walls), [caster, foe_front, foe_behind],
+        spell_registry=_REGISTRY,
+    )
+    result = eng._execute(
+        caster, Action(kind="cast_spell", target_id="foe_front", spell_name="Line Sparks")
+    )
+    assert result.ok
+    assert foe_front.hp == 100 - 4
+    assert foe_behind.hp == 100
+
+
 # ---------------------------------------------------------------------------
 # burst: unchanged circle around the resolved center
 # ---------------------------------------------------------------------------