type_vtk_vtu_initialize Subroutine

    subroutine type_vtk_vtu_initialize(self, file_name, cell_id_array_name, field_names, field_values)
        !> Read VTK file using C++ backend with the handle pattern
        implicit none
        class(type_vtk), intent(inout) :: self
        character(*), intent(in) :: file_name
        character(*), intent(in) :: cell_id_array_name
        character(*), intent(in), optional :: field_names(:)
        real(real64), intent(inout), allocatable, optional :: field_values(:, :)

        ! --- ローカル変数 ---
        character(len=256) :: c_file_name
        character(len=256) :: c_cell_id_array_name
        character(len=256) :: c_field_name
        integer(c_int) :: ierr
        integer(int32) :: i

        ! --- 生データ格納用の一時配列 ---
        integer(int64), allocatable :: raw_connectivity(:)
        integer(int64), allocatable :: raw_offsets(:)
        integer(int32), allocatable :: raw_cell_types(:)
        integer(int32), allocatable :: raw_cell_entity_ids(:)
        integer(int64) :: total_conn_size
        integer(int32) :: connectivity_first, connectivity_last, num_nodes_in_cell
        character(50, kind=c_char) :: f_format, f_dataset
        integer(c_int) :: len_f_format, len_f_dataset

        !----------------------------------------------------------------!
        ! 1. C++リーダーの初期化とハンドルの取得
        !----------------------------------------------------------------!

        ! 自分が "vtu" リーダーであることを記録 (finalizerでの呼び分けに使う)
        self%reader_type = "vtu"

        ! もし既にハンドルが有効なら、古いC++オブジェクトを解放してメモリリークを防ぐ
        if (c_associated(self%handle)) then
            call vtu_finalize(self%handle)
            self%handle = c_null_ptr
        end if

        ! C言語で扱えるように文字列を準備
        c_file_name = trim(file_name)//c_null_char
        c_cell_id_array_name = trim(cell_id_array_name)//c_null_char

        ! C++リーダーを初期化し、返されたポインタをハンドルとして保存
        self%handle = vtu_initialize(c_file_name, ierr)

        ! ハンドルが有効か、エラーコードが0かを確認
        if (.not. c_associated(self%handle) .or. ierr /= 0) then
            write (*, *) "C++ VTU Reader failed to initialize. Error code: ", ierr
            stop "VTU Initialization Failed"
        end if

        !----------------------------------------------------------------!
        ! 2. ハンドルを使って各種データを取得
        !----------------------------------------------------------------!

        ! ヘッダー情報の取得
        len_f_dataset = 50
        len_f_format = 50
        call vtu_read_header(self%handle, f_format, len_f_dataset, f_dataset, len_f_format)
        allocate (character(len=len_trim(f_format)) :: self%format)
        self%format = trim(adjustl(f_format))
        allocate (character(len=len_trim(f_dataset)) :: self%dataset)
        self%dataset = trim(adjustl(f_dataset))

        ! ポイントデータの取得
        call vtu_get_num_points(self%handle, self%num_points)
        if (self%num_points > 0) then
            call self%POINTS%initialize(self%num_points)
            call vtu_get_points(self%handle, self%POINTS%x, self%POINTS%y, self%POINTS%z)
        end if

        ! セルデータの取得
        call vtu_get_num_cells(self%handle, self%num_total_cells)
        if (self%num_total_cells > 0) then
            ! 生データをCから取得するためのメモリ確保
            call vtu_get_total_connectivity_size(self%handle, total_conn_size)
            call allocate_array(raw_connectivity, total_conn_size)
            call allocate_array(raw_offsets, self%num_total_cells + 1_int64)
            call allocate_array(raw_cell_types, self%num_total_cells)
            call allocate_array(raw_cell_entity_ids, self%num_total_cells)

            ! 生データをCから取得
            call vtu_get_cell_info(self%handle, raw_connectivity, raw_offsets, raw_cell_types)
            call vtu_get_cell_ids(self%handle, c_cell_id_array_name, raw_cell_entity_ids)

            ! Fortran構造体にデータを格納し直す
            allocate (self%cells(self%num_total_cells))
            do i = 1, self%num_total_cells
                self%cells(i)%cell_type = raw_cell_types(i)
                self%cells(i)%cell_entity_id = raw_cell_entity_ids(i)

                connectivity_first = raw_offsets(i) + 1
                connectivity_last = raw_offsets(i + 1)
                num_nodes_in_cell = connectivity_last - connectivity_first + 1

                call allocate_array(self%cells(i)%connectivity, num_nodes_in_cell)
                self%cells(i)%connectivity(:) = int(raw_connectivity(connectivity_first:connectivity_last), kind=int32)
                call self%cells(i)%set(num_nodes_in_cell)
            end do
        end if

        ! 初期条件フィールドの読み込み (オプション)
        if (present(field_names) .and. present(field_values)) then
            if (self%num_points > 0 .and. size(field_names) > 0) then
                allocate (field_values(self%num_points, size(field_names)))
                do i = 1, size(field_names)
                    c_field_name = trim(field_names(i))//c_null_char
                    call vtu_get_point_data(self%handle, c_field_name, field_values(:, i))
                end do
            end if
        end if

        !----------------------------------------------------------------!
        ! 3. 後片付け (C++オブジェクトのfinalizeは呼ばない)
        !----------------------------------------------------------------!
        call deallocate_array(raw_connectivity)
        call deallocate_array(raw_offsets)
        call deallocate_array(raw_cell_types)
        call deallocate_array(raw_cell_entity_ids)

    end subroutine type_vtk_vtu_initialize