Add visibility system for time and weather effects

Night, fog, and storms now reduce viewport size. Night reduces by 6
width and 2 height (21x11 -> 15x9). Thick fog reduces by 8 width and 4
height. Storm reduces by 4 width and 2 height. Effects stack but clamp
to minimum 7x5. Dawn and dusk subtly dim by 2 width.

src/mudlib/commands/look.py

@@ -17,6 +17,7 @@ from mudlib.render.room import (
 )
 from mudlib.seasons import get_season
 from mudlib.thing import Thing
+from mudlib.visibility import get_visibility
 from mudlib.weather import (
     WeatherCondition,
     get_current_weather,
@@ -90,16 +91,38 @@ async def cmd_look(player: Player, args: str) -> None:
         await player.writer.drain()
         return
 
+    # Compute environment state once
+    hour = None
+    day = None
+    weather = None
+    season = None
+    try:
+        hour = get_game_hour()
+        day = get_game_day()
+        weather = get_current_weather()
+        season = get_season(day)
+    except RuntimeError:
+        pass
+
+    # Use hour/weather for visibility
+    if hour is not None and weather is not None:
+        effective_width, effective_height = get_visibility(
+            hour, weather, VIEWPORT_WIDTH, VIEWPORT_HEIGHT
+        )
+    else:
+        effective_width = VIEWPORT_WIDTH
+        effective_height = VIEWPORT_HEIGHT
+
     # Get the viewport from the zone
-    viewport = zone.get_viewport(player.x, player.y, VIEWPORT_WIDTH, VIEWPORT_HEIGHT)
+    viewport = zone.get_viewport(player.x, player.y, effective_width, effective_height)
 
     # Calculate center position
-    center_x = VIEWPORT_WIDTH // 2
+    center_x = effective_width // 2
-    center_y = VIEWPORT_HEIGHT // 2
+    center_y = effective_height // 2
 
     # Get nearby entities (players and mobs) from the zone
     # Viewport half-diagonal distance for range
-    viewport_range = VIEWPORT_WIDTH // 2 + VIEWPORT_HEIGHT // 2
+    viewport_range = effective_width // 2 + effective_height // 2
     nearby = zone.contents_near(player.x, player.y, viewport_range)
 
     # Build a list of (relative_x, relative_y) for other entities
@@ -129,13 +152,13 @@ async def cmd_look(player: Player, args: str) -> None:
         rel_y = dy + center_y
 
         # Check if within viewport bounds
-        if 0 <= rel_x < VIEWPORT_WIDTH and 0 <= rel_y < VIEWPORT_HEIGHT:
+        if 0 <= rel_x < effective_width and 0 <= rel_y < effective_height:
             entity_positions.append((rel_x, rel_y))
 
     # Build the output with ANSI coloring
     # priority: player @ > other players * > mobs * > effects > terrain
-    half_width = VIEWPORT_WIDTH // 2
+    half_width = effective_width // 2
-    half_height = VIEWPORT_HEIGHT // 2
+    half_height = effective_height // 2
 
     output_lines = []
     for y, row in enumerate(viewport):
@@ -169,13 +192,7 @@ async def cmd_look(player: Player, args: str) -> None:
     output.append(render_where(zone.description))
 
     # Atmosphere line (between Where and viewport)
-    try:
+    if hour is not None and weather is not None and season is not None:
-        hour = get_game_hour()
-        day = get_game_day()
-        weather = get_current_weather()
-        season = get_season(day)
-
-        # Get weather description (empty string for clear)
         if weather.condition == WeatherCondition.clear:
             weather_desc = ""
         else:
@@ -183,9 +200,6 @@ async def cmd_look(player: Player, args: str) -> None:
 
         atmosphere = render_atmosphere(hour, weather_desc, season)
         output.append(atmosphere)
-    except RuntimeError:
-        # Game time not initialized, skip atmosphere
-        pass
 
