mma.f90

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!===========================================================================!
!                       Method of Moving Asymptotes                         !
! This implementation is based on the following documents:                  !
!        1-https://people.kth.se/~krille/mmagcmma.pdf                       !
!        2-https://people.kth.se/~krille/originalmma.pdf                    !
!        2-https://comsolyar.com/wp-content/uploads/2020/03/gcmma.pdf       !
! ------------------------------------------------------------------------- !
!                                                                           !
! This module solves the following original optimization problem:           !
!                                                                           !
!      Minimize  f_0(x) + a_0*z + sum( c_i*y_i + 0.5*d_i*(y_i)^2 )          !
!    subject to  f_i(x) - a_i*z - y_i <= 0,  i = 1,...,m                    !
!                xmin_j <= x_j <= xmax_j,    j = 1,...,n                    !
!                z >= 0,   y_i >= 0,         i = 1,...,m                    !
!                                                                           !
! by first creating the following convex approximation of the original      !
! problem:                                                                  !
!                                                                           !
!      Minimize sum_{j = 1,...,n} (p0j / (upp_j-x_j) + q0j / (x_j-low_j)) + !
!                        a0*z + sum_i = 1,...,m(c_i*y_i + 0.5*d_i*y_i^2)    !
!    subject to sum_{j = 1,...,n} (pij / (upp_j-x_j) + qij / (x_j-low_j)) + !
!                    a_i*z + y_i <= b_i,                       i = 1,...,m  !
!               xmin_j <= alpha_j <= x_j <= beta_j <= xmax_j   j = 1,...,n  !
!               y_i>=0                                         i = 1,...,m  !
!               z>=0                                                        !
!                                                                           !
! note that based on eq(3.5) there should be r0 in the approximated problem !
! however since it is just a constant added to a minimization problem, it   !
! is ignored.                                                               !
! A primal-dual algorithm is then employed to solve the approximated        !
! problem using interior point method.                                      !
!===========================================================================!
 
module mma
  ! Inclusions from Neko
  use num_types, only: rp, dp, sp
  use json_module, only: json_file
  use json_utils, only: json_get_or_default
  use vector, only: vector_t
  use matrix, only: matrix_t
  use comm, only: pe_rank, neko_comm, pe_size, mpi_real_precision
  use utils, only: neko_error, filename_suffix
  use neko_config, only: neko_bcknd_device, neko_bcknd_cuda, neko_bcknd_hip, &
       neko_bcknd_opencl
  use device, only: device_memcpy, host_to_device, device_to_host
  use, intrinsic :: iso_c_binding, only: c_ptr
  use logger, only: neko_log
  use mpi_f08, only: mpi_sum, mpi_allreduce, mpi_integer
  use scratch_registry, only: scratch_registry_t
 
  implicit none
  private
 
  type, public :: mma_t
     private
     integer :: n, m, n_global, max_iter
     real(kind=rp) :: a0, asyinit, asyincr, asydecr, epsimin, &
          residumax, residunorm
     real(kind=rp) :: move_limit = 0.2_rp
     type(vector_t) :: xold1, xold2, low, upp, alpha, beta, a, c, d, xmax, xmin
     logical :: is_initialized = .false.
     logical :: is_updated = .false.
     type(scratch_registry_t) :: scratch
     character(len=:), allocatable :: subsolver, bcknd
 
     ! Internal dummy variables for MMA
     type(vector_t) :: p0j, q0j
     type(matrix_t) :: pij, qij
     type(vector_t) :: bi
 
     !---nesessary for KKT check after updating df0dx, fval, dfdx --------
     real(kind=rp) :: z, zeta
     type(vector_t) :: y, lambda, s, mu
     type(vector_t) :: xsi, eta
   contains
     generic, public :: init => init_from_json, init_from_components
     procedure, public, pass(this) :: init_from_json => mma_init_from_json
     procedure, public, pass(this) :: init_from_components => &
          mma_init_from_components
     procedure, public, pass(this) :: free => mma_free
     procedure, public, pass(this) :: get_n => mma_get_n
     procedure, public, pass(this) :: get_m => mma_get_m
     procedure, public, pass(this) :: get_residumax => mma_get_residumax
     procedure, public, pass(this) :: get_residunorm => mma_get_residunorm
     procedure, public, pass(this) :: get_max_iter => mma_get_max_iter
     procedure, public, pass(this) :: get_backend_and_subsolver => &
          mma_get_backend_and_subsolver
 
