// Copyright Epic Games, Inc. All Rights Reserved.

#include "StaticMeshBuilder.h"

#include "BuildOptimizationHelper.h"
#include "Components.h"
#include "Engine/StaticMesh.h"
#include "IMeshReductionInterfaces.h"
#include "IMeshReductionManagerModule.h"
#include "MeshBuild.h"
#include "MeshDescriptionHelper.h"
#include "Misc/ScopedSlowTask.h"
#include "Modules/ModuleManager.h"
#include "PhysicsEngine/BodySetup.h"
#include "StaticMeshAttributes.h"
#include "StaticMeshOperations.h"
#include "StaticMeshResources.h"
#include "Math/Bounds.h"
#include "NaniteBuilder.h"
#include "Rendering/NaniteResources.h"

DEFINE_LOG_CATEGORY(LogStaticMeshBuilder);

void BuildAllBufferOptimizations(
	struct FStaticMeshLODResources& StaticMeshLOD,
	const struct FMeshBuildSettings& LODBuildSettings,
	TArray< uint32 >& IndexBuffer,
	bool bNeeds32BitIndices,
	TArray< FStaticMeshBuildVertex >& StaticMeshBuildVertices
);

FStaticMeshBuilder::FStaticMeshBuilder()
{

}

static bool UseNativeQuadraticReduction()
{
	// Are we using our tool, or simplygon?  The tool is only changed during editor restarts
	IMeshReduction* ReductionModule = FModuleManager::Get().LoadModuleChecked<IMeshReductionManagerModule>("MeshReductionInterface").GetStaticMeshReductionInterface();

	FString VersionString = ReductionModule->GetVersionString();
	TArray<FString> SplitVersionString;
	VersionString.ParseIntoArray(SplitVersionString, TEXT("_"), true);

	bool bUseQuadricSimplier = SplitVersionString[0].Equals("QuadricMeshReduction");
	return bUseQuadricSimplier;
}


/**
 * Compute bounding box and sphere from position buffer
 */
static void ComputeBoundsFromPositionBuffer(const FPositionVertexBuffer& UsePositionBuffer, FBoxSphereBounds& BoundsOut)
{
	// Calculate the bounding box.
	FBounds3f Bounds;
	for (uint32 VertexIndex = 0; VertexIndex < UsePositionBuffer.GetNumVertices(); VertexIndex++)
	{
		Bounds += UsePositionBuffer.VertexPosition(VertexIndex);
	}
	
	// Calculate the bounding sphere, using the center of the bounding box as the origin.
	FVector3f Center = Bounds.GetCenter();
	float RadiusSqr = 0.0f;
	for (uint32 VertexIndex = 0; VertexIndex < UsePositionBuffer.GetNumVertices(); VertexIndex++)
	{
		RadiusSqr = FMath::Max(	RadiusSqr, ( UsePositionBuffer.VertexPosition(VertexIndex) - Center ).SizeSquared() );
	}

	BoundsOut.Origin = FVector(Center);
	BoundsOut.BoxExtent = FVector(Bounds.GetExtent());
	BoundsOut.SphereRadius = FMath::Sqrt( RadiusSqr );
}


/**
 * Compute bounding box and sphere from vertices
 */
static void ComputeBoundsFromVertexList(const TArray<FStaticMeshBuildVertex>& Vertices, FBoxSphereBounds& BoundsOut)
{
	// Calculate the bounding box.
	FBounds3f Bounds;
	for (int32 VertexIndex = 0; VertexIndex < Vertices.Num(); VertexIndex++)
	{
		Bounds += Vertices[VertexIndex].Position;
	}
	
	// Calculate the bounding sphere, using the center of the bounding box as the origin.
	FVector3f Center = Bounds.GetCenter();
	float RadiusSqr = 0.0f;
	for (int32 VertexIndex = 0; VertexIndex < Vertices.Num(); VertexIndex++)
	{
		RadiusSqr = FMath::Max(	RadiusSqr, ( Vertices[VertexIndex].Position - Center ).SizeSquared() );
	}

	BoundsOut.Origin = FVector(Center);
	BoundsOut.BoxExtent = FVector(Bounds.GetExtent());
	BoundsOut.SphereRadius = FMath::Sqrt( RadiusSqr );
}

static void CorrectFallbackSettings( FMeshNaniteSettings& NaniteSettings, int32 NumTris )
{
	static const auto CVarFallbackThreshold = IConsoleManager::Get().FindTConsoleVariableDataInt(TEXT("r.Nanite.Builder.FallbackTriangleThreshold"));

	switch( NaniteSettings.FallbackTarget )
	{
	case ENaniteFallbackTarget::Auto:
		NaniteSettings.FallbackPercentTriangles = 1.0f;
		NaniteSettings.FallbackRelativeError = NumTris <= CVarFallbackThreshold->GetValueOnAnyThread() ? 0.0f : 1.0f;
		break;
	case ENaniteFallbackTarget::PercentTriangles:
		NaniteSettings.FallbackRelativeError = 0.0f;
		break;
	case ENaniteFallbackTarget::RelativeError:
		NaniteSettings.FallbackPercentTriangles = 1.0f;
		break;
	}
}

static bool BuildNanite(
	UStaticMesh* StaticMesh,
	FStaticMeshSourceModel& SourceModel,
	FStaticMeshLODResourcesArray& LODResources,
	FStaticMeshVertexFactoriesArray& LODVertexFactories,
	Nanite::FResources& NaniteResources,
	FMeshNaniteSettings& NaniteSettings, 
	TArrayView<float> PercentTriangles,
	FBoxSphereBounds& BoundsOut
)
{
	TRACE_CPUPROFILER_EVENT_SCOPE( FStaticMeshBuilder::BuildNanite );

	if( !SourceModel.IsMeshDescriptionValid() )
	{
		return false;
	}

	FMeshDescription MeshDescription = *SourceModel.GetOrCacheMeshDescription();

	FMeshBuildSettings& BuildSettings = StaticMesh->GetSourceModel(0).BuildSettings;
	FStaticMeshLODResources& StaticMeshLOD = LODResources[0];

	CorrectFallbackSettings( NaniteSettings, MeshDescription.Triangles().Num() );

	// compute tangents, lightmap UVs, etc

	// Until the simplifier supports tangents, only 100% fallback meshes will need them
	const bool bNeedTangents = PercentTriangles.Num() > 0 && NaniteSettings.FallbackPercentTriangles == 1.0f && NaniteSettings.FallbackRelativeError == 0.0f;

	FMeshDescriptionHelper MeshDescriptionHelper( &BuildSettings );
	MeshDescriptionHelper.SetupRenderMeshDescription( StaticMesh, MeshDescription, true, bNeedTangents );

