sky(phase-3a): per-Dereth-day weather roll + ACDREAM_DAY_GROUP override

Diagnosed the "retail shows early night, acdream shows early day" time
mismatch: server time sync was working correctly (ticks=291079558 →
dayFraction=0.8546 → EvensongAndHalf, hour 14 of 16). The mismatch was
the hardcoded DayGroup index.

Dereth's SkyDesc carries 20 DayGroups (Sunny / Clear / Cloudy / Storm /
etc), each weighted at ChanceOfOccur=5.0. Retail rolls one per server
day by `ChanceOfOccur` as a PDF (r12 §11). We were always rendering
DayGroup 0 = "Sunny" regardless of day, so at EvensongAndHalf we showed
SkyTime[7]@0.84 — sun still 20° above the western horizon, warm golden
— i.e. pre-sunset rather than the dimmer pre-night appearance retail
shows after rolling a cloudier group.

Fix (Phase 3a):
- LoadedSkyDesc.SelectDayGroupIndex(dayIndex) — deterministic roller:
  SplitMix64 hash of dayIndex → normalize to [0, sumChances) → walk the
  cumulative distribution. Same dayIndex on every client = same weather
  on every client, zero network sync needed.
- LoadedSkyDesc.ActiveDayGroup(ticks) / BuildProviderForDay(ticks) —
  convenience wrappers that compute dayIndex from raw server ticks.
- ACDREAM_DAY_GROUP=<N> env var override. Set to 10 "Clear", 12 "Cloudy",
  etc. for A/B visual verification against retail.
- SyncFromServer gains a [sky-dump] log: `ticks=X dayFraction=Y
  calendar=PY{year} {month} {day} {hour}` so the time-sync state is
  auditable from a single grep.
- GameWindow: tracks _loadedSkyDayIndex + _activeDayGroup. Calls
  RefreshSkyForCurrentDay on every server sync — swaps WorldTime's
  provider + caches the group only when the day index crosses a
  boundary (idempotent within a single day). SkyRenderer.Render now
  consumes _activeDayGroup instead of the legacy DefaultDayGroup.

Observed (this session, local ACE):
  server sent ticks=291079558 → PY106 ColdMeet 10 EvensongAndHalf
  SplitMix64(day 38197) will deterministically pick one of 20 groups.

Build + 717 tests green. Ready for user visual verification with
retail side-by-side.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -295,6 +295,15 @@ public sealed class GameWindow : IDisposable
     private AcDream.App.Rendering.Sky.SkyRenderer? _skyRenderer;
     private AcDream.Core.World.LoadedSkyDesc? _loadedSkyDesc;
 
+    // Phase 3a — retail-faithful per-Dereth-day weather roll. The active
+    // DayGroup is re-picked deterministically whenever the server clock
+    // crosses a DayTicks boundary. <c>long.MinValue</c> sentinel means
+    // "no day rolled yet" so the first RefreshSkyForCurrentDay call
+    // unconditionally installs a provider. See r12 §11 for the roller
+    // semantics.
+    private long _loadedSkyDayIndex = long.MinValue;
+    private AcDream.Core.World.DayGroupData? _activeDayGroup;
+
     // Current rain/snow emitter handles — spawned on weather-kind change
     // and stopped when the kind leaves Rain/Snow. Non-zero == active.
     private int _rainEmitterHandle;
@@ -868,18 +877,16 @@ public sealed class GameWindow : IDisposable
             _loadedSkyDesc = AcDream.Core.World.SkyDescLoader.LoadFromRegion(region);
             if (_loadedSkyDesc is not null)
             {
-                WorldTime.SetProvider(_loadedSkyDesc.BuildDefaultProvider());
                 WorldTime.TickSize = _loadedSkyDesc.TickSize > 0 ? _loadedSkyDesc.TickSize : 1.0;
                 Console.WriteLine(
                     $"sky: loaded Region 0x13000000 — {_loadedSkyDesc.DayGroups.Count} day groups, " +
                     $"TickSize={_loadedSkyDesc.TickSize}, LightTickSize={_loadedSkyDesc.LightTickSize}");
-                if (_loadedSkyDesc.DefaultDayGroup is not null)
-                {
-                    Console.WriteLine(
-                        $"sky: default group '{_loadedSkyDesc.DefaultDayGroup.Name}' has " +
-                        $"{_loadedSkyDesc.DefaultDayGroup.SkyObjects.Count} sky objects, " +
-                        $"{_loadedSkyDesc.DefaultDayGroup.SkyTimes.Count} keyframes");
-                }
+
+                // Initial DayGroup roll using whatever WorldTime currently
+                // has (either the hardcoded boot seed or a pre-arrived
+                // server sync). RefreshSkyForCurrentDay will re-roll when
+                // ServerTimeUpdated delivers the real ConnectRequest tick.
+                RefreshSkyForCurrentDay();
             }
         }
 