     generic, public :: update => update_vector, update_cpu, update_device
     procedure, pass(this) :: update_vector => mma_update_vector
     procedure, pass(this) :: update_cpu => mma_update_cpu
     procedure, pass(this) :: update_device => mma_update_device
 
     generic, public :: kkt => kkt_vector, kkt_cpu, kkt_device
     procedure, pass(this) :: kkt_vector => mma_kkt_vector
     procedure, pass(this) :: kkt_cpu => mma_kkt_cpu
     procedure, pass(this) :: kkt_device => mma_kkt_device
 
     procedure, pass(this) :: save_checkpoint => mma_save_checkpoint
     procedure, pass(this) :: load_checkpoint => mma_load_checkpoint
 
     ! Private utilities
     procedure, pass(this) :: copy_from => mma_copy_from
 
  end type mma_t
 
  ! ========================================================================== !
  ! Default parameters
 
  real(kind=rp), parameter :: a0_default = 1.0_rp
  real(kind=rp), parameter :: a_default = 0.0_rp
  real(kind=rp), parameter :: c_default = 1000.0_rp
  real(kind=rp), parameter :: d_default = 0.0_rp
  real(kind=rp), parameter :: xmin_default = 0.0_rp
  real(kind=rp), parameter :: xmax_default = 1.0_rp
 
  real(kind=rp), parameter :: asyinit_default = 0.2_rp
  real(kind=rp), parameter :: asyincr_default = 1.05_rp
  real(kind=rp), parameter :: asydecr_default = 0.65_rp
  real(kind=rp), parameter :: move_limit_default = 0.2_rp
 
  integer, parameter :: max_iter_default = 100
  character(len=*), parameter :: subsolver_default = "dip"
  real(kind=rp), parameter :: scale_default = 1.0_rp
  logical, parameter :: auto_scale_default = .false.
 
  ! ========================================================================== !
  ! interface for cpu backend module subroutines
 
  interface
     ! CPU update function, runs one iteration of MMA
     module subroutine mma_update_cpu(this, iter, x, df0dx, fval, dfdx)
       class(mma_t), intent(inout) :: this
       integer, intent(in) :: iter
       real(kind=rp), dimension(this%n), intent(inout) :: x
       real(kind=rp), dimension(this%n), intent(in) :: df0dx
       real(kind=rp), dimension(this%m), intent(in) :: fval
       real(kind=rp), dimension(this%m, this%n), intent(in) :: dfdx
     end subroutine mma_update_cpu
 
     ! CPU KKT check for convergence
     module subroutine mma_kkt_cpu(this, x, df0dx, fval, dfdx)
       class(mma_t), intent(inout) :: this
       real(kind=rp), dimension(this%n), intent(in) :: x
       real(kind=rp), dimension(this%n), intent(in) :: df0dx
       real(kind=rp), dimension(this%m), intent(in) :: fval
       real(kind=rp), dimension(this%m, this%n), intent(in) :: dfdx
     end subroutine mma_kkt_cpu
 
     ! ========================================================================== !
     ! interface for device backend module subroutines
 
     ! Device update function, runs one iteration of MMA
     module subroutine mma_update_device(this, iter, x, df0dx, fval, dfdx)
       class(mma_t), intent(inout) :: this
       integer, intent(in) :: iter
       type(c_ptr), intent(inout) :: x
       type(c_ptr), intent(in) :: df0dx, fval, dfdx
     end subroutine mma_update_device
 
     module subroutine mma_kkt_device(this, x, df0dx, fval, dfdx)
       class(mma_t), intent(inout) :: this
       type(c_ptr), intent(in) :: x, df0dx, fval, dfdx
     end subroutine mma_kkt_device
 
  end interface
 
  ! ========================================================================== !
  ! Interface for IO routines
 
  interface
     module subroutine mma_save_checkpoint_hdf5(object, filename, overwrite)
       class(mma_t), intent(inout) :: object
       character(len=*), intent(in) :: filename
       logical, intent(in), optional :: overwrite
     end subroutine mma_save_checkpoint_hdf5
 