	//Build new vertex buffers
	TArray< FStaticMeshBuildVertex > StaticMeshBuildVertices;

	//Because we will remove MeshVertex that are redundant, we need a remap
	//Render data Wedge map is only set for LOD 0???
	TArray<int32> RemapVerts;

	TArray<int32>& WedgeMap = StaticMeshLOD.WedgeMap;
	WedgeMap.Reset();

	//Prepare the PerSectionIndices array so we can optimize the index buffer for the GPU
	TArray<TArray<uint32> > PerSectionIndices;
	PerSectionIndices.AddDefaulted( MeshDescription.PolygonGroups().Num() );
	StaticMeshLOD.Sections.Empty( MeshDescription.PolygonGroups().Num() );

	//Build the vertex and index buffer
	UE::Private::StaticMeshBuilder::BuildVertexBuffer(
		StaticMesh,
		MeshDescription,
		BuildSettings,
		WedgeMap,
		StaticMeshLOD.Sections,
		PerSectionIndices,
		StaticMeshBuildVertices,
		MeshDescriptionHelper.GetOverlappingCorners(),
		RemapVerts,
		bNeedTangents
	);

	TVertexInstanceAttributesRef<FVector2f> VertexInstanceUVs = MeshDescription.VertexInstanceAttributes().GetAttributesRef<FVector2f>(MeshAttribute::VertexInstance::TextureCoordinate);
	const uint32 NumTextureCoord = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;

	// Only the render data and vertex buffers will be used from now on unless we have more than one source models
	// This will help with memory usage for Nanite Mesh by releasing memory before doing the build
	MeshDescription.Empty();
	
	// TODO get bounds from Nanite which computes them anyways!!!!
	ComputeBoundsFromVertexList(StaticMeshBuildVertices, BoundsOut);

	// Concatenate the per-section index buffers.
	TArray<uint32> CombinedIndices;
	bool bNeeds32BitIndices = false;
	UE::Private::StaticMeshBuilder::BuildCombinedSectionIndices(PerSectionIndices, StaticMeshLOD.Sections, CombinedIndices, bNeeds32BitIndices);

	// Nanite build requires the section material indices to have already been resolved from the SectionInfoMap
	// as the indices are baked into the FMaterialTriangles.
	for (int32 SectionIndex = 0; SectionIndex < StaticMeshLOD.Sections.Num(); SectionIndex++)
	{
		StaticMeshLOD.Sections[SectionIndex].MaterialIndex = StaticMesh->GetSectionInfoMap().Get(0, SectionIndex).MaterialIndex;
	}

	// Make sure to not keep the large WedgeMap from the input mesh around.
	// No need to calculate a new one for the coarse mesh, because Nanite meshes don't need it yet.
	WedgeMap.Empty();

	Nanite::IBuilderModule& NaniteBuilderModule = Nanite::IBuilderModule::Get();

	// Setup the input data
	Nanite::IBuilderModule::FVertexMeshData InputMeshData;
	Swap( InputMeshData.Vertices, StaticMeshBuildVertices );
	Swap( InputMeshData.TriangleIndices, CombinedIndices );
	InputMeshData.Sections = StaticMeshLOD.Sections;
									
	// Request output LODs for each LOD resource
	TArray< Nanite::IBuilderModule::FVertexMeshData, TInlineAllocator<4> > OutputLODMeshData;
	OutputLODMeshData.SetNum( PercentTriangles.Num() );

	for( int32 LodIndex = 0; LodIndex < OutputLODMeshData.Num(); LodIndex++ )
	{
		OutputLODMeshData[ LodIndex ].PercentTriangles = PercentTriangles[ LodIndex ];
	}

	if( !NaniteBuilderModule.Build( NaniteResources, InputMeshData, OutputLODMeshData, NumTextureCoord, NaniteSettings ) )
	{
		UE_LOG(LogStaticMesh, Error, TEXT("Failed to build Nanite for HiRes static mesh. See previous line(s) for details."));
		return false;
	}

	// Copy over the output data to the static mesh LOD data
	// Certain output LODs might be empty if the builder decided it wasn't needed (then remove these LODs again)
	int ValidLODCount = 0;
	for (int32 LodIndex = 0; LodIndex < OutputLODMeshData.Num(); ++LodIndex)
	{
		Nanite::IBuilderModule::FVertexMeshData& ProxyMeshData = OutputLODMeshData[LodIndex];
				
		bool bHasValidSections = false;
		for (FStaticMeshSection& Section : ProxyMeshData.Sections)
		{
			if (Section.NumTriangles > 0)
			{
				bHasValidSections = true;
				break;
			}
		}

		// Valid valid sections then copy over data to the LODResource
		if (bHasValidSections)
		{
			// Add new LOD resource if not created yet
			if (ValidLODCount >= LODResources.Num())
			{
				LODResources.Add(new FStaticMeshLODResources);
				new (LODVertexFactories) FStaticMeshVertexFactories(GMaxRHIFeatureLevel);
			}

			FStaticMeshLODResources& ProxyLOD = LODResources[ValidLODCount];
			ProxyLOD.Sections.Empty(ProxyMeshData.Sections.Num());
			for (FStaticMeshSection& Section : ProxyMeshData.Sections)
			{
				ProxyLOD.Sections.Add(Section);
			}

			TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build::BufferInit);
			ProxyLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseHighPrecisionTangentBasis(BuildSettings.bUseHighPrecisionTangentBasis);
			ProxyLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseFullPrecisionUVs(BuildSettings.bUseFullPrecisionUVs);
			FStaticMeshVertexBufferFlags StaticMeshVertexBufferFlags;
			StaticMeshVertexBufferFlags.bNeedsCPUAccess = true;
			StaticMeshVertexBufferFlags.bUseBackwardsCompatibleF16TruncUVs = BuildSettings.bUseBackwardsCompatibleF16TruncUVs;
			ProxyLOD.VertexBuffers.StaticMeshVertexBuffer.Init(ProxyMeshData.Vertices, NumTextureCoord, StaticMeshVertexBufferFlags);
			ProxyLOD.VertexBuffers.PositionVertexBuffer.Init(ProxyMeshData.Vertices);
			ProxyLOD.VertexBuffers.ColorVertexBuffer.Init(ProxyMeshData.Vertices);

			// Why is the 'bNeeds32BitIndices' used from the original index buffer? Is that needed?
			const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;
			ProxyLOD.IndexBuffer.SetIndices(ProxyMeshData.TriangleIndices, IndexBufferStride);

			BuildAllBufferOptimizations(ProxyLOD, BuildSettings, ProxyMeshData.TriangleIndices, bNeeds32BitIndices, ProxyMeshData.Vertices);

