refactor(runtime): own world environment state

src/AcDream.Core/World/SkyDescLoader.cs

@@ -221,48 +221,22 @@ public sealed class LoadedSkyDesc
     /// </summary>
     public int SelectDayGroupIndex(int year, int secondsPerDay, int dayOfYear)
     {
-        if (DayGroups.Count == 0) return 0;
-
         // Env-var override has absolute priority.
         var env = System.Environment.GetEnvironmentVariable("ACDREAM_DAY_GROUP");
+        int? forcedIndex = null;
         if (int.TryParse(env, System.Globalization.NumberStyles.Integer,
                 System.Globalization.CultureInfo.InvariantCulture, out var forced)
             && forced >= 0 && forced < DayGroups.Count)
         {
-            return forced;
+            forcedIndex = forced;
         }
 
-        if (DayGroups.Count == 1) return 0;
-
-        // --- Retail SkyDesc::CalcPresentDayGroup @ 0x00500E10 ---
-
-        // Step 1: deterministic per-day seed.
-        int seed = unchecked(year * secondsPerDay + dayOfYear);
-
-        // Step 2: 32-bit signed LCG (retail uses x86 silent wrap; force
-        // unchecked in C#). `0x8ABE1652` is stored as `-0x7541E9AE` in the
-        // decompile — same bit pattern.
-        int mixed = unchecked(seed * 0x6A42FDB2 + unchecked((int)0x8ABE1652));
-
-        // Step 3: signed-int → float with +2^32 fixup for negative values
-        // (retail x87 converts `int` to `float` and then adds
-        // `_DAT_0079920c` ≈ 4294967296.0f when `iVar4 < 0`).
-        float hashF = (float)mixed;
-        if (mixed < 0) hashF += 4294967296.0f;
-
-        // Step 4: scale to [0, dayGroupCount). `_DAT_007c6f10` is
-        // `1.0f / 2^32` per reuse pattern (see research doc §2.4).
-        const float kInv2Pow32 = 1.0f / 4294967296.0f;
-        float countF = (float)DayGroups.Count;
-        int index = (int)System.MathF.Floor(countF * hashF * kInv2Pow32);
-
-        // Step 5: safety clamp for float rounding at the upper edge.
-        // Retail does `if (count <= index) index = 0;`. Using the same
-        // "snap to 0 on overflow" instead of clamping to count-1.
-        if (index >= DayGroups.Count) index = 0;
-        if (index < 0) index = 0;
-
-        return index;
+        return SkyDayGroupSelector.SelectIndex(
+            DayGroups.Count,
+            year,
+            secondsPerDay,
+            dayOfYear,
+            forcedIndex);
     }
 
     /// <summary>