     # Viewport
     output.append("\r\n".join(output_lines))

src/mudlib/visibility.py

@@ -0,0 +1,54 @@
+"""Visibility calculations based on time and weather."""
+
+from mudlib.weather import WeatherCondition, WeatherState
+
+
+def get_visibility(
+    hour: int,
+    weather: WeatherState,
+    base_width: int = 21,
+    base_height: int = 11,
+) -> tuple[int, int]:
+    """Calculate effective viewport dimensions.
+
+    Night, fog, and storms reduce visibility. Multiple effects stack
+    but never reduce below minimum (7x5).
+
+    Returns:
+        Tuple of (effective_width, effective_height)
+    """
+    width = base_width
+    height = base_height
+
+    # Time-based reductions
+    if hour >= 20 or hour <= 4:
+        # Night (hours 20-4)
+        width -= 6
+        height -= 2
+    elif 5 <= hour <= 6:
+        # Dawn (hours 5-6)
+        width -= 2
+    elif 18 <= hour <= 19:
+        # Dusk (hours 18-19)
+        width -= 2
+
+    # Weather-based reductions
+    if weather.condition == WeatherCondition.fog:
+        if weather.intensity >= 0.7:
+            # Thick fog
+            width -= 8
+            height -= 4
+        elif weather.intensity >= 0.4:
+            # Moderate fog
+            width -= 4
+            height -= 2
+    elif weather.condition == WeatherCondition.storm:
+        # Storm
+        width -= 4
+        height -= 2
+
+    # Clamp to minimum
+    width = max(7, width)
+    height = max(5, height)
+
+    return width, height

tests/test_look_command.py

@@ -337,3 +337,83 @@ async def test_look_atmosphere_between_where_and_viewport():
     bracket_pos = output.find("[")
 
     assert where_pos < bracket_pos < location_pos
+
+
+@pytest.mark.asyncio
+async def test_look_night_reduces_viewport():
+    """Look at night should show smaller viewport than day."""
+
+    # Set time to noon (hour 12) - epoch was 12 hours ago
+    # elapsed_game_hours = (time.time() - epoch) / 60 / 1.0 = 12
+    # so epoch = time.time() - 12 * 60
+    init_game_time(epoch=time.time() - 12 * 60, real_minutes_per_game_hour=1.0)
+    init_weather(condition=WeatherCondition.clear, intensity=0.5)
+
+    mock_reader = MagicMock()
+    mock_writer = MagicMock()
+    mock_writer.write = MagicMock()
+    mock_writer.drain = AsyncMock()
+
+    terrain = [["." for _ in range(50)] for _ in range(50)]
+    zone = Zone(
+        name="test_zone",
+        description="The Test Zone",
+        width=50,
+        height=50,
+        terrain=terrain,
+        impassable={"^", "~"},
+    )
+
+    player = Player(
+        name="TestPlayer", x=25, y=25, reader=mock_reader, writer=mock_writer
+    )
+    player.location = zone
+    zone._contents.append(player)
+
+    await cmd_look(player, "")
+    day_output = get_output(player)
+
+    # Find viewport section (between atmosphere and Location:)
+    day_lines = day_output.split("\r\n")
+    # Viewport is after atmosphere (containing '[') and before Location:
+    viewport_start_idx = None
+    viewport_end_idx = None
+    for i, line in enumerate(day_lines):
+        if "[" in line and viewport_start_idx is None:
+            viewport_start_idx = i + 1
+        if line.startswith("Location:"):
+            viewport_end_idx = i
+            break
+
+    assert viewport_start_idx is not None
+    assert viewport_end_idx is not None
+    day_viewport_height = viewport_end_idx - viewport_start_idx
+
+    # Reset writer
+    mock_writer.reset_mock()
+
+    # Set time to night (hour 22) - epoch was 22 hours ago
+    init_game_time(epoch=time.time() - 22 * 60, real_minutes_per_game_hour=1.0)
+
+    await cmd_look(player, "")
+    night_output = get_output(player)
+
+    # Find viewport section
+    night_lines = night_output.split("\r\n")
+    viewport_start_idx = None
+    viewport_end_idx = None
+    for i, line in enumerate(night_lines):
+        if "[" in line and viewport_start_idx is None:
+            viewport_start_idx = i + 1
+        if line.startswith("Location:"):
+            viewport_end_idx = i
+            break
+
+    assert viewport_start_idx is not None
+    assert viewport_end_idx is not None
+    night_viewport_height = viewport_end_idx - viewport_start_idx
+
+    # Night should have fewer viewport lines (9 vs 11)
+    assert night_viewport_height < day_viewport_height
+    assert day_viewport_height == 11  # full viewport at noon
+    assert night_viewport_height == 9  # reduced viewport at night

tests/test_visibility.py

@@ -0,0 +1,135 @@
+"""Tests for visibility calculations."""
+
+from mudlib.visibility import get_visibility
+from mudlib.weather import WeatherCondition, WeatherState
+
+
+def test_clear_day_full_visibility():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 21
+    assert height == 11
+
+
+def test_night_reduces_visibility():
+    hour = 22  # night (20-4)
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 15  # 21 - 6
+    assert height == 9  # 11 - 2
+
+
+def test_thick_fog_during_day():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.fog, intensity=0.8)
+    width, height = get_visibility(hour, weather)
+    assert width == 13  # 21 - 8
+    assert height == 7  # 11 - 4
+
+
+def test_night_plus_thick_fog_clamps_to_minimum():
+    hour = 22  # night
+    weather = WeatherState(condition=WeatherCondition.fog, intensity=0.9)
+    width, height = get_visibility(hour, weather)
+    # Night: -6 width, -2 height (21x11 -> 15x9)
+    # Thick fog: -8 width, -4 height (15x9 -> 7x5)
+    # Should clamp to minimum 7x5
+    assert width == 7
+    assert height == 5
+
+
+def test_storm_reduces_visibility():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.storm, intensity=0.7)
+    width, height = get_visibility(hour, weather)
+    assert width == 17  # 21 - 4
+    assert height == 9  # 11 - 2
+
+
+def test_dawn_subtle_dimming():
+    hour = 5  # dawn (5-6)
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 19  # 21 - 2
+    assert height == 11  # 11 - 0
+
+
+def test_dusk_subtle_dimming():
+    hour = 18  # dusk (18-19)
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 19  # 21 - 2
+    assert height == 11  # 11 - 0
+
+
+def test_moderate_fog():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.fog, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 17  # 21 - 4
+    assert height == 9  # 11 - 2
+
+
+def test_light_fog_no_reduction():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.fog, intensity=0.3)
+    width, height = get_visibility(hour, weather)
+    assert width == 21  # no reduction for light fog
+    assert height == 11
+
+
+def test_cloudy_no_visibility_reduction():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.cloudy, intensity=0.7)
+    width, height = get_visibility(hour, weather)
+    assert width == 21
+    assert height == 11
+
+
+def test_rain_no_visibility_reduction():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.rain, intensity=0.7)
+    width, height = get_visibility(hour, weather)
+    assert width == 21
+    assert height == 11
+
+
+def test_snow_no_visibility_reduction():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.snow, intensity=0.7)
+    width, height = get_visibility(hour, weather)
+    assert width == 21
+    assert height == 11
+
+
+def test_custom_base_dimensions():
+    hour = 12  # noon
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather, base_width=31, base_height=21)
+    assert width == 31
+    assert height == 21
+
+
+def test_night_custom_base():
+    hour = 22  # night
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather, base_width=31, base_height=21)
+    assert width == 25  # 31 - 6
+    assert height == 19  # 21 - 2
+
+
+def test_midnight_is_night():
+    hour = 0  # midnight
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 15  # reduced for night
+    assert height == 9
+
+
+def test_early_morning_is_night():
+    hour = 3  # early morning
+    weather = WeatherState(condition=WeatherCondition.clear, intensity=0.5)
+    width, height = get_visibility(hour, weather)
+    assert width == 15  # reduced for night
+    assert height == 9