refactor(runtime): own world environment state

src/AcDream.Core/World/SkyDayGroupSelector.cs

@@ -0,0 +1,40 @@
+namespace AcDream.Core.World;
+
+/// <summary>
+/// Retail-verbatim day-group selection from
+/// <c>SkyDesc::CalcPresentDayGroup @ 0x00500E10</c>.
+/// </summary>
+public static class SkyDayGroupSelector
+{
+    public static int SelectIndex(
+        int dayGroupCount,
+        int absoluteYear,
+        int daysPerYear,
+        int dayOfYear,
+        int? forcedIndex = null)
+    {
+        if (dayGroupCount <= 0)
+            return 0;
+
+        if (forcedIndex is >= 0 && forcedIndex < dayGroupCount)
+            return forcedIndex.Value;
+
+        if (dayGroupCount == 1)
+            return 0;
+
+        int seed = unchecked(absoluteYear * daysPerYear + dayOfYear);
+        int mixed = unchecked(
+            seed * 0x6A42FDB2 + unchecked((int)0x8ABE1652));
+
+        float hash = mixed;
+        if (mixed < 0)
+            hash += 4294967296.0f;
+
+        const float inverseTwoTo32 = 1.0f / 4294967296.0f;
+        int index = (int)MathF.Floor(
+            (float)dayGroupCount * hash * inverseTwoTo32);
+
+        // Retail snaps the floating-point upper-edge overflow to zero.
+        return index < 0 || index >= dayGroupCount ? 0 : index;
+    }
+}