     module subroutine mma_load_checkpoint_hdf5(object, filename)
       class(mma_t), intent(inout) :: object
       character(len=*), intent(in) :: filename
     end subroutine mma_load_checkpoint_hdf5
  end interface
 
contains
 
  ! ========================================================================== !
  ! Initializers and destructors
 
  subroutine mma_init_from_json(this, x, n, m, json, scale, auto_scale)
    ! ----------------------------------------------------- !
    ! Initializing the mma object and all the parameters    !
    ! required for MMA method. (a_i, c_i, d_i, ...)         !
    ! x: the design varaibles(DV), n: number of DV,         !
    ! m: number of constraints                              !
    !                                                       !
    ! Note that residumax & residunorm of the KKT conditions!
    ! are initialized with huge(0.0_rp). This is done to    !
    ! avoid unnecessary extra computation of KKT norms for  !
    ! the initial design.                                   !
    ! ----------------------------------------------------- !
    class(mma_t), intent(inout) :: this
    integer, intent(in) :: n, m
    type(vector_t), intent(in) :: x
 
    type(json_file), intent(inout) :: json
 
    ! Read the scaling info for fval and dfdx from json
    real(kind=rp), intent(out) :: scale
    logical, intent(out) :: auto_scale
    ! -------------------------------------------------------------------!
    !      Internal parameters for MMA                                   !
    !      Minimize  f_0(x) + a_0*z + sum( c_i*y_i + 0.5*d_i*(y_i)^2 )   !
    !    subject to  f_i(x) - a_i*z - y_i <= 0,  i = 1,...,m             !
    !                xmin_j <= x_j <= xmax_j,    j = 1,...,n             !
    !                z >= 0,   y_i >= 0,         i = 1,...,m             !
    ! -------------------------------------------------------------------!
    real(kind=rp), dimension(n) :: xmax, xmin
    real(kind=rp), dimension(m) :: a, c, d
    character(len=:), allocatable :: subsolver, bcknd, bcknd_default
 
    ! For reading the values from json and then set the value for the arrays
    real(kind=rp) :: a0 , xmax_const, xmin_const, a_const, c_const, d_const
    real(kind=rp) :: move_limit
 
    integer :: max_iter, n_global, ierr
    real(kind=rp) :: epsimin, asyinit, asyincr, asydecr
 
    call mpi_allreduce(n, n_global, 1, mpi_integer, &
         mpi_sum, neko_comm, ierr)
 
    ! Assign default values for the backend based on the NEKO_BCKND_DEVICE
    if (neko_bcknd_device .eq. 1) then
       bcknd_default = "device"
    else
       bcknd_default = "cpu"
    end if
 
    ! ------------------------------------------------------------------------ !
    ! Assign defaults if nothing is parsed
    ! based on the Cpp Code by Niels
    call json_get_or_default(json, 'mma.epsimin', epsimin, &
         1.0e-9_rp * sqrt(real(m + n_global, rp)))
    call json_get_or_default(json, 'mma.max_iter', max_iter, max_iter_default)
 
    ! Following parameters are set based on eq.3.8:--------
    call json_get_or_default(json, 'mma.asyinit', asyinit, asyinit_default)
    call json_get_or_default(json, 'mma.asyincr', asyincr, asyincr_default)
    call json_get_or_default(json, 'mma.asydecr', asydecr, asydecr_default)
 
    call json_get_or_default(json, 'mma.backend', bcknd, bcknd_default)
    call json_get_or_default(json, 'mma.subsolver', subsolver, subsolver_default)
 
    call json_get_or_default(json, 'mma.xmin', xmin_const, xmin_default)
    call json_get_or_default(json, 'mma.xmax', xmax_const, xmax_default)
    call json_get_or_default(json, 'mma.a0', a0, a0_default)
    call json_get_or_default(json, 'mma.a', a_const, a_default)
    call json_get_or_default(json, 'mma.c', c_const, c_default)
    call json_get_or_default(json, 'mma.d', d_const, d_default)
    call json_get_or_default(json, 'mma.move_limit', move_limit, move_limit_default)
 
    call json_get_or_default(json, 'mma.scale', scale, scale_default)
    call json_get_or_default(json, 'mma.auto_scale', auto_scale, auto_scale_default)
 
    ! Initialize the MMA object with the parsed parameters
    a = a_const
    c = c_const
    d = d_const
    xmin = xmin_const
    xmax = xmax_const
 
    ! ------------------------------------------------------------------------ !
    ! Initialize the MMA object with the parameters read from json
 
    call this%init(x, n, m, a0, a, c, d, xmin, xmax, &
         max_iter, epsimin, asyinit, asyincr, asydecr, bcknd, subsolver, &
         move_limit)
 
  end subroutine mma_init_from_json
 
  subroutine mma_free(this)
    class(mma_t), intent(inout) :: this
    ! Deallocate the internal vectors
    call this%xold1%free()
    call this%xold2%free()
    call this%alpha%free()
    call this%beta%free()
    call this%a%free()
    call this%c%free()
    call this%d%free()
    call this%low%free()
    call this%upp%free()
    call this%xmax%free()
    call this%xmin%free()
    call this%p0j%free()
    call this%q0j%free()
    call this%bi%free()
    call this%y%free()
    call this%lambda%free()
    call this%s%free()
    call this%mu%free()
    call this%xsi%free()
    call this%eta%free()
 
    ! Deallocate the internal dummy matrices
    call this%pij%free()
    call this%qij%free()
    call this%scratch%free()
 
    this%is_initialized = .false.
    this%is_updated = .false.
  end subroutine mma_free
 
  subroutine mma_init_from_components(this, x, n, m, a0, a, c, d, xmin, xmax, &
       max_iter, epsimin, asyinit, asyincr, asydecr, bcknd, subsolver, &
       move_limit)
    ! ----------------------------------------------------- !
    ! Initializing the mma object and all the parameters    !
    ! required for MMA method. (a_i, c_i, d_i, ...)         !
    ! x: the design varaibles(DV), n: number of DV,         !
    ! m: number of constraints                              !
    !                                                       !
    ! Note that residumax & residunorm of the KKT conditions!
    ! are initialized with huge(0.0_rp). This is done to    !
    ! avoid unnecessary extra computation of KKT norms for  !
    ! the initial design.                                   !
    ! ----------------------------------------------------- !
    class(mma_t), intent(inout) :: this
    integer, intent(in) :: n, m
    type(vector_t), intent(in) :: x
    ! -------------------------------------------------------------------!
    !      Internal parameters for MMA                                   !
    !      Minimize  f_0(x) + a_0*z + sum( c_i*y_i + 0.5*d_i*(y_i)^2 )   !
    !    subject to  f_i(x) - a_i*z - y_i <= 0,  i = 1,...,m             !
    !                xmin_j <= x_j <= xmax_j,    j = 1,...,n             !
    !                z >= 0,   y_i >= 0,         i = 1,...,m             !
    ! -------------------------------------------------------------------!
    real(kind=rp), intent(in), dimension(n) :: xmax, xmin
    real(kind=rp), intent(in), dimension(m) :: a, c, d
    real(kind=rp), intent(in) :: a0
    integer, intent(in), optional :: max_iter
    real(kind=rp), intent(in), optional :: epsimin, asyinit, asyincr, asydecr
    real(kind=rp), intent(in), optional :: move_limit
    character(len=*), intent(in), optional :: bcknd, subsolver
    character(len=256) :: log_msg
    integer :: i, ierr
 
    call this%free()
    call this%scratch%init()
 
    this%n = n
    this%m = m
 
    call this%xold1%init(n)
    call this%xold2%init(n)
    this%xold1 = x
    this%xold2 = x
 
    call this%alpha%init(n)
    call this%beta%init(n)
 
    call this%a%init(m)
    call this%c%init(m)
    call this%d%init(m)
    call this%low%init(n)
    call this%upp%init(n)
    call this%xmax%init(n)
    call this%xmin%init(n)
 
    !internal dummy variables for MMA
    call this%p0j%init(n)
    call this%q0j%init(n)
    call this%pij%init(m, n)
    call this%qij%init(m, n)
    call this%bi%init(m)
 
    ! Necessary for KKT check after updating df0dx, fval, dfdx
    call this%y%init(m)
    call this%lambda%init(m)
    call this%s%init(m)
    call this%mu%init(m)
    call this%xsi%init(n)
    call this%eta%init(n)
    this%z = 0.0_rp
    this%zeta = 0.0_rp
 
    this%a0 = a0
    this%a%x = a
    this%c%x = c
    this%d%x = d
 
    ! Set the bounds for the design variable based on the problem
    this%xmax%x = xmax
    this%xmin%x = xmin
 
    if (neko_bcknd_device .eq. 1) then
       call this%a%copy_from(host_to_device, sync = .false.)
       call this%c%copy_from(host_to_device, sync = .false.)
       call this%d%copy_from(host_to_device, sync = .false.)
       call this%xmax%copy_from(host_to_device, sync = .false.)
       call this%xmin%copy_from(host_to_device, sync = .true.)
    end if
 
    ! Set KKT norms to a large number for the initial design
    this%residumax = huge(0.0_rp)
    this%residunorm = huge(0.0_rp)
 
    ! Sync parameters across MPI
    call mpi_allreduce(n, this%n_global, 1, mpi_integer, mpi_sum, neko_comm, &
         ierr)
 
    ! ------------------------------------------------------------------------ !
    ! Assign defaults if nothing is parsed
 
    ! Based on the Cpp Code by Niels
    if (.not. present(max_iter)) this%max_iter = max_iter_default
    if (.not. present(epsimin)) then
       this%epsimin = 1.0e-9_rp * sqrt(real(this%m + this%n_global, rp))
    end if
 
    ! Following parameters are set based on eq.3.8
    if (.not. present(asyinit)) this%asyinit = asyinit_default
    if (.not. present(asyincr)) this%asyincr = asyincr_default
    if (.not. present(asydecr)) this%asydecr = asydecr_default
    if (.not. present(move_limit)) this%move_limit = move_limit_default
 
    ! Set default backend based on NEKO_BCKND_DEVICE
    if (.not. present(bcknd) .and. neko_bcknd_device .eq. 0) then
       this%bcknd = "cpu"
    else if (.not. present(bcknd)) then
       this%bcknd = "device"
    end if
 
    ! Set default subsolver
    if (.not. present(subsolver)) this%subsolver = subsolver_default
 
    ! Assign values from inputs when present
    if (present(max_iter)) this%max_iter = max_iter
    if (present(epsimin)) this%epsimin = epsimin
    if (present(asyinit)) this%asyinit = asyinit
    if (present(asyincr)) this%asyincr = asyincr
    if (present(asydecr)) this%asydecr = asydecr
    if (present(move_limit)) this%move_limit = move_limit
    if (present(bcknd)) this%bcknd = bcknd
    if (present(subsolver)) this%subsolver = subsolver
 
    ! The backend selects the implementation of the update and the KKT check
    if (this%bcknd .ne. "cpu" .and. this%bcknd .ne. "device") then
       call neko_error("MMA: unknown backend '" // trim(this%bcknd) // &
            "', expected 'cpu' or 'device'.")
    end if
 
    ! The subsolvers reduce epsilon towards epsimin by factors of 10
    if (.not. (this%epsimin .gt. 0.0_rp)) then
       call neko_error("MMA: epsimin must be positive.")
    end if
 
    call neko_log%section('MMA Parameters')
 
    write(log_msg, '(A10,1X,A)') 'backend   ', trim(this%bcknd)
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,A)') 'subsolver ', trim(this%subsolver)
    call neko_log%message(log_msg)
 
    write(log_msg, '(A10,1X,I0)') 'n         ', this%n_global
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,I0)') 'm         ', this%m
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,I0)') 'max_iter  ', this%max_iter
    call neko_log%message(log_msg)
 
    write(log_msg, '(A10,1X,E11.5)') 'epsimin   ', this%epsimin
    call neko_log%message(log_msg)
 
    write(log_msg, '(A10,1X,E11.5)') 'asyinit   ', this%asyinit
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,E11.5)') 'asyincr   ', this%asyincr
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,E11.5)') 'asydecr   ', this%asydecr
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,E11.5)') 'a0        ', this%a0
    call neko_log%message(log_msg)
    write(log_msg, '(A10,1X,E11.5)') 'movelimit ', this%move_limit
    call neko_log%message(log_msg)
 
    call neko_log%message('Parameters a')
    do i = 1, this%m
       write(log_msg, '(3X,A,I2,A,E11.5)') 'a(', i, ') = ', this%a%x(i)
       call neko_log%message(log_msg)
    end do
    call neko_log%message('Parameters c')
    do i = 1, this%m
       write(log_msg, '(3X,A,I2,A,E11.5)') 'c(', i, ') = ', this%c%x(i)
       call neko_log%message(log_msg)
    end do
    call neko_log%message('Parameters d')
    do i = 1, this%m
       write(log_msg, '(3X,A,I2,A,E11.5)') 'd(', i, ') = ', this%d%x(i)
       call neko_log%message(log_msg)
    end do
 
    call neko_log%end_section()
 
    ! The object is correctly initialized
    this%is_initialized = .true.
  end subroutine mma_init_from_components
 
  ! ========================================================================== !
  ! Updator and KKT checker
 
  subroutine mma_update_vector(this, iter, x, df0dx, fval, dfdx)
    class(mma_t), intent(inout) :: this
    integer, intent(in) :: iter
    type(vector_t), intent(inout) :: x
    type(vector_t), intent(inout) :: df0dx, fval
    type(matrix_t), intent(inout) :: dfdx
 
    ! Select backend type
    select case (this%bcknd)
    case ("cpu")
       if (neko_bcknd_device .eq. 1) then
          call x%copy_from(device_to_host, sync = .false.)
          call df0dx%copy_from(device_to_host, sync = .false.)
          call fval%copy_from(device_to_host, sync = .false.)
          call dfdx%copy_from(device_to_host, sync = .true.)
       end if
 
       call mma_update_cpu(this, iter, x%x, df0dx%x, fval%x, dfdx%x)
 
       if (neko_bcknd_device .eq. 1) then
          call x%copy_from(host_to_device, sync = .true.)
       end if
 
    case ("device")
       call mma_update_device(this, iter, x%x_d, df0dx%x_d, fval%x_d, dfdx%x_d)
    case default
       call neko_error('mma_update_vector: Unknown backend: ' // &
            trim(this%bcknd))
    end select
 
  end subroutine mma_update_vector
 
  subroutine mma_kkt_vector(this, x, df0dx, fval, dfdx)
    class(mma_t), intent(inout) :: this
    type(vector_t), intent(inout) :: x, df0dx, fval
    type(matrix_t), intent(inout) :: dfdx
 
    ! Select backend type
    select case (this%bcknd )
    case ("cpu")
       if (neko_bcknd_device .eq. 1) then
          call device_memcpy(x%x, x%x_d, this%n, device_to_host, &
               sync = .false.)
          call device_memcpy(df0dx%x, df0dx%x_d, this%n, device_to_host, &
               sync = .false.)
          call device_memcpy(fval%x, fval%x_d, this%m, device_to_host, &
               sync = .false.)
          call device_memcpy(dfdx%x, dfdx%x_d, this%m * this%n, device_to_host,&
               sync = .true.)
       end if
 
       call mma_kkt_cpu(this, x%x, df0dx%x, fval%x, dfdx%x)
    case ("device")
       call mma_kkt_device(this, x%x_d, df0dx%x_d, fval%x_d, dfdx%x_d)
    case default
       call neko_error('mma_KKT_vector: Unknown backend: ' // &
            trim(this%bcknd))
    end select
  end subroutine mma_kkt_vector
 
  ! ========================================================================== !
  ! IO Functions
 
  subroutine mma_save_checkpoint(this, filename, overwrite)
    class(mma_t), intent(inout) :: this
    character(len=*), intent(in) :: filename
    logical, intent(in), optional :: overwrite
    character(len=12) :: file_ext
 
    ! Get the file extension
    call filename_suffix(filename, file_ext)
 
    select case (trim(file_ext))
    case ('h5', 'hdf5', 'hf5')
       call mma_save_checkpoint_hdf5(this, filename, overwrite)
    case default
       call neko_error('mma_save_checkpoint: Unsupported file format: ' // &
            trim(file_ext))
    end select
 
  end subroutine mma_save_checkpoint
 
  subroutine mma_load_checkpoint(this, filename)
    class(mma_t), intent(inout) :: this
    character(len=*), intent(in) :: filename
    character(len=12) :: file_ext
 
    ! Get the file extension
    call filename_suffix(filename, file_ext)
 
    select case (trim(file_ext))
    case ('h5', 'hdf5', 'hf5')
       call mma_load_checkpoint_hdf5(this, filename)
    case default
       call neko_error('mma_load_checkpoint: Unsupported file format: ' // &
            trim(file_ext))
    end select
  end subroutine mma_load_checkpoint
 
  ! ========================================================================== !
  ! Getters and setters
 
  pure function mma_get_n(this) result(n)
    class(mma_t), intent(in) :: this
    integer :: n
    n = this%n
  end function mma_get_n
 
  pure function mma_get_m(this) result(m)
    class(mma_t), intent(in) :: this
    integer :: m
    m = this%m
  end function mma_get_m
 
  pure function mma_get_residumax(this) result(residumax)
    class(mma_t), intent(in) :: this
    real(kind=rp) :: residumax
    residumax = this%residumax
  end function mma_get_residumax
 
  pure function mma_get_residunorm(this) result(residunorm)
    class(mma_t), intent(in) :: this
    real(kind=rp) :: residunorm
    residunorm = this%residunorm
  end function mma_get_residunorm
 
  pure function mma_get_max_iter(this) result(max_iter_value)
    class(mma_t), intent(in) :: this
    integer :: max_iter_value
    max_iter_value = this%max_iter
  end function mma_get_max_iter
 
  pure function mma_get_backend_and_subsolver(this) result(backend_subsolver)
    class(mma_t), intent(in) :: this
    character(len=:), allocatable :: backend_subsolver
    character(len=:), allocatable :: backend
 
    if (neko_bcknd_cuda .eq. 1) then
       backend = "cuda"
    else if (neko_bcknd_hip .eq. 1) then
       backend = "hip"
    else if (neko_bcknd_opencl .eq. 1) then
       backend = "opencl"
    else
       backend = "cpu"
    end if
 
    backend_subsolver = 'backend:' // trim(backend) // ', subsolver:' // &
         trim(this%subsolver)
  end function mma_get_backend_and_subsolver
 
  ! ========================================================================== !
  ! Private utilities
 
  subroutine mma_copy_from(this, direction, sync)
    class(mma_t), intent(inout) :: this
    integer, intent(in) :: direction
    logical, intent(in) :: sync
 
    call this%xold1%copy_from(direction, sync = .false.)
    call this%xold2%copy_from(direction, sync = .false.)
    call this%xmax%copy_from(direction, sync = .false.)
    call this%xmin%copy_from(direction, sync = .false.)
 
    call this%low%copy_from(direction, sync = .false.)
    call this%upp%copy_from(direction, sync = .false.)
 
    call this%a%copy_from(direction, sync = .false.)
    call this%c%copy_from(direction, sync = .false.)
    call this%d%copy_from(direction, sync = .false.)
    call this%y%copy_from(direction, sync = .false.)
    call this%s%copy_from(direction, sync = .false.)
 
    call this%p0j%copy_from(direction, sync = .false.)
    call this%q0j%copy_from(direction, sync = .false.)
    call this%pij%copy_from(direction, sync = .false.)
    call this%qij%copy_from(direction, sync = .false.)
    call this%bi%copy_from(direction, sync = .false.)
 
    call this%alpha%copy_from(direction, sync = .false.)
    call this%beta%copy_from(direction, sync = .false.)
    call this%lambda%copy_from(direction, sync = .false.)
    call this%mu%copy_from(direction, sync = .false.)
    call this%xsi%copy_from(direction, sync = .false.)
    call this%eta%copy_from(direction, sync = sync)
 
  end subroutine mma_copy_from
 
  ! ========================================================================== !
  ! Dummy implementations for module procedures
 
#if !HAVE_HDF5
  module subroutine mma_save_checkpoint_hdf5(object, filename, overwrite)
    class(mma_t), intent(inout) :: object
    character(len=*), intent(in) :: filename
    logical, intent(in), optional :: overwrite
    call neko_error('mma: HDF5 support not enabled rebuild with HAVE_HDF5')
  end subroutine mma_save_checkpoint_hdf5
 
  module subroutine mma_load_checkpoint_hdf5(object, filename)
    class(mma_t), intent(inout) :: object
    character(len=*), intent(in) :: filename
    call neko_error('mma: HDF5 support not enabled rebuild with HAVE_HDF5')
  end subroutine mma_load_checkpoint_hdf5
#endif
 
end module mma