feat(core): add SetupMesh.Flatten for single-level part hierarchy

tests/AcDream.Core.Tests/Meshing/SetupMeshTests.cs

@@ -0,0 +1,110 @@
+using System.Numerics;
+using AcDream.Core.Meshing;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Tests.Meshing;
+
+public class SetupMeshTests
+{
+    [Fact]
+    public void Flatten_SinglePartSetup_YieldsOneMeshRef()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u },
+            DefaultScale = { Vector3.One },
+            PlacementFrames =
+            {
+                [Placement.Default] = new AnimationFrame(1)
+                {
+                    Frames =
+                    {
+                        new Frame
+                        {
+                            Origin = new Vector3(0, 0, 0),
+                            Orientation = Quaternion.Identity,
+                        },
+                    },
+                },
+            },
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        var single = Assert.Single(refs);
+        Assert.Equal(0x01000100u, single.GfxObjId);
+        // Identity-ish transform
+        Assert.Equal(Matrix4x4.Identity, single.PartTransform);
+    }
+
+    [Fact]
+    public void Flatten_TwoPartSetup_YieldsTwoMeshRefs()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u, 0x01000200u },
+            DefaultScale = { Vector3.One, Vector3.One },
+            PlacementFrames =
+            {
+                [Placement.Default] = new AnimationFrame(2)
+                {
+                    Frames =
+                    {
+                        new Frame { Origin = new(0, 0, 0), Orientation = Quaternion.Identity },
+                        new Frame { Origin = new(10, 0, 0), Orientation = Quaternion.Identity },
+                    },
+                },
+            },
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Equal(2, refs.Count);
+        Assert.Equal(0x01000100u, refs[0].GfxObjId);
+        Assert.Equal(0x01000200u, refs[1].GfxObjId);
+        // Second part is translated by 10 on X.
+        Assert.Equal(10f, refs[1].PartTransform.Translation.X);
+    }
+
+    [Fact]
+    public void Flatten_PartScale_IsAppliedToTransform()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u },
+            DefaultScale = { new Vector3(2, 3, 4) },
+            PlacementFrames =
+            {
+                [Placement.Default] = new AnimationFrame(1)
+                {
+                    Frames = { new Frame { Orientation = Quaternion.Identity } },
+                },
+            },
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        // The transform's M11 = 2 (scale X), M22 = 3, M33 = 4
+        Assert.Equal(2f, refs[0].PartTransform.M11);
+        Assert.Equal(3f, refs[0].PartTransform.M22);
+        Assert.Equal(4f, refs[0].PartTransform.M33);
+    }
+
+    [Fact]
+    public void Flatten_MissingPlacementFrame_UsesIdentity()
+    {
+        var setup = new Setup
+        {
+            Parts = { 0x01000100u },
+            DefaultScale = { Vector3.One },
+            // PlacementFrames deliberately empty
+        };
+
+        var refs = SetupMesh.Flatten(setup);
+
+        Assert.Single(refs);
+        Assert.Equal(Matrix4x4.Identity, refs[0].PartTransform);
+    }
+}