@@ -1024,7 +1031,15 @@ public sealed class GameWindow : IDisposable
             // Phase G.1: keep the client's day/night clock in sync with
             // server time. Fires once from ConnectRequest (initial seed)
             // and repeatedly on TimeSync-flagged packets.
-            _liveSession.ServerTimeUpdated += ticks => WorldTime.SyncFromServer(ticks);
+            // Phase 3a: also re-roll the active DayGroup if the Dereth-day
+            // index changed — retail rolls one weather preset per server
+            // day (r12 §11), deterministic from the day index so retail
+            // and acdream converge without a wire message.
+            _liveSession.ServerTimeUpdated += ticks =>
+            {
+                WorldTime.SyncFromServer(ticks);
+                RefreshSkyForCurrentDay();
+            };
 
             // Phase F.1-H.1: wire every parsed GameEvent into the right
             // Core state class (chat, combat, spellbook, items). After
@@ -3612,7 +3627,7 @@ public sealed class GameWindow : IDisposable
             if (!cameraInsideCell)
             {
                 _skyRenderer?.Render(camera, camPos, (float)WorldTime.DayFraction,
-                    _loadedSkyDesc?.DefaultDayGroup, kf);
+                    _activeDayGroup, kf);
             }
 
             _terrain?.Draw(camera, frustum, neverCullLandblockId: playerLb);
@@ -4280,6 +4295,56 @@ public sealed class GameWindow : IDisposable
         ae.CurrFrame = ae.LowFrame;
     }
 
+    /// <summary>
+    /// Phase 3a — re-roll the active DayGroup whenever the current
+    /// Dereth-day index differs from what we last installed. Idempotent
+    /// within the same server-day. Swaps both the
+    /// <see cref="AcDream.Core.World.SkyStateProvider"/> feeding
+    /// <see cref="WorldTime"/> (for lighting interp) and the cached
+    /// <see cref="_activeDayGroup"/> (for the sky-object render loop).
+    ///
+    /// <para>
+    /// Honors <c>ACDREAM_DAY_GROUP=N</c> — when set, every call picks
+    /// group N regardless of day index. Useful for A/B testing each
+    /// weather preset against retail. See
+    /// <see cref="AcDream.Core.World.LoadedSkyDesc.SelectDayGroupIndex"/>
+    /// for the roller.
+    /// </para>
+    /// </summary>
+    private void RefreshSkyForCurrentDay()
+    {
+        if (_loadedSkyDesc is null || _loadedSkyDesc.DayGroups.Count == 0)
+            return;
+
+        long dayIndex = (long)System.Math.Floor(
+            WorldTime.NowTicks / AcDream.Core.World.DerethDateTime.DayTicks);
+
+        int idx = _loadedSkyDesc.SelectDayGroupIndex(dayIndex);
+        var grp = idx >= 0 && idx < _loadedSkyDesc.DayGroups.Count
+            ? _loadedSkyDesc.DayGroups[idx]
+            : null;
+
+        bool dayChanged = dayIndex != _loadedSkyDayIndex;
+        bool groupChanged = !ReferenceEquals(grp, _activeDayGroup);
+
+        if (!dayChanged && !groupChanged) return;
+
+        _loadedSkyDayIndex = dayIndex;
+        _activeDayGroup = grp;
+
+        if (grp is not null && grp.SkyTimes.Count > 0)
+        {
+            WorldTime.SetProvider(
+                new AcDream.Core.World.SkyStateProvider(
+                    grp.SkyTimes.Select(s => s.Keyframe).ToList()));
+
+            Console.WriteLine(
+                $"sky: day {dayIndex} → DayGroup[{idx}] \"{grp.Name}\" " +
+                $"(Chance={grp.ChanceOfOccur:F2}, {grp.SkyObjects.Count} objects, " +
+                $"{grp.SkyTimes.Count} keyframes)");
+        }
+    }
+
     /// <summary>
     /// Derive the current sun (directional light, slot 0 of the UBO)
     /// from the interpolated <see cref="AcDream.Core.World.SkyKeyframe"/>,