			ProxyLOD.MaxDeviation = ProxyMeshData.MaxDeviation;

			ValidLODCount++;
		}
	}

	return true;
}


bool FStaticMeshBuilder::Build(FStaticMeshRenderData& StaticMeshRenderData, UStaticMesh* StaticMesh, const FStaticMeshLODGroup& LODGroup, bool bGenerateCoarseMeshStreamingLODs, bool bTargetSupportsNanite)
{
	const bool bNaniteBuildEnabled = StaticMesh->IsNaniteEnabled();
	const bool bHaveHiResSourceModel = StaticMesh->IsHiResMeshDescriptionValid();
	int32 NumTasks = (bNaniteBuildEnabled && bHaveHiResSourceModel) ? (StaticMesh->GetNumSourceModels() + 1) : (StaticMesh->GetNumSourceModels());
	FScopedSlowTask SlowTask(NumTasks, NSLOCTEXT("StaticMeshEditor", "StaticMeshBuilderBuild", "Building static mesh render data."));
	SlowTask.MakeDialog();

	// The tool can only been switch by restarting the editor
	static bool bIsThirdPartyReductiontool = !UseNativeQuadraticReduction();

	if (!StaticMesh->IsMeshDescriptionValid(0))
	{
		//Warn the user that there is no mesh description data
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot find a valid mesh description to build the asset."));
		return false;
	}

	if (StaticMeshRenderData.LODResources.Num() > 0)
	{
		//At this point the render data is suppose to be empty
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot build static mesh render data twice [%s]."), *StaticMesh->GetFullName());
		
		//Crash in debug
		checkSlow(StaticMeshRenderData.LODResources.Num() == 0);

		return false;
	}

	TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build);

	const int32 NumSourceModels = StaticMesh->GetNumSourceModels();
	StaticMeshRenderData.AllocateLODResources(NumSourceModels);

	TArray<FMeshDescription> MeshDescriptions;
	MeshDescriptions.SetNum(NumSourceModels);

	const FMeshSectionInfoMap BeforeBuildSectionInfoMap = StaticMesh->GetSectionInfoMap();
	const FMeshSectionInfoMap BeforeBuildOriginalSectionInfoMap = StaticMesh->GetOriginalSectionInfoMap();
	FMeshNaniteSettings NaniteSettings = StaticMesh->NaniteSettings;

	bool bNaniteDataBuilt = false;		// true once we have finished building Nanite, which can happen in multiple places
	int32 NaniteBuiltLevels = 0;

	// Bounds of the pre-Nanite mesh
	FBoxSphereBounds HiResBounds;
	bool bHaveHiResBounds = false;

	// Do nanite build for HiRes SourceModel if we have one. In that case we skip the inline nanite build
	// below that would happen with LOD0 build
	if (bHaveHiResSourceModel && bNaniteBuildEnabled && bTargetSupportsNanite)
	{
		SlowTask.EnterProgressFrame(1);

		bool bBuildSuccess = BuildNanite(
			StaticMesh,
			StaticMesh->GetHiResSourceModel(),
			StaticMeshRenderData.LODResources,
			StaticMeshRenderData.LODVertexFactories,
			StaticMeshRenderData.NaniteResources,
			NaniteSettings,
			TArrayView< float >(),
			HiResBounds );

		if( bBuildSuccess )
		{
			bHaveHiResBounds = true;
			bNaniteDataBuilt = true;
		}
	}

	// If we want Nanite built, and have not already done it, do it based on LOD0 built render data.
	// This will replace the output VertexBuffers/etc with the fractional Nanite cut to be stored as LOD0 RenderData.
	// NOTE: We still want to do this for targets that do not support Nanite (if no hi-res source model was provided)
	// so that it generates the fallback, in which case the Nanite bulk will be stripped
	if (bNaniteBuildEnabled && ((bTargetSupportsNanite && !bNaniteDataBuilt) || (!bTargetSupportsNanite && !bHaveHiResSourceModel)))
	{
		TArray< float, TInlineAllocator<4> > PercentTriangles;
		for (int32 LodIndex = 0; LodIndex < NumSourceModels; ++LodIndex)
		{
			FStaticMeshSourceModel& SrcModel = StaticMesh->GetSourceModel( LodIndex );
			
			// As soon as we hit an artist provided LOD stop
			if( LodIndex > 0 && SrcModel.IsMeshDescriptionValid() )
				break;
			
			FMeshReductionSettings ReductionSettings = LODGroup.GetSettings( SrcModel.ReductionSettings, LodIndex );
			PercentTriangles.Add( ReductionSettings.PercentTriangles );
		}
		
		SlowTask.EnterProgressFrame( PercentTriangles.Num() );

		bool bBuildSuccess = BuildNanite(
			StaticMesh,
			StaticMesh->GetSourceModel(0),
			StaticMeshRenderData.LODResources,
			StaticMeshRenderData.LODVertexFactories,
			StaticMeshRenderData.NaniteResources,
			NaniteSettings,
			PercentTriangles,
			HiResBounds );
		check( bBuildSuccess );

		bHaveHiResBounds = true;
		bNaniteDataBuilt = true;
		NaniteBuiltLevels = PercentTriangles.Num();

		if (!bTargetSupportsNanite)
		{
			// Strip the Nanite bulk for the target platform
			StaticMeshRenderData.NaniteResources = Nanite::FResources();
		}
	}

	// Build render data for each LOD, starting from where Nanite left off.
	for (int32 LodIndex = NaniteBuiltLevels; LodIndex < NumSourceModels; ++LodIndex)
	{
		TRACE_CPUPROFILER_EVENT_SCOPE_STR("FStaticMeshBuilder::Build LOD");
		SlowTask.EnterProgressFrame(1);
		FScopedSlowTask BuildLODSlowTask(3);
		BuildLODSlowTask.EnterProgressFrame(1);

		FStaticMeshSourceModel& SrcModel = StaticMesh->GetSourceModel(LodIndex);

		float MaxDeviation = 0.0f;
		FMeshBuildSettings& LODBuildSettings = SrcModel.BuildSettings;
		bool bIsMeshDescriptionValid = StaticMesh->CloneMeshDescription(LodIndex, MeshDescriptions[LodIndex]);
		FMeshDescriptionHelper MeshDescriptionHelper(&LODBuildSettings);

		FMeshReductionSettings ReductionSettings = LODGroup.GetSettings(SrcModel.ReductionSettings, LodIndex);

