aabb_tree.f90

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! LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
! ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
! POSSIBILITY OF SUCH DAMAGE.
!
! ============================================================================ !
! Original C++ Implementation from:
! https://github.com/JamesRandall/SimpleVoxelEngine/blob/master/voxelEngine/src/AABBTree.h
!
! Translated to Fortran by:
! @author Tim Felle Olsen
! @date 9 Feb 2024
!
! C++ Code License:
! The MIT License (MIT)
!
! Copyright (c) 2017 James Randall
!
! Permission is hereby granted, free of charge, to any person obtaining a copy of
! this software and associated documentation files (the "Software"), to deal in
! the Software without restriction, including without limitation the rights to
! use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
! the Software, and to permit persons to whom the Software is furnished to do so,
! subject to the following conditions:
!
! The above copyright notice and this permission notice shall be included in all
! copies or substantial portions of the Software.
!
! THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
! IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
! FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
! COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
! IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
! CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
! ============================================================================ !
 
module aabb_tree
  use aabb, only : aabb_t, get_aabb, merge
  use tri, only : tri_t
  use num_types, only : rp, dp
  use stack, only : stack_i4_t
  use utils, only : neko_error
  implicit none
  private
 
  integer, parameter, public :: aabb_null_node = -1
 
  ! ========================================================================== !
  ! Type definitions
  ! ========================================================================== !
 
  type, public :: aabb_node_t
     private
     type(aabb_t), public :: aabb
     integer :: object_index = -1
 
     ! tree links
     integer :: parent_node_index = aabb_null_node
     integer :: left_node_index = aabb_null_node
     integer :: right_node_index = aabb_null_node
 
     ! node linked list link
     integer :: next_node_index = aabb_null_node
 
   contains
     procedure, pass(this), public :: init => aabb_node_init
 
     ! Getters
     procedure, pass(this), public :: get_aabb => aabb_node_get_aabb
     procedure, pass(this), public :: get_object_index => &
          aabb_node_get_object_index
     procedure, pass(this), public :: get_parent_index => &
          aabb_node_get_parent_index
     procedure, pass(this), public :: get_left_index => &
          aabb_node_get_left_index
     procedure, pass(this), public :: get_right_index => &
          aabb_node_get_right_index
 
     ! Unary operations
     procedure, pass(this), public :: min_distance => aabb_node_min_distance
 
     ! Boolean operators
     procedure, pass(this), public :: is_leaf => aabb_node_is_leaf
     procedure, pass(this), public :: is_valid => aabb_node_is_valid
 
     ! Comparison operators
     generic :: operator(.lt.) => less
     generic :: operator(.gt.) => greater
 
     procedure, pass(this) :: less => aabb_node_less
     procedure, pass(this) :: greater => aabb_node_greater
 
  end type aabb_node_t
 
  type, public :: aabb_tree_t
     private
     type(aabb_node_t), allocatable :: nodes(:)
     integer :: root_node_index = aabb_null_node
     integer :: allocated_node_count = 0
     integer :: next_free_node_index = aabb_null_node
     integer :: node_capacity = 0
     integer :: growth_size = 1
 
   contains
 
     ! Initializers
     procedure, pass(this), public :: init => aabb_tree_init
     procedure, pass(this), public :: build_generic => aabb_tree_build_tree
     procedure, pass(this), public :: build_from_aabb => aabb_tree_build_tree_aabb
     procedure, pass(this), public :: insert_object => &
          aabb_tree_insert_object
     generic :: build => build_generic
 
