NumericFactory/FactoryNumeric/Packages/com.unity.hlod/Editor/MeshCombiner.cs

using System.Collections.Generic;
using System.Linq;
using Unity.Collections;
using Unity.HLODSystem.Utils;
using UnityEngine;

namespace Unity.HLODSystem
{
    public class MeshCombiner
    {
        public struct CombineInfo
        {
            public Matrix4x4 Transform;
            public WorkingMesh Mesh;
            public int MeshIndex;

        }
        public WorkingMesh CombineMesh(Allocator allocator, List<CombineInfo> infos)
        {
            //I didn't consider animation mesh combine.
            int verticesCount = 0;
            int normalCount = 0;
            int tangentCount = 0;
            int UV1Count = 0;
            int UV2Count = 0;
            int UV3Count = 0;
            int UV4Count = 0;
            int colorCount = 0;

            int trianglesCount = 0;
            
            List<Dictionary<int,int>> remappers = new List<Dictionary<int, int>>(infos.Count);
            
            for (int i = 0; i < infos.Count; ++i)
            {
                int[] meshIndices = infos[i].Mesh.GetTriangles(infos[i].MeshIndex);
                Dictionary<int, int> remapper = CalculateMeshRemap(meshIndices);

                verticesCount += (infos[i].Mesh.vertices.Length > 0) ? remapper.Count : 0;
                normalCount += (infos[i].Mesh.normals.Length > 0) ? remapper.Count : 0;
                tangentCount += (infos[i].Mesh.tangents.Length > 0) ? remapper.Count : 0;
                UV1Count += (infos[i].Mesh.uv.Length > 0) ? remapper.Count : 0;
                UV2Count += (infos[i].Mesh.uv2.Length > 0) ? remapper.Count : 0;
                UV3Count += (infos[i].Mesh.uv3.Length > 0) ? remapper.Count : 0;
                UV4Count += (infos[i].Mesh.uv4.Length > 0) ? remapper.Count : 0;
                colorCount += (infos[i].Mesh.colors.Length > 0) ? remapper.Count : 0;

                trianglesCount += meshIndices.Length;
                
                remappers.Add(remapper);
            }
            
            WorkingMesh combinedMesh = new WorkingMesh(allocator, verticesCount, trianglesCount, 1, 0);
            
            List<Vector3> vertices = new List<Vector3>(verticesCount);
            List<Vector3> normals = new List<Vector3>(verticesCount);
            List<Vector4> tangents = new List<Vector4>(verticesCount);
            List<Vector2> uv1s = new List<Vector2>(verticesCount);
            List<Vector2> uv2s = new List<Vector2>(verticesCount);
            List<Vector2> uv3s = new List<Vector2>(verticesCount);
            List<Vector2> uv4s = new List<Vector2>(verticesCount);
            List<Color> colors = new List<Color>(colorCount);
            
            List<int> triangles = new List<int>(trianglesCount);

            for (int i = 0; i < infos.Count; ++i)
            {
                WorkingMesh mesh = infos[i].Mesh;
                Dictionary<int, int> remapper = remappers[i];
                int startIndex = vertices.Count;

                if (verticesCount > 0)
                {
                    FillBuffer(ref vertices, mesh.vertices, remapper, Vector3.zero);
                    for (int vi = startIndex; vi < vertices.Count; ++vi)
                    {
                        vertices[vi] = infos[i].Transform.MultiplyPoint(vertices[vi]);
                    }
                }

                if (normalCount > 0)
                {
                    FillBuffer(ref normals, mesh.normals, remapper, Vector3.up);
                    for (int ni = startIndex; ni < normals.Count; ++ni)
                    {
                        normals[ni] = infos[i].Transform.MultiplyVector(normals[ni]);
                    }
                }

                if (tangentCount > 0)
                {
                    FillBuffer(ref tangents, mesh.tangents, remapper, new Vector4(1, 0, 0, 1));
                    for (int ti = startIndex; ti < tangents.Count; ++ti)
                    {
                        Vector3 tanVec = new Vector3(tangents[ti].x, tangents[ti].y, tangents[ti].z);
                        tanVec = infos[i].Transform.MultiplyVector(tanVec);
                        Vector4 transTan = new Vector4(tanVec.x, tanVec.y, tanVec.z, tangents[ti].w);
                        tangents[ti] = transTan;
                    }
                }

                if ( UV1Count > 0 )
                    FillBuffer(ref uv1s, mesh.uv, remapper, Vector2.zero);
                if ( UV2Count > 0 )
                    FillBuffer(ref uv2s, mesh.uv2, remapper, Vector2.zero);
                if ( UV3Count > 0 )
                    FillBuffer(ref uv3s, mesh.uv3, remapper, Vector2.zero);
                if ( UV4Count > 0 )
                    FillBuffer(ref uv4s, mesh.uv4, remapper, Vector2.zero);
                if ( colorCount > 0 )
                    FillBuffer(ref colors, mesh.colors, remapper, Color.white);

                FillIndices(ref triangles, mesh.GetTriangles(infos[i].MeshIndex), remapper, startIndex);

            }

            combinedMesh.name = "CombinedMesh";
            combinedMesh.vertices = vertices.ToArray();
            combinedMesh.normals = normals.ToArray();
            combinedMesh.tangents = tangents.ToArray();
            combinedMesh.uv = uv1s.ToArray();
            combinedMesh.uv2 = uv2s.ToArray();
            combinedMesh.uv3 = uv3s.ToArray();
            combinedMesh.uv4 = uv4s.ToArray();
            combinedMesh.colors = colors.ToArray();
            
            combinedMesh.SetTriangles(triangles.ToArray(), 0);

            return combinedMesh;
        }
        
        private void FillBuffer<T>(ref List<T> buffer, T[] source, Dictionary<int,int> remapper, T defaultValue)
        { 
            int startIndex = buffer.Count;
            buffer.AddRange(Enumerable.Repeat(defaultValue, remapper.Count));
            
            if (source == null || source.Length == 0)
            {

                return;
            }

            foreach (var pair in remapper)
            {
                buffer[pair.Value + startIndex] = source[pair.Key];
            }
        }

        private void FillIndices(ref List<int> buffer, int[] source, Dictionary<int,int> remapper, int startIndex )
        {
            for (int i = 0; i < source.Length; ++i)
            {
                int newIndex = remapper[source[i]] + startIndex;
                buffer.Add(newIndex);
            }
        }

        
        //first original index
        //second new index
        private Dictionary<int, int> CalculateMeshRemap(int[] indices)
        {
            Dictionary<int, int> remapper = new Dictionary<int, int>();

            for (int i = 0; i < indices.Length; ++i)
            {
                if (remapper.ContainsKey(indices[i]))
                    continue;
                
                remapper.Add(indices[i], remapper.Count);
            }

            return remapper;
        }
    }
}