		// Make sure we do not reduce a non custom LOD by itself
		const int32 BaseReduceLodIndex = FMath::Clamp<int32>(ReductionSettings.BaseLODModel, NaniteBuiltLevels, bIsMeshDescriptionValid ? LodIndex : LodIndex - 1);
		// Use simplifier if a reduction in triangles or verts has been requested.
		bool bUseReduction = StaticMesh->IsReductionActive(LodIndex);

		if (bIsMeshDescriptionValid)
		{
			MeshDescriptionHelper.SetupRenderMeshDescription(StaticMesh, MeshDescriptions[LodIndex], false, true);
			//Make sure the cache is good before looking for the active reduction
			if (SrcModel.CacheMeshDescriptionTrianglesCount == MAX_uint32)
			{
				SrcModel.CacheMeshDescriptionTrianglesCount = static_cast<uint32>(MeshDescriptions[LodIndex].Triangles().Num());
			}
			if (SrcModel.CacheMeshDescriptionVerticesCount == MAX_uint32)
			{
				SrcModel.CacheMeshDescriptionVerticesCount = static_cast<uint32>(FStaticMeshOperations::GetUniqueVertexCount(MeshDescriptions[LodIndex], MeshDescriptionHelper.GetOverlappingCorners()));
			}
			//Get back the reduction status once we apply all build settings, vertex count can change depending on the build settings
			bUseReduction = StaticMesh->IsReductionActive(LodIndex);
		}
		else
		{
			if (bUseReduction)
			{
				// Initialize an empty mesh description that the reduce will fill
				FStaticMeshAttributes(MeshDescriptions[LodIndex]).Register();
			}
			else
			{
				//Duplicate the lodindex 0 we have a 100% reduction which is like a duplicate
				MeshDescriptions[LodIndex] = MeshDescriptions[BaseReduceLodIndex];
				//Set the overlapping threshold
				float ComparisonThreshold = StaticMesh->GetSourceModel(BaseReduceLodIndex).BuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;
				MeshDescriptionHelper.FindOverlappingCorners(MeshDescriptions[LodIndex], ComparisonThreshold);
				if (LodIndex > 0)
				{
					
					//Make sure the SectionInfoMap is taken from the Base RawMesh
					int32 SectionNumber = StaticMesh->GetOriginalSectionInfoMap().GetSectionNumber(BaseReduceLodIndex);
					for (int32 SectionIndex = 0; SectionIndex < SectionNumber; ++SectionIndex)
					{
						//Keep the old data if its valid
						bool bHasValidLODInfoMap = StaticMesh->GetSectionInfoMap().IsValidSection(LodIndex, SectionIndex);
						//Section material index have to be remap with the ReductionSettings.BaseLODModel SectionInfoMap to create
						//a valid new section info map for the reduced LOD.
						if (!bHasValidLODInfoMap && StaticMesh->GetSectionInfoMap().IsValidSection(BaseReduceLodIndex, SectionIndex))
						{
							//Copy the BaseLODModel section info to the reduce LODIndex.
							FMeshSectionInfo SectionInfo = StaticMesh->GetSectionInfoMap().Get(BaseReduceLodIndex, SectionIndex);
							FMeshSectionInfo OriginalSectionInfo = StaticMesh->GetOriginalSectionInfoMap().Get(BaseReduceLodIndex, SectionIndex);
							StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
							StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, SectionIndex, OriginalSectionInfo);
						}
					}
				}
			}

			if (LodIndex > 0)
			{
				LODBuildSettings = StaticMesh->GetSourceModel(BaseReduceLodIndex).BuildSettings;
			}
		}

		// Reduce LODs
		if (bUseReduction)
		{
			TRACE_CPUPROFILER_EVENT_SCOPE_STR("FStaticMeshBuilder::Build - Reduce LOD");
			
			float OverlappingThreshold = LODBuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;
			FOverlappingCorners OverlappingCorners;
			FStaticMeshOperations::FindOverlappingCorners(OverlappingCorners, MeshDescriptions[BaseReduceLodIndex], OverlappingThreshold);

			int32 OldSectionInfoMapCount = StaticMesh->GetSectionInfoMap().GetSectionNumber(LodIndex);

			TFunction<void(const FMeshDescription&, const FMeshDescription&)> CheckReduction = [&](const FMeshDescription& InitMesh, const FMeshDescription& ReducedMesh)
			{
				FBox BBoxInitMesh = InitMesh.ComputeBoundingBox();
				double BBoxInitMeshSize = (BBoxInitMesh.Max - BBoxInitMesh.Min).Length();

				FBox BBoxReducedMesh = ReducedMesh.ComputeBoundingBox();
				double BBoxReducedMeshSize = (BBoxReducedMesh.Max - BBoxReducedMesh.Min).Length();

				constexpr double ThresholdForAbnormalGrowthOfBBox = UE_DOUBLE_SQRT_3; // the reduced mesh must stay in the bounding sphere 
				if (BBoxReducedMeshSize > BBoxInitMeshSize * ThresholdForAbnormalGrowthOfBBox)
				{
					UE_LOG(LogStaticMeshBuilder, Warning, TEXT("The generation of LOD could have generated spikes on the mesh for %s"), *StaticMesh->GetName());
				}
			};

			if (LodIndex == BaseReduceLodIndex)
			{
				//When using LOD 0, we use a copy of the mesh description since reduce do not support inline reducing
				FMeshDescription BaseMeshDescription = MeshDescriptions[BaseReduceLodIndex];
				MeshDescriptionHelper.ReduceLOD(BaseMeshDescription, MeshDescriptions[LodIndex], ReductionSettings, OverlappingCorners, MaxDeviation);
				CheckReduction(BaseMeshDescription, MeshDescriptions[LodIndex]);
			}
			else
			{
				MeshDescriptionHelper.ReduceLOD(MeshDescriptions[BaseReduceLodIndex], MeshDescriptions[LodIndex], ReductionSettings, OverlappingCorners, MaxDeviation);
				CheckReduction(MeshDescriptions[BaseReduceLodIndex], MeshDescriptions[LodIndex]);
			}
			

			const TPolygonGroupAttributesRef<FName> PolygonGroupImportedMaterialSlotNames = MeshDescriptions[LodIndex].PolygonGroupAttributes().GetAttributesRef<FName>(MeshAttribute::PolygonGroup::ImportedMaterialSlotName);
			const TPolygonGroupAttributesRef<FName> BasePolygonGroupImportedMaterialSlotNames = MeshDescriptions[BaseReduceLodIndex].PolygonGroupAttributes().GetAttributesRef<FName>(MeshAttribute::PolygonGroup::ImportedMaterialSlotName);
			// Recompute adjacency information. Since we change the vertices when we reduce
			MeshDescriptionHelper.FindOverlappingCorners(MeshDescriptions[LodIndex], OverlappingThreshold);
			