     ! Getters
     procedure, pass(this), public :: get_size => aabb_tree_get_size
 
     procedure, pass(this), public :: get_root_index => &
          aabb_tree_get_root_index
     procedure, pass(this), public :: get_parent_index => &
          aabb_tree_get_parent_index
     procedure, pass(this), public :: get_left_index => &
          aabb_tree_get_left_index
     procedure, pass(this), public :: get_right_index => &
          aabb_tree_get_right_index
 
     procedure, pass(this), public :: get_node => aabb_tree_get_node
     procedure, pass(this), public :: get_root_node => &
          aabb_tree_get_root_node
     procedure, pass(this), public :: get_parent_node => &
          aabb_tree_get_parent_node
     procedure, pass(this), public :: get_left_node => &
          aabb_tree_get_left_node
     procedure, pass(this), public :: get_right_node => &
          aabb_tree_get_right_node
 
     procedure, pass(this), public :: get_aabb => aabb_tree_get_aabb
 
     procedure, pass(this), public :: query_overlaps => &
          aabb_tree_query_overlaps
 
     procedure, pass(this), public :: print => aabb_tree_print
 
     ! ----------------------------------------------------------------------- !
     ! Internal methods
 
     procedure, pass(this) :: allocate_node => aabb_tree_allocate_node
     procedure, pass(this) :: deallocate_node => aabb_tree_deallocate_node
     procedure, pass(this) :: resize_node_pool => aabb_tree_resize_node_pool
     procedure, pass(this) :: insert_leaf => aabb_tree_insert_leaf
 
     procedure, pass(this) :: fix_upwards_tree => aabb_tree_fix_upwards_tree
 
     procedure, pass(this) :: valid_tree => aabb_tree_valid_tree
 
  end type aabb_tree_t
 
contains
 
  ! ========================================================================== !
  ! Definitions of node methods
  ! ========================================================================== !
 
  subroutine aabb_node_init(this)
    class(aabb_node_t), intent(inout) :: this
 
    this%object_index = -1
    this%parent_node_index = aabb_null_node
    this%left_node_index = aabb_null_node
    this%right_node_index = aabb_null_node
    this%next_node_index = aabb_null_node
  end subroutine aabb_node_init
 
  ! -------------------------------------------------------------------------- !
  ! Getters
 
  pure function aabb_node_get_aabb(this) result(res)
    class(aabb_node_t), intent(in) :: this
    type(aabb_t) :: res
 
    res = this%aabb
  end function aabb_node_get_aabb
 
  pure function aabb_node_get_object_index(this) result(object_index)
    class(aabb_node_t), intent(in) :: this
    integer :: object_index
 
    object_index = this%object_index
  end function aabb_node_get_object_index
 
  pure function aabb_node_get_parent_index(this) result(parent_index)
    class(aabb_node_t), intent(in) :: this
    integer :: parent_index
 
    parent_index = this%parent_node_index
  end function aabb_node_get_parent_index
 
  pure function aabb_node_get_left_index(this) result(left_index)
    class(aabb_node_t), intent(in) :: this
    integer :: left_index
 
    left_index = this%left_node_index
  end function aabb_node_get_left_index
 
  pure function aabb_node_get_right_index(this) result(right_index)
    class(aabb_node_t), intent(in) :: this
    integer :: right_index
 
    right_index = this%right_node_index
  end function aabb_node_get_right_index
 
  function aabb_node_min_distance(this, p) result(distance)
    class(aabb_node_t), intent(in) :: this
    real(kind=dp), dimension(3), intent(in) :: p
    real(kind=dp) :: distance
 
    distance = 0.5_rp * this%aabb%get_diameter() &
         - norm2(this%aabb%get_center() - p)
  end function aabb_node_min_distance
 
