refactor(content): share EnvCell geometry identity

src/AcDream.Core/Rendering/Wb/EnvCellGeometryIdentity.cs

@@ -0,0 +1,29 @@
+using System.Collections.Generic;
+
+namespace AcDream.Core.Rendering.Wb;
+
+/// <summary>
+/// Computes WorldBuilder's content identity for an EnvCell shell.
+/// Equal environment, cell-structure, and ordered surface tuples share one
+/// prepared geometry payload even when they occur under different cell IDs.
+/// </summary>
+public static class EnvCellGeometryIdentity
+{
+    /// <summary>
+    /// Source: WorldBuilder <c>EnvCellRenderManager.GetEnvCellGeomId</c>,
+    /// lines 94-103. Bit 33 separates content identities from per-cell IDs,
+    /// whose synthetic geometry requests use bit 32.
+    /// </summary>
+    public static ulong Compute(
+        uint environmentId,
+        ushort cellStructure,
+        IReadOnlyList<ushort> surfaces)
+    {
+        long hash = 17;
+        hash = unchecked(hash * 31 + environmentId);
+        hash = unchecked(hash * 31 + cellStructure);
+        for (int i = 0; i < surfaces.Count; i++)
+            hash = unchecked(hash * 31 + surfaces[i]);
+        return (ulong)hash | 0x2_0000_0000UL;
+    }
+}