			//Make sure the static mesh SectionInfoMap is up to date with the new reduce LOD
			//We have to remap the material index with the ReductionSettings.BaseLODModel sectionInfoMap
			//Set the new SectionInfoMap for this reduced LOD base on the ReductionSettings.BaseLODModel SectionInfoMap
			TArray<int32> BaseUniqueMaterialIndexes;
			//Find all unique Material in used order
			for (const FPolygonGroupID PolygonGroupID : MeshDescriptions[BaseReduceLodIndex].PolygonGroups().GetElementIDs())
			{
				int32 MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(BasePolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
				if (MaterialIndex == INDEX_NONE)
				{
					MaterialIndex = PolygonGroupID.GetValue();
				}
				BaseUniqueMaterialIndexes.AddUnique(MaterialIndex);
			}
			TArray<int32> UniqueMaterialIndex;
			//Find all unique Material in used order
			for (const FPolygonGroupID PolygonGroupID : MeshDescriptions[LodIndex].PolygonGroups().GetElementIDs())
			{
				int32 MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(PolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
				if (MaterialIndex == INDEX_NONE)
				{
					MaterialIndex = PolygonGroupID.GetValue();
				}
				UniqueMaterialIndex.AddUnique(MaterialIndex);
			}

			//If the reduce did not output the same number of section use the base LOD sectionInfoMap
			bool bIsOldMappingInvalid = OldSectionInfoMapCount != MeshDescriptions[LodIndex].PolygonGroups().Num();

			bool bValidBaseSectionInfoMap = BeforeBuildSectionInfoMap.GetSectionNumber(BaseReduceLodIndex) > 0;
			//All used material represent a different section
			for (int32 SectionIndex = 0; SectionIndex < UniqueMaterialIndex.Num(); ++SectionIndex)
			{
				//Keep the old data
				bool bHasValidLODInfoMap = !bIsOldMappingInvalid && BeforeBuildSectionInfoMap.IsValidSection(LodIndex, SectionIndex);
				//Section material index have to be remap with the ReductionSettings.BaseLODModel SectionInfoMap to create
				//a valid new section info map for the reduced LOD.

				//Find the base LOD section using this material
				if (!bHasValidLODInfoMap)
				{
					bool bSectionInfoSet = false;
					if (bValidBaseSectionInfoMap)
					{
						for (int32 BaseSectionIndex = 0; BaseSectionIndex < BaseUniqueMaterialIndexes.Num(); ++BaseSectionIndex)
						{
							if (UniqueMaterialIndex[SectionIndex] == BaseUniqueMaterialIndexes[BaseSectionIndex])
							{
								//Copy the base sectionInfoMap
								FMeshSectionInfo SectionInfo = BeforeBuildSectionInfoMap.Get(BaseReduceLodIndex, BaseSectionIndex);
								FMeshSectionInfo OriginalSectionInfo = BeforeBuildOriginalSectionInfoMap.Get(BaseReduceLodIndex, BaseSectionIndex);
								StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
								StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, BaseSectionIndex, OriginalSectionInfo);
								bSectionInfoSet = true;
								break;
							}
						}
					}

					if (!bSectionInfoSet)
					{
						//Just set the default section info in case we did not found any match with the Base Lod
						FMeshSectionInfo SectionInfo;
						SectionInfo.MaterialIndex = SectionIndex;
						StaticMesh->GetSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
						StaticMesh->GetOriginalSectionInfoMap().Set(LodIndex, SectionIndex, SectionInfo);
					}
				}
			}
		}
		BuildLODSlowTask.EnterProgressFrame(1);
		const FPolygonGroupArray& PolygonGroups = MeshDescriptions[LodIndex].PolygonGroups();

		FStaticMeshLODResources& StaticMeshLOD = StaticMeshRenderData.LODResources[LodIndex];
		StaticMeshLOD.MaxDeviation = MaxDeviation;

		//Build new vertex buffers
		TArray< FStaticMeshBuildVertex > StaticMeshBuildVertices;

		StaticMeshLOD.Sections.Empty(PolygonGroups.Num());
		TArray<int32> RemapVerts; //Because we will remove MeshVertex that are redundant, we need a remap
									//Render data Wedge map is only set for LOD 0???

		TArray<int32>& WedgeMap = StaticMeshLOD.WedgeMap;
		WedgeMap.Reset();

		//Prepare the PerSectionIndices array so we can optimize the index buffer for the GPU
		TArray<TArray<uint32> > PerSectionIndices;
		PerSectionIndices.AddDefaulted(MeshDescriptions[LodIndex].PolygonGroups().Num());

		//Build the vertex and index buffer
		UE::Private::StaticMeshBuilder::BuildVertexBuffer(
			StaticMesh,
			MeshDescriptions[LodIndex],
			LODBuildSettings,
			WedgeMap,
			StaticMeshLOD.Sections,
			PerSectionIndices,
			StaticMeshBuildVertices,
			MeshDescriptionHelper.GetOverlappingCorners(),
			RemapVerts,
			true
		);

		TVertexInstanceAttributesRef<FVector2f> VertexInstanceUVs = MeshDescriptions[LodIndex].VertexInstanceAttributes().GetAttributesRef<FVector2f>(MeshAttribute::VertexInstance::TextureCoordinate);
		const uint32 NumTextureCoord = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;

		// Only the render data and vertex buffers will be used from now on unless we have more than one source models
		// This will help with memory usage for Nanite Mesh by releasing memory before doing the build
		if (NumSourceModels == 1)
		{
			MeshDescriptions.Empty();
		}

		// Concatenate the per-section index buffers.
		TArray<uint32> CombinedIndices;
		bool bNeeds32BitIndices = false;
		UE::Private::StaticMeshBuilder::BuildCombinedSectionIndices(PerSectionIndices, StaticMeshLOD.Sections, CombinedIndices, bNeeds32BitIndices);

		{
			TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::Build::BufferInit);
			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseHighPrecisionTangentBasis(LODBuildSettings.bUseHighPrecisionTangentBasis);
			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.SetUseFullPrecisionUVs(LODBuildSettings.bUseFullPrecisionUVs);
			FStaticMeshVertexBufferFlags StaticMeshVertexBufferFlags;
			StaticMeshVertexBufferFlags.bNeedsCPUAccess = true;
			StaticMeshVertexBufferFlags.bUseBackwardsCompatibleF16TruncUVs = LODBuildSettings.bUseBackwardsCompatibleF16TruncUVs;
			StaticMeshLOD.VertexBuffers.StaticMeshVertexBuffer.Init(StaticMeshBuildVertices, NumTextureCoord, StaticMeshVertexBufferFlags);
			StaticMeshLOD.VertexBuffers.PositionVertexBuffer.Init(StaticMeshBuildVertices);
			StaticMeshLOD.VertexBuffers.ColorVertexBuffer.Init(StaticMeshBuildVertices);

			const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;
			StaticMeshLOD.IndexBuffer.SetIndices(CombinedIndices, IndexBufferStride);

			// post-process the index buffer
			BuildLODSlowTask.EnterProgressFrame(1);
			BuildAllBufferOptimizations(StaticMeshLOD, LODBuildSettings, CombinedIndices, bNeeds32BitIndices, StaticMeshBuildVertices);
		}

	} //End of LOD for loop

	{
		TRACE_CPUPROFILER_EVENT_SCOPE_STR("FStaticMeshBuilder::Build - Calculate Bounds");

		// Calculate the bounding box of LOD0 buffer
		FPositionVertexBuffer& BasePositionVertexBuffer = StaticMeshRenderData.LODResources[0].VertexBuffers.PositionVertexBuffer;
		ComputeBoundsFromPositionBuffer(BasePositionVertexBuffer, StaticMeshRenderData.Bounds);
		// combine with high-res bounds if it was computed
		if (bHaveHiResBounds)
		{
			StaticMeshRenderData.Bounds = StaticMeshRenderData.Bounds + HiResBounds;
		}
	}


	return true;
}

bool FStaticMeshBuilder::BuildMeshVertexPositions(
	UStaticMesh* StaticMesh,
	TArray<uint32>& BuiltIndices,
	TArray<FVector3f>& BuiltVertices)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(FStaticMeshBuilder::BuildMeshVertexPositions);

	FStaticMeshSourceModel& SourceModel = StaticMesh->IsHiResMeshDescriptionValid() ? StaticMesh->GetHiResSourceModel() : StaticMesh->GetSourceModel(0);
	if (!SourceModel.IsMeshDescriptionValid())
	{
		//Warn the user that there is no mesh description data
		UE_LOG(LogStaticMeshBuilder, Error, TEXT("Cannot find a valid mesh description to build the asset."));
		return false;
	}

	const int32 NumSourceModels = StaticMesh->GetNumSourceModels();
	if (NumSourceModels > 0)
	{
		FMeshDescription MeshDescription;
		const bool bIsMeshDescriptionValid = SourceModel.CloneMeshDescription(MeshDescription);
		if (bIsMeshDescriptionValid)
		{
			FElementIDRemappings Remappings;
			MeshDescription.Compact(Remappings);

			const FMeshBuildSettings& BuildSettings = SourceModel.BuildSettings;

			const FStaticMeshConstAttributes Attributes(MeshDescription);
			TArrayView<const FVector3f> VertexPositions = Attributes.GetVertexPositions().GetRawArray();
			TArrayView<const FVertexID> VertexIndices = Attributes.GetTriangleVertexIndices().GetRawArray();
			const FVector3f BuildScale3D = (FVector3f)BuildSettings.BuildScale3D;

			BuiltVertices.Reserve(VertexPositions.Num());
			for (int32 VertexIndex = 0; VertexIndex < VertexPositions.Num(); ++VertexIndex)
			{
				BuiltVertices.Add(VertexPositions[VertexIndex] * BuildScale3D);
			}

			BuiltIndices.Reserve(VertexIndices.Num());
			for (int32 TriangleIndex = 0; TriangleIndex < VertexIndices.Num() / 3; ++TriangleIndex)
			{
				const uint32 I0 = VertexIndices[TriangleIndex * 3 + 0];
				const uint32 I1 = VertexIndices[TriangleIndex * 3 + 1];
				const uint32 I2 = VertexIndices[TriangleIndex * 3 + 2];

				if (!ensureMsgf(I0 != INDEX_NONE && I1 != INDEX_NONE && I2 != INDEX_NONE,
					TEXT("Mesh '%s' has triangles with uninitialized vertex indices"), *StaticMesh->GetName()))
				{
					continue;
				}

				const FVector3f V0 = BuiltVertices[I0];
				const FVector3f V1 = BuiltVertices[I1];
				const FVector3f V2 = BuiltVertices[I2];

				const FVector3f TriangleNormal = ((V1 - V2) ^ (V0 - V2));
				const bool bDegenerateTriangle = TriangleNormal.SizeSquared() < SMALL_NUMBER;
				if (!bDegenerateTriangle)
				{
					BuiltIndices.Add(I0);
					BuiltIndices.Add(I1);
					BuiltIndices.Add(I2);
				}
			}
		}
	}

	return true;
}

bool AreVerticesEqual(FStaticMeshBuildVertex const& A, FStaticMeshBuildVertex const& B, float ComparisonThreshold)
{
	if (   !A.Position.Equals(B.Position, ComparisonThreshold)
		|| !NormalsEqual(A.TangentX, B.TangentX)
		|| !NormalsEqual(A.TangentY, B.TangentY)
		|| !NormalsEqual(A.TangentZ, B.TangentZ)
		|| A.Color != B.Color)
	{
		return false;
	}

	// UVs
	for (int32 UVIndex = 0; UVIndex < MAX_STATIC_TEXCOORDS; UVIndex++)
	{
		if (!UVsEqual(A.UVs[UVIndex], B.UVs[UVIndex]))
		{
			return false;
		}
	}

	return true;
}

namespace UE::Private::StaticMeshBuilder
{

void BuildVertexBuffer(
	UStaticMesh* StaticMesh,
	const FMeshDescription& MeshDescription,
	const FMeshBuildSettings& BuildSettings,
	TArray<int32>& OutWedgeMap,
	FStaticMeshSectionArray& OutSections,
	TArray<TArray<uint32>>& OutPerSectionIndices,
	TArray< FStaticMeshBuildVertex>& StaticMeshBuildVertices,
	const FOverlappingCorners& OverlappingCorners,
	TArray<int32>& RemapVerts,
	bool bNeedTangents
)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(BuildVertexBuffer);

	TArray<int32> RemapVertexInstanceID;
	// set up vertex buffer elements
	const int32 NumVertexInstances = MeshDescription.VertexInstances().GetArraySize();
	StaticMeshBuildVertices.Reserve(NumVertexInstances);

	FStaticMeshConstAttributes Attributes(MeshDescription);

	TPolygonGroupAttributesConstRef<FName> PolygonGroupImportedMaterialSlotNames = Attributes.GetPolygonGroupMaterialSlotNames();
	TVertexAttributesConstRef<FVector3f> VertexPositions = Attributes.GetVertexPositions();
	TVertexInstanceAttributesConstRef<FVector3f> VertexInstanceNormals = Attributes.GetVertexInstanceNormals();
	TVertexInstanceAttributesConstRef<FVector3f> VertexInstanceTangents = Attributes.GetVertexInstanceTangents();
	TVertexInstanceAttributesConstRef<float> VertexInstanceBinormalSigns = Attributes.GetVertexInstanceBinormalSigns();
	TVertexInstanceAttributesConstRef<FVector4f> VertexInstanceColors = Attributes.GetVertexInstanceColors();
	TVertexInstanceAttributesConstRef<FVector2f> VertexInstanceUVs = Attributes.GetVertexInstanceUVs();

	const bool bHasColors = VertexInstanceColors.IsValid();

	const uint32 NumTextureCoord = VertexInstanceUVs.IsValid() ? VertexInstanceUVs.GetNumChannels() : 0;
	const FVector3f BuildScale(BuildSettings.BuildScale3D);