src/AcDream.Core/World/SkyDescLoader.cs

@@ -143,15 +143,99 @@ public sealed class LoadedSkyDesc
     public IReadOnlyList<DayGroupData> DayGroups = Array.Empty<DayGroupData>();
 
     /// <summary>
-    /// Default day group — currently group 0 per WorldBuilder's
-    /// <c>SkyboxRenderManager.Render</c>. Weather integration later picks
-    /// the current day's group by <c>ChanceOfOccur</c>.
+    /// Pick a <see cref="DayGroupData"/> deterministically for a given
+    /// in-game day. Matches r12 §11: retail rolls one DayGroup per Derethian
+    /// day using <see cref="DayGroupData.ChanceOfOccur"/> as a PDF weight,
+    /// seeded by the day index so all clients in the same server-day see
+    /// the same weather. Use <see cref="DerethDateTime.DayTicks"/>-floored
+    /// ticks as <paramref name="dayIndex"/> to stay in sync with the
+    /// server's <c>PortalYearTicks</c>.
+    ///
+    /// <para>
+    /// Honors the <c>ACDREAM_DAY_GROUP</c> environment variable as a
+    /// force-override (useful for visual verification: set it to 12 =
+    /// "Cloudy" or 10 = "Clear" to confirm the weather-selection
+    /// hypothesis without running through the roller).
+    /// </para>
     /// </summary>
-    public DayGroupData? DefaultDayGroup =>
-        DayGroups.Count > 0 ? DayGroups[0] : null;
+    public int SelectDayGroupIndex(long dayIndex)
+    {
+        if (DayGroups.Count == 0) return 0;
+
+        // Env-var override has absolute priority.
+        var env = System.Environment.GetEnvironmentVariable("ACDREAM_DAY_GROUP");
+        if (int.TryParse(env, System.Globalization.NumberStyles.Integer,
+                System.Globalization.CultureInfo.InvariantCulture, out var forced)
+            && forced >= 0 && forced < DayGroups.Count)
+        {
+            return forced;
+        }
+
+        if (DayGroups.Count == 1) return 0;
+
+        // SplitMix64-style deterministic hash of the day index so all
+        // clients roll the same weather for the same Dereth-day without
+        // any network synchronization (r12 §11, "client-local weather
+        // model"). The constants are the canonical SplitMix64 mixer from
+        // the JDK's Splittable Random — high avalanche quality.
+        ulong h = (ulong)dayIndex;
+        h = (h ^ (h >> 30)) * 0xbf58476d1ce4e5b9UL;
+        h = (h ^ (h >> 27)) * 0x94d049bb133111ebUL;
+        h ^= (h >> 31);
+
+        // Sum ChanceOfOccur to get the weighted total.
+        double total = 0;
+        for (int i = 0; i < DayGroups.Count; i++)
+            if (DayGroups[i].ChanceOfOccur > 0)
+                total += DayGroups[i].ChanceOfOccur;
+
+        // Fall back to uniform if weights are missing / zero.
+        if (total <= 0)
+            return (int)(h % (ulong)DayGroups.Count);
+
+        // Pick = h normalized to [0, total), then cumulative-distribution walk.
+        double pick = (h / (double)ulong.MaxValue) * total;
+        double cum = 0;
+        for (int i = 0; i < DayGroups.Count; i++)
+        {
+            if (DayGroups[i].ChanceOfOccur > 0)
+                cum += DayGroups[i].ChanceOfOccur;
+            if (pick < cum) return i;
+        }
+        return DayGroups.Count - 1;
+    }
 
