//original code by Bunny83 from https://answers.unity.com/questions/1213025/separating-submeshes-into-unique-meshes.html
namespace NOT_Lonely
{
    using UnityEngine;
    using System.Collections.Generic;

    public static class MeshExtension
    {
        private class Vertices
        {
            List<Vector3> verts = null;
            List<Vector2> uv1 = null;
            List<Vector3> normals = null;
            List<Vector4> tangents = null;
            List<Color32> colors = null;

            public Vertices()
            {
                verts = new List<Vector3>();
            }
            public Vertices(Mesh aMesh)
            {
                verts = CreateList(aMesh.vertices);
                uv1 = CreateList(aMesh.uv);
                normals = CreateList(aMesh.normals);
                tangents = CreateList(aMesh.tangents);
                colors = CreateList(aMesh.colors32);
            }

            private List<T> CreateList<T>(T[] aSource)
            {
                if (aSource == null || aSource.Length == 0)
                    return null;
                return new List<T>(aSource);
            }
            private void Copy<T>(ref List<T> aDest, List<T> aSource, int aIndex)
            {
                if (aSource == null)
                    return;
                if (aDest == null)
                    aDest = new List<T>();
                aDest.Add(aSource[aIndex]);
            }
            public int Add(Vertices aOther, int aIndex)
            {
                int i = verts.Count;
                Copy(ref verts, aOther.verts, aIndex);
                Copy(ref uv1, aOther.uv1, aIndex);
                Copy(ref normals, aOther.normals, aIndex);
                Copy(ref tangents, aOther.tangents, aIndex);
                Copy(ref colors, aOther.colors, aIndex);
                return i;
            }
            public void AssignTo(Mesh aTarget)
            {
                if (verts.Count > 65535)
                    aTarget.indexFormat = UnityEngine.Rendering.IndexFormat.UInt32;
                aTarget.SetVertices(verts);
                if (uv1 != null) aTarget.SetUVs(0, uv1);
                if (normals != null) aTarget.SetNormals(normals);
                if (tangents != null) aTarget.SetTangents(tangents);
                if (colors != null) aTarget.SetColors(colors);
            }
        }

        public static Mesh GetSubmesh(this Mesh aMesh, int aSubMeshIndex)
        {
            if (aSubMeshIndex < 0 || aSubMeshIndex >= aMesh.subMeshCount)
                return null;
            int[] indices = aMesh.GetTriangles(aSubMeshIndex);
            Vertices source = new Vertices(aMesh);
            Vertices dest = new Vertices();
            Dictionary<int, int> map = new Dictionary<int, int>();
            int[] newIndices = new int[indices.Length];
            for (int i = 0; i < indices.Length; i++)
            {
                int o = indices[i];
                int n;
                if (!map.TryGetValue(o, out n))
                {
                    n = dest.Add(source, o);
                    map.Add(o, n);
                }
                newIndices[i] = n;
            }
            Mesh m = new Mesh();
            dest.AssignTo(m);
            m.triangles = newIndices;
            return m;
        }
    }
}