	TMap<FPolygonGroupID, int32> PolygonGroupToSectionIndex;

	for (const FPolygonGroupID PolygonGroupID : MeshDescription.PolygonGroups().GetElementIDs())
	{
		int32& SectionIndex = PolygonGroupToSectionIndex.FindOrAdd(PolygonGroupID);
		SectionIndex = OutSections.Add(FStaticMeshSection());
		FStaticMeshSection& StaticMeshSection = OutSections[SectionIndex];
		StaticMeshSection.MaterialIndex = StaticMesh->GetMaterialIndexFromImportedMaterialSlotName(PolygonGroupImportedMaterialSlotNames[PolygonGroupID]);
		if (StaticMeshSection.MaterialIndex == INDEX_NONE)
		{
			StaticMeshSection.MaterialIndex = PolygonGroupID.GetValue();
		}
	}

	int32 ReserveIndicesCount = MeshDescription.Triangles().Num() * 3;

	//Fill the remap array
	RemapVerts.AddZeroed(ReserveIndicesCount);
	for (int32& RemapIndex : RemapVerts)
	{
		RemapIndex = INDEX_NONE;
	}

	//Initialize the wedge map array tracking correspondence between wedge index and rendering vertex index
	OutWedgeMap.Reset();
	OutWedgeMap.AddZeroed(ReserveIndicesCount);

	float VertexComparisonThreshold = BuildSettings.bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;

	int32 WedgeIndex = 0;
	for (const FTriangleID TriangleID : MeshDescription.Triangles().GetElementIDs())
	{
		const FPolygonGroupID PolygonGroupID = MeshDescription.GetTrianglePolygonGroup(TriangleID);
		const int32 SectionIndex = PolygonGroupToSectionIndex[PolygonGroupID];
		TArray<uint32>& SectionIndices = OutPerSectionIndices[SectionIndex];

		TArrayView<const FVertexID> VertexIDs = MeshDescription.GetTriangleVertices(TriangleID);

		FVector3f CornerPositions[3];
		for (int32 TriVert = 0; TriVert < 3; ++TriVert)
		{
			CornerPositions[TriVert] = VertexPositions[VertexIDs[TriVert]];
		}
		FOverlappingThresholds OverlappingThresholds;
		OverlappingThresholds.ThresholdPosition = VertexComparisonThreshold;
		// Don't process degenerate triangles.
		if (PointsEqual(CornerPositions[0], CornerPositions[1], OverlappingThresholds)
			|| PointsEqual(CornerPositions[0], CornerPositions[2], OverlappingThresholds)
			|| PointsEqual(CornerPositions[1], CornerPositions[2], OverlappingThresholds))
		{
			WedgeIndex += 3;
			continue;
		}

		TArrayView<const FVertexInstanceID> VertexInstanceIDs = MeshDescription.GetTriangleVertexInstances(TriangleID);
		for (int32 TriVert = 0; TriVert < 3; ++TriVert, ++WedgeIndex)
		{
			const FVertexInstanceID VertexInstanceID = VertexInstanceIDs[TriVert];
			const FVector3f& VertexPosition = CornerPositions[TriVert];
			const FVector3f& VertexInstanceNormal = VertexInstanceNormals[VertexInstanceID];
			const FVector3f& VertexInstanceTangent = VertexInstanceTangents[VertexInstanceID];
			const float VertexInstanceBinormalSign = VertexInstanceBinormalSigns[VertexInstanceID];

			FStaticMeshBuildVertex StaticMeshVertex;

			StaticMeshVertex.Position = VertexPosition * BuildScale;
			if (bNeedTangents)
			{
				StaticMeshVertex.TangentX = VertexInstanceTangent / BuildScale;
				StaticMeshVertex.TangentY = ( (VertexInstanceNormal ^ VertexInstanceTangent) * VertexInstanceBinormalSign ) / BuildScale;
				StaticMeshVertex.TangentX.Normalize();
				StaticMeshVertex.TangentY.Normalize();
			}
			else
			{
				StaticMeshVertex.TangentX = FVector3f(1.0f, 0.0f, 0.0f);
				StaticMeshVertex.TangentY = FVector3f(0.0f, 1.0f, 0.0f);
			}
			StaticMeshVertex.TangentZ = VertexInstanceNormal / BuildScale;
			StaticMeshVertex.TangentZ.Normalize();

			if (bHasColors)
			{
				const FVector4f& VertexInstanceColor = VertexInstanceColors[VertexInstanceID];
				const FLinearColor LinearColor(VertexInstanceColor);
				StaticMeshVertex.Color = LinearColor.ToFColor(true);
			}
			else
			{
				StaticMeshVertex.Color = FColor::White;
			}

			const uint32 MaxNumTexCoords = FMath::Min<int32>(MAX_MESH_TEXTURE_COORDS_MD, MAX_STATIC_TEXCOORDS);
			for (uint32 UVIndex = 0; UVIndex < MaxNumTexCoords; ++UVIndex)
			{
				if (UVIndex < NumTextureCoord)
				{
					StaticMeshVertex.UVs[UVIndex] = VertexInstanceUVs.Get(VertexInstanceID, UVIndex);
				}
				else
				{
					StaticMeshVertex.UVs[UVIndex] = FVector2f(0.0f, 0.0f);
				}
			}


			//Never add duplicated vertex instance
			//Use WedgeIndex since OverlappingCorners has been built based on that
			const TArray<int32>& DupVerts = OverlappingCorners.FindIfOverlapping(WedgeIndex);

			int32 Index = INDEX_NONE;
			for (int32 k = 0; k < DupVerts.Num(); k++)
			{
				if (DupVerts[k] >= WedgeIndex)
				{
					break;
				}
				int32 Location = RemapVerts.IsValidIndex(DupVerts[k]) ? RemapVerts[DupVerts[k]] : INDEX_NONE;
				if (Location != INDEX_NONE && AreVerticesEqual(StaticMeshVertex, StaticMeshBuildVertices[Location], VertexComparisonThreshold))
				{
					Index = Location;
					break;
				}
			}
			if (Index == INDEX_NONE)
			{
				Index = StaticMeshBuildVertices.Add(StaticMeshVertex);
			}
			RemapVerts[WedgeIndex] = Index;
			OutWedgeMap[WedgeIndex] = Index;
			SectionIndices.Add(Index);
		}
	}