     /// <summary>
-    /// Build a shader-facing <see cref="SkyStateProvider"/> for the default day group.
+    /// Convenience: compute the day index from current server ticks and
+    /// look up today's rolled DayGroup. Safe to call off the render thread.
+    /// </summary>
+    public DayGroupData? ActiveDayGroup(double serverTicks)
+    {
+        long dayIndex = (long)System.Math.Floor(serverTicks / DerethDateTime.DayTicks);
+        int idx = SelectDayGroupIndex(dayIndex);
+        return idx < DayGroups.Count ? DayGroups[idx] : null;
+    }
+
+    /// <summary>
+    /// Legacy accessor — kept for callers that don't yet know the server
+    /// tick count. Rolls against <c>dayIndex=0</c> (so env-var override
+    /// works but unforced selection always returns the same group).
+    /// Prefer <see cref="ActiveDayGroup(double)"/> which syncs to the
+    /// server clock.
+    /// </summary>
+    public DayGroupData? DefaultDayGroup
+    {
+        get
+        {
+            int idx = SelectDayGroupIndex(0);
+            return DayGroups.Count > 0 ? DayGroups[idx] : null;
+        }
+    }
+
+    /// <summary>
+    /// Build a shader-facing <see cref="SkyStateProvider"/> for the
+    /// default day group. For retail-faithful day-rolling, rebuild this
+    /// whenever <see cref="ActiveDayGroup"/> changes (once per Dereth-day).
     /// </summary>
     public SkyStateProvider BuildDefaultProvider()
     {
@@ -159,6 +243,18 @@ public sealed class LoadedSkyDesc
         if (grp is null || grp.SkyTimes.Count == 0) return SkyStateProvider.Default();
         return new SkyStateProvider(grp.SkyTimes.Select(s => s.Keyframe).ToList());
     }
+
+    /// <summary>
+    /// Build a <see cref="SkyStateProvider"/> for the DayGroup rolled for
+    /// <paramref name="serverTicks"/>. Call on day rollover to swap the
+    /// interpolator to the new weather regime.
+    /// </summary>
+    public SkyStateProvider BuildProviderForDay(double serverTicks)
+    {
+        var grp = ActiveDayGroup(serverTicks);
+        if (grp is null || grp.SkyTimes.Count == 0) return SkyStateProvider.Default();
+        return new SkyStateProvider(grp.SkyTimes.Select(s => s.Keyframe).ToList());
+    }
 }
 
 /// <summary>

src/AcDream.Core/World/SkyState.cs

@@ -289,8 +289,17 @@ public sealed class WorldTimeService
     public void SyncFromServer(double serverTicks)
     {
         _lastSyncedTicks = serverTicks;
-        _lastSyncedWallClockUtc = DateTime.UtcNow;
+        _lastSyncedWallClockUtc = System.DateTime.UtcNow;
         _debugDayFractionOverride = null;
+
+        if (System.Environment.GetEnvironmentVariable("ACDREAM_DUMP_SKY") == "1")
+        {
+            var df = DerethDateTime.DayFraction(serverTicks);
+            var cal = DerethDateTime.ToCalendar(serverTicks);
+            System.Console.WriteLine(
+                $"[sky-dump] SyncFromServer: ticks={serverTicks:F1} dayFraction={df:F4} " +
+                $"calendar=PY{cal.Year} {cal.Month} {cal.Day} {cal.Hour}");
+        }
     }
 
     /// <summary>