AstarBuildGizmos.cs

void BuildGizmos() {
    var occupiedComb = new List<CombineInstance>();
    var freeComb = new List<CombineInstance>();
    int xLen = _runtimeGrid.Len0, yLen = _runtimeGrid.Len1, zLen = _runtimeGrid.Len2;
    var sharedMesh = Cube();
    sharedMesh.hideFlags = HideFlags.DontSave;

    for (var x = 0; x < xLen; x++)
    for (var y = 0; y < yLen; y++)
    for (var z = 0; z < zLen; z++) {
        var posLS = new Vector3(x, y, z);
        var xyz = new int3(x, y, z);
        if (_runtimeGrid.isWalkable.Get(xyz)) {
            freeComb.Add(new CombineInstance() {
                mesh = sharedMesh,
                transform = Matrix4x4.TRS(posLS, Quaternion.identity, Vector3.one)
            });
        }
        else {
            occupiedComb.Add(new CombineInstance() {
                mesh = sharedMesh,
                transform = Matrix4x4.TRS(posLS, Quaternion.identity, Vector3.one),
            });
        }
    }

    const int maxVert = 1 << 16;
    var cubeVertCount = sharedMesh.vertices.Length;

    __occupiedMeshes = Combine(occupiedComb);
    __freeMeshes = Combine(freeComb);

    Mesh[] Combine(List<CombineInstance> arg) {
        var meshCount = Mathf.CeilToInt((float) cubeVertCount * arg.Count / maxVert);
        var chunk = arg.Count / meshCount;
        var result = new Mesh[meshCount];
        for (int i = 0; i < meshCount; i++) {
            var combine = arg.Skip(i * chunk).Take(chunk).ToArray();
            var mesh = new Mesh(){hideFlags = HideFlags.DontSave};
            mesh.CombineMeshes(combine);
            mesh.Optimize();
            result[i] = mesh;
        }

        return result;
    }

    Mesh Cube() {
        var mesh = new Mesh();

        const float length = 1f;
        const float width = 1f;
        const float height = 1f;

        var p0 = new Vector3(-length * .5f, -width * .5f, height * .5f);
        var p1 = new Vector3(length * .5f, -width * .5f, height * .5f);
        var p2 = new Vector3(length * .5f, -width * .5f, -height * .5f);
        var p3 = new Vector3(-length * .5f, -width * .5f, -height * .5f);

        var p4 = new Vector3(-length * .5f, width * .5f, height * .5f);
        var p5 = new Vector3(length * .5f, width * .5f, height * .5f);
        var p6 = new Vector3(length * .5f, width * .5f, -height * .5f);
        var p7 = new Vector3(-length * .5f, width * .5f, -height * .5f);

        var vertices = new[] {
            p0, p1, p2, p3, // Bottom
            p7, p4, p0, p3, // Left
            p4, p5, p1, p0, // Front
            p6, p7, p3, p2, // Back
            p5, p6, p2, p1, // Right
            p7, p6, p5, p4 // Top
        };


        var up = Vector3.up;
        var down = Vector3.down;
        var front = Vector3.forward;
        var back = Vector3.back;
        var left = Vector3.left;
        var right = Vector3.right;

        Vector3[] normales = {
            down, down, down, down, // Bottom
            left, left, left, left, // Left
            front, front, front, front, // Front
            back, back, back, back, // Back
            right, right, right, right, // Right
            up, up, up, up // Top
        };

        var _00 = new Vector2(0f, 0f);
        var _10 = new Vector2(1f, 0f);
        var _01 = new Vector2(0f, 1f);
        var _11 = new Vector2(1f, 1f);

        var uvs = new[] {
            _11, _01, _00, _10, // Bottom
            _11, _01, _00, _10, // Left
            _11, _01, _00, _10, // Front
            _11, _01, _00, _10, // Back
            _11, _01, _00, _10, // Right
            _11, _01, _00, _10, // Top
        };

        var triangles = new[] {
            // Bottom
            3, 1, 0,
            3, 2, 1,

            // Left
            3 + 4 * 1, 1 + 4 * 1, 0 + 4 * 1,
            3 + 4 * 1, 2 + 4 * 1, 1 + 4 * 1,

            // Front
            3 + 4 * 2, 1 + 4 * 2, 0 + 4 * 2,
            3 + 4 * 2, 2 + 4 * 2, 1 + 4 * 2,

            // Back
            3 + 4 * 3, 1 + 4 * 3, 0 + 4 * 3,
            3 + 4 * 3, 2 + 4 * 3, 1 + 4 * 3,

            // Right
            3 + 4 * 4, 1 + 4 * 4, 0 + 4 * 4,
            3 + 4 * 4, 2 + 4 * 4, 1 + 4 * 4,

            // Top
            3 + 4 * 5, 1 + 4 * 5, 0 + 4 * 5,
            3 + 4 * 5, 2 + 4 * 5, 1 + 4 * 5,
        };


        mesh.vertices = vertices;
        mesh.normals = normales;
        mesh.uv = uvs;
        mesh.triangles = triangles;
        return mesh;
    }
}