	//Optimize before setting the buffer
	if (NumVertexInstances < 100000 * 3)
	{
		BuildOptimizationHelper::CacheOptimizeVertexAndIndexBuffer(StaticMeshBuildVertices, OutPerSectionIndices, OutWedgeMap);
		//check(OutWedgeMap.Num() == MeshDescription->VertexInstances().Num());
	}
}

/**
 * Utility function used inside FStaticMeshBuilder::Build() per-LOD loop to populate
 * the Sections in a FStaticMeshLODResources from PerSectionIndices, as well as
 * concatenate all section indices into CombinedIndicesOut.
 * Returned bNeeds32BitIndicesOut indicates whether max vert index is larger than max int16
 */
void BuildCombinedSectionIndices(
	const TArray<TArray<uint32>>& PerSectionIndices,
	FStaticMeshSectionArray& SectionsOut,
	TArray<uint32>& CombinedIndicesOut,
	bool& bNeeds32BitIndicesOut)
{
	bNeeds32BitIndicesOut = false;
	for (int32 SectionIndex = 0; SectionIndex < SectionsOut.Num(); SectionIndex++)
	{
		FStaticMeshSection& Section = SectionsOut[SectionIndex];
		const TArray<uint32>& SectionIndices = PerSectionIndices[SectionIndex];
		Section.FirstIndex = 0;
		Section.NumTriangles = 0;
		Section.MinVertexIndex = 0;
		Section.MaxVertexIndex = 0;

		if (SectionIndices.Num())
		{
			Section.FirstIndex = CombinedIndicesOut.Num();
			Section.NumTriangles = SectionIndices.Num() / 3;

			CombinedIndicesOut.AddUninitialized(SectionIndices.Num());
			uint32* DestPtr = &CombinedIndicesOut[Section.FirstIndex];
			uint32 const* SrcPtr = SectionIndices.GetData();

			Section.MinVertexIndex = *SrcPtr;
			Section.MaxVertexIndex = *SrcPtr;

			for (int32 Index = 0; Index < SectionIndices.Num(); Index++)
			{
				uint32 VertIndex = *SrcPtr++;

				bNeeds32BitIndicesOut |= (VertIndex > MAX_uint16);
				Section.MinVertexIndex = FMath::Min<uint32>(VertIndex, Section.MinVertexIndex);
				Section.MaxVertexIndex = FMath::Max<uint32>(VertIndex, Section.MaxVertexIndex);
				*DestPtr++ = VertIndex;
			}
		}
	}
}

} // namespace UE::Private::StaticMeshBuilder

void BuildAllBufferOptimizations(FStaticMeshLODResources& StaticMeshLOD, const FMeshBuildSettings& LODBuildSettings, TArray< uint32 >& IndexBuffer, bool bNeeds32BitIndices, TArray< FStaticMeshBuildVertex >& StaticMeshBuildVertices)
{
	TRACE_CPUPROFILER_EVENT_SCOPE(BuildAllBufferOptimizations);

	if (StaticMeshLOD.AdditionalIndexBuffers == nullptr)
	{
		StaticMeshLOD.AdditionalIndexBuffers = new FAdditionalStaticMeshIndexBuffers();
	}

	const EIndexBufferStride::Type IndexBufferStride = bNeeds32BitIndices ? EIndexBufferStride::Force32Bit : EIndexBufferStride::Force16Bit;

	// Build the reversed index buffer.
	if (LODBuildSettings.bBuildReversedIndexBuffer)
	{
		TArray<uint32> InversedIndices;
		const int32 IndexCount = IndexBuffer.Num();
		InversedIndices.AddUninitialized(IndexCount);

		for (int32 SectionIndex = 0; SectionIndex < StaticMeshLOD.Sections.Num(); ++SectionIndex)
		{
			const FStaticMeshSection& SectionInfo = StaticMeshLOD.Sections[SectionIndex];
			const int32 SectionIndexCount = SectionInfo.NumTriangles * 3;

			for (int32 i = 0; i < SectionIndexCount; ++i)
			{
				InversedIndices[SectionInfo.FirstIndex + i] = IndexBuffer[SectionInfo.FirstIndex + SectionIndexCount - 1 - i];
			}
		}
		StaticMeshLOD.AdditionalIndexBuffers->ReversedIndexBuffer.SetIndices(InversedIndices, IndexBufferStride);
	}

	// Build the depth-only index buffer.
	TArray<uint32> DepthOnlyIndices;
	{
		BuildOptimizationHelper::BuildDepthOnlyIndexBuffer(
			DepthOnlyIndices,
			StaticMeshBuildVertices,
			IndexBuffer,
			StaticMeshLOD.Sections
		);

		if (DepthOnlyIndices.Num() < 50000 * 3)
		{
			BuildOptimizationThirdParty::CacheOptimizeIndexBuffer(DepthOnlyIndices);
		}

		StaticMeshLOD.DepthOnlyIndexBuffer.SetIndices(DepthOnlyIndices, IndexBufferStride);
	}

	// Build the inversed depth only index buffer.
	if (LODBuildSettings.bBuildReversedIndexBuffer)
	{
		TArray<uint32> ReversedDepthOnlyIndices;
		const int32 IndexCount = DepthOnlyIndices.Num();
		ReversedDepthOnlyIndices.AddUninitialized(IndexCount);
		for (int32 i = 0; i < IndexCount; ++i)
		{
			ReversedDepthOnlyIndices[i] = DepthOnlyIndices[IndexCount - 1 - i];
		}
		StaticMeshLOD.AdditionalIndexBuffers->ReversedDepthOnlyIndexBuffer.SetIndices(ReversedDepthOnlyIndices, IndexBufferStride);
	}

	// Build a list of wireframe edges in the static mesh.
	{
		TArray<BuildOptimizationHelper::FMeshEdge> Edges;
		TArray<uint32> WireframeIndices;

		BuildOptimizationHelper::FStaticMeshEdgeBuilder(IndexBuffer, StaticMeshBuildVertices, Edges).FindEdges();
		WireframeIndices.Empty(2 * Edges.Num());
		for (int32 EdgeIndex = 0; EdgeIndex < Edges.Num(); EdgeIndex++)
		{
			BuildOptimizationHelper::FMeshEdge&	Edge = Edges[EdgeIndex];
			WireframeIndices.Add(Edge.Vertices[0]);
			WireframeIndices.Add(Edge.Vertices[1]);
		}
		StaticMeshLOD.AdditionalIndexBuffers->WireframeIndexBuffer.SetIndices(WireframeIndices, IndexBufferStride);
	}
}