  ! -------------------------------------------------------------------------- !
  ! Boolean operators
 
  pure function aabb_node_is_leaf(this) result(res)
    class(aabb_node_t), intent(in) :: this
    logical :: res
 
    res = this%left_node_index == aabb_null_node .and. &
         this%right_node_index == aabb_null_node
  end function aabb_node_is_leaf
 
  pure function aabb_node_is_valid(this) result(valid)
    class(aabb_node_t), intent(in) :: this
    logical :: valid
 
    if (this%is_leaf()) then
       valid = &
       & this%left_node_index .eq. aabb_null_node .and. &
       & this%right_node_index .eq. aabb_null_node .and. &
       & this%object_index .gt. 0
    else
       valid = &
       & this%left_node_index .ne. aabb_null_node .and. &
       & this%right_node_index .ne. aabb_null_node .and. &
       & this%object_index .eq. -1
    end if
 
  end function aabb_node_is_valid
 
  ! -------------------------------------------------------------------------- !
  ! Comparison operators
 
  pure function aabb_node_less(this, other) result(res)
    class(aabb_node_t), intent(in) :: this
    class(aabb_node_t), intent(in) :: other
    logical :: res
 
    res = this%aabb .lt. other%aabb
 
  end function aabb_node_less
 
  pure function aabb_node_greater(this, other) result(res)
    class(aabb_node_t), intent(in) :: this
    class(aabb_node_t), intent(in) :: other
    logical :: res
 
    res = this%aabb .gt. other%aabb
 
  end function aabb_node_greater
 
  ! ========================================================================== !
  ! Definitions of tree methods
  ! ========================================================================== !
 
  subroutine aabb_tree_init(this, initial_capacity)
    class(aabb_tree_t), intent(inout) :: this
    integer, intent(in) :: initial_capacity
    integer :: i, nonzero_capacity
 
    if (initial_capacity < 1) then
       nonzero_capacity = 1
    else
       nonzero_capacity = initial_capacity
    end if
 
    this%root_node_index = aabb_null_node
    this%allocated_node_count = 0
    this%next_free_node_index = 1
    this%node_capacity = nonzero_capacity
    this%growth_size = nonzero_capacity
 
    if (allocated(this%nodes)) deallocate(this%nodes)
    allocate(this%nodes(nonzero_capacity))
 
    do i = 1, nonzero_capacity
       this%nodes(i)%next_node_index = i + 1
    end do
    this%nodes(nonzero_capacity)%next_node_index = aabb_null_node
  end subroutine aabb_tree_init
 
  subroutine aabb_tree_build_tree_aabb(this, objects, padding)
    class(aabb_tree_t), intent(inout) :: this
    type(aabb_t), intent(in) :: objects(:)
    real(kind=dp), optional, intent(in) :: padding
 
    integer :: i_obj, i_node, i
    logical :: done
 
    integer :: start_layer, end_layer
 
    type(aabb_t), allocatable :: box_list(:)
    integer, dimension(:), allocatable :: sorted_indices
 
    real(kind=dp) :: aabb_padding
 
    if (allocated(box_list)) deallocate(box_list)
    allocate(box_list(size(objects)))
 
    call this%init(size(objects) * 2)
    if (size(objects) .eq. 0) then
       return
    end if
 
    ! ------------------------------------------------------------------------ !
    ! Start by sorting the list of objects, then build a balanced binary tree
    ! from the sorted list
 
 
    if (present(padding)) then
       aabb_padding = padding
    else
       aabb_padding = 0.0_dp
    end if
 
    do i_obj = 1, size(objects)
       box_list(i_obj) = get_aabb(objects(i_obj), aabb_padding)
    end do
    call sort(box_list, sorted_indices)
 
    do i = 1, size(sorted_indices)
       i_obj = sorted_indices(i)
       i_node = this%allocate_node()
       this%nodes(i_node)%aabb = box_list(i_obj)
       this%nodes(i_node)%object_index = i_obj
    end do
 
 
    start_layer = 1
    end_layer = size(objects)
    done = .false.
    do while (.not. done)
 
       ! build the next layer
       do i = start_layer, end_layer - 1, 2
          i_node = this%allocate_node()
 
          this%nodes(i_node)%aabb = merge(this%nodes(i)%aabb, &
               this%nodes(i + 1)%aabb)
 
          this%nodes(i_node)%left_node_index = i
          this%nodes(i_node)%right_node_index = i + 1
 
          this%nodes(i)%parent_node_index = i_node
          this%nodes(i + 1)%parent_node_index = i_node
       end do
 
       ! if the number of nodes is odd, we need to create a new node to hold the
       ! last node
       if (mod(end_layer - start_layer, 2) .eq. 0) then
          i_node = this%allocate_node()
          this%nodes(i_node)%aabb = this%nodes(end_layer)%aabb
          this%nodes(i_node)%left_node_index = end_layer
          this%nodes(i_node)%right_node_index = aabb_null_node
 
          this%nodes(end_layer)%parent_node_index = i_node
       end if
 
       ! move to the next layer
       start_layer = end_layer + 1
       end_layer = this%allocated_node_count
 
       ! If there is only one node left, we are done
       done = start_layer .eq. end_layer
    end do
 
    ! The last node allocated is the root node
    this%root_node_index = this%allocated_node_count
 
    if (this%get_size() .ne. size(objects)) then
       print *, "this%get_size() = ", this%get_size()
       print *, "size(objects) = ", size(objects)
       call neko_error("Invalid tree size")
    end if
 
  end subroutine aabb_tree_build_tree_aabb
 
 
 
  subroutine aabb_tree_build_tree(this, objects, padding)
    class(aabb_tree_t), intent(inout) :: this
    class(*), target, intent(in) :: objects(:)
    real(kind=dp), optional, intent(in) :: padding
 
    integer :: i_obj, i_node, i
    logical :: done
 
    integer :: start_layer, end_layer
 
    type(aabb_t), allocatable :: box_list(:)
    integer, dimension(:), allocatable :: sorted_indices
 
    real(kind=dp) :: aabb_padding
 
    if (allocated(box_list)) deallocate(box_list)
    allocate(box_list(size(objects)))
 
    call this%init(size(objects) * 2)
    if (size(objects) .eq. 0) then
       return
    end if
 
    ! ------------------------------------------------------------------------ !
    ! Start by sorting the list of objects, then build a balanced binary tree
    ! from the sorted list
 
 
    if (present(padding)) then
       aabb_padding = padding
    else
       aabb_padding = 0.0_dp
    end if
 
    do i_obj = 1, size(objects)
       box_list(i_obj) = get_aabb(objects(i_obj), aabb_padding)
    end do
    call sort(box_list, sorted_indices)
 
    do i = 1, size(sorted_indices)
       i_obj = sorted_indices(i)
       i_node = this%allocate_node()
       this%nodes(i_node)%aabb = box_list(i_obj)
       this%nodes(i_node)%object_index = i_obj
    end do
 
 
    start_layer = 1
    end_layer = size(objects)
    done = .false.
    do while (.not. done)
 
       ! build the next layer
       do i = start_layer, end_layer - 1, 2
          i_node = this%allocate_node()
 
          this%nodes(i_node)%aabb = merge(this%nodes(i)%aabb, &
               this%nodes(i + 1)%aabb)
 
          this%nodes(i_node)%left_node_index = i
          this%nodes(i_node)%right_node_index = i + 1
 
          this%nodes(i)%parent_node_index = i_node
          this%nodes(i + 1)%parent_node_index = i_node
       end do
 
       ! if the number of nodes is odd, we need to create a new node to hold the
       ! last node
       if (mod(end_layer - start_layer, 2) .eq. 0) then
          i_node = this%allocate_node()
          this%nodes(i_node)%aabb = this%nodes(end_layer)%aabb
          this%nodes(i_node)%left_node_index = end_layer
          this%nodes(i_node)%right_node_index = aabb_null_node
 
          this%nodes(end_layer)%parent_node_index = i_node
       end if
 
       ! move to the next layer
       start_layer = end_layer + 1
       end_layer = this%allocated_node_count
 
       ! If there is only one node left, we are done
       done = start_layer .eq. end_layer
    end do
 
    ! The last node allocated is the root node
    this%root_node_index = this%allocated_node_count
 
    if (this%get_size() .ne. size(objects)) then
       print *, "this%get_size() = ", this%get_size()
       print *, "size(objects) = ", size(objects)
       call neko_error("Invalid tree size")
    end if
 
  end subroutine aabb_tree_build_tree
 
  subroutine sort(array, indices)
    type(aabb_t), dimension(:), intent(in) :: array
    integer, intent(inout), dimension(:), allocatable :: indices
    logical, dimension(:), allocatable :: visited
 
    integer :: i, imin
    integer :: minidx
 
    allocate(indices(size(array)))
    allocate(visited(size(array)))
 
    visited = .false.
    indices = 0
    do i = 1, size(array)
       minidx = -1
       do imin = 1, size(array)
          if (.not. visited(imin) .and. minidx .eq. -1) minidx = imin
          if (minidx .gt. -1) then
             if (visited(imin) .and. array(imin) .lt. array(minidx)) minidx = imin
          end if
       end do
 
       indices(i) = minidx
       visited(minidx) = .true.
    end do
 
  end subroutine sort
 
  ! -------------------------------------------------------------------------- !
  ! Getters
 
  function aabb_tree_get_size(this) result(size)
    class(aabb_tree_t), intent(in) :: this
    integer :: size
 
    type(stack_i4_t) :: simple_stack
    integer :: idx, tmp
 
    call simple_stack%init(this%allocated_node_count)
    size = 0
    tmp = this%get_root_index()
    if (tmp .ne. aabb_null_node) then
       call simple_stack%push(tmp)
    end if
 
    do while (.not. simple_stack%is_empty())
       idx = simple_stack%pop()
       if (idx .eq. aabb_null_node) cycle
 
       if (this%nodes(idx)%is_leaf()) then
          size = size + 1
       else
          tmp = this%get_left_index(idx)
          call simple_stack%push(tmp)
          tmp = this%get_right_index(idx)
          call simple_stack%push(tmp)
       end if
    end do
 
  end function aabb_tree_get_size
 
  ! -------------------------------------------------------------------------- !
  ! Get index of nodes
 
  pure function aabb_tree_get_root_index(this) result(root_index)
    class(aabb_tree_t), intent(in) :: this
    integer :: root_index
 
    root_index = this%root_node_index
  end function aabb_tree_get_root_index
 
  pure function aabb_tree_get_parent_index(this, node_index) &
       result(parent_index)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    integer :: parent_index
 
    parent_index = this%nodes(node_index)%parent_node_index
  end function aabb_tree_get_parent_index
 
  pure function aabb_tree_get_left_index(this, node_index) &
       result(left_index)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    integer :: left_index
 
    left_index = this%nodes(node_index)%left_node_index
  end function aabb_tree_get_left_index
 
  pure function aabb_tree_get_right_index(this, node_index) &
       result(right_index)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    integer :: right_index
 
    right_index = this%nodes(node_index)%right_node_index
  end function aabb_tree_get_right_index
 
  ! -------------------------------------------------------------------------- !
  ! Get nodes
 
  pure function aabb_tree_get_node(this, node_index) result(node)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    type(aabb_node_t) :: node
 
    node = this%nodes(node_index)
  end function aabb_tree_get_node
 
  pure function aabb_tree_get_root_node(this) result(root_node)
    class(aabb_tree_t), intent(in) :: this
    type(aabb_node_t) :: root_node
 
    root_node = this%nodes(this%root_node_index)
  end function aabb_tree_get_root_node
 
  pure function aabb_tree_get_parent_node(this, node_index) &
       result(parent_node)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    type(aabb_node_t) :: parent_node
 
    parent_node = this%nodes(this%nodes(node_index)%parent_node_index)
  end function aabb_tree_get_parent_node
 
  pure function aabb_tree_get_left_node(this, node_index) result(left_node)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    type(aabb_node_t) :: left_node
 
    left_node = this%nodes(this%nodes(node_index)%left_node_index)
  end function aabb_tree_get_left_node
 
  pure function aabb_tree_get_right_node(this, node_index) &
       result(right_node)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    type(aabb_node_t) :: right_node
 
    right_node = this%nodes(this%nodes(node_index)%right_node_index)
  end function aabb_tree_get_right_node
 
  pure function aabb_tree_get_aabb(this, node_index) result(out_box)
    class(aabb_tree_t), intent(in) :: this
    integer, intent(in) :: node_index
    type(aabb_t) :: out_box
 
    out_box = this%nodes(node_index)%aabb
  end function aabb_tree_get_aabb
 
  ! -------------------------------------------------------------------------- !
 
  subroutine aabb_tree_insert_object(this, object, object_index)
    class(aabb_tree_t), intent(inout) :: this
    class(*), intent(in) :: object
    integer, intent(in) :: object_index
 
    integer :: node_index
 
    node_index = this%allocate_node()
    this%nodes(node_index)%aabb = get_aabb(object)
    this%nodes(node_index)%object_index = object_index
 
    call this%insert_leaf(node_index)
  end subroutine aabb_tree_insert_object
 
  subroutine aabb_tree_query_overlaps(this, object, object_index, overlaps)
    class(aabb_tree_t), intent(in) :: this
    class(*), intent(in) :: object
    integer, intent(in) :: object_index
    type(stack_i4_t), intent(inout) :: overlaps
 
    type(stack_i4_t) :: simple_stack
    type(aabb_t) :: object_box
 
    integer :: root_index, left_index, right_index
    integer :: node_index, tmp_index
 
    object_box = get_aabb(object)
    root_index = this%get_root_index()
 
    call simple_stack%init()
    call simple_stack%push(root_index)
 
    do while (.not. simple_stack%is_empty())
       node_index = simple_stack%pop()
 
       if (node_index == aabb_null_node) cycle
 
       if (this%nodes(node_index)%aabb%overlaps(object_box)) then
          if (this%nodes(node_index)%is_leaf()) then
             if (this%nodes(node_index)%object_index .ne. object_index) then
                tmp_index = this%nodes(node_index)%object_index
                call overlaps%push(tmp_index)
             end if
          else
             left_index = this%get_left_index(node_index)
             if (left_index .ne. aabb_null_node) then
                call simple_stack%push(left_index)
             end if
             right_index = this%get_right_index(node_index)
             if (right_index .ne. aabb_null_node) then
                call simple_stack%push(right_index)
             end if
          end if
       end if
    end do
    call simple_stack%free()
  end subroutine aabb_tree_query_overlaps
 
  ! -------------------------------------------------------------------------- !
  ! Internal methods
 
  function aabb_tree_allocate_node(this) result(node_index)
    class(aabb_tree_t), intent(inout) :: this
    integer :: node_index
 
    if (this%next_free_node_index == aabb_null_node) then
       call this%resize_node_pool(this%node_capacity + this%growth_size)
    end if
 
    node_index = this%next_free_node_index
 
    associate(new_node => this%nodes(node_index))
      this%next_free_node_index = new_node%next_node_index
 
      new_node%parent_node_index = aabb_null_node
      new_node%left_node_index = aabb_null_node
      new_node%right_node_index = aabb_null_node
 
      this%next_free_node_index = new_node%next_node_index
      this%allocated_node_count = this%allocated_node_count + 1
 
    end associate
  end function aabb_tree_allocate_node
 
  subroutine aabb_tree_deallocate_node(this, node_index)
    class(aabb_tree_t), intent(inout) :: this
    integer, intent(in) :: node_index
 
    this%nodes(node_index)%next_node_index = this%next_free_node_index
    this%next_free_node_index = node_index
    this%allocated_node_count = this%allocated_node_count - 1
  end subroutine aabb_tree_deallocate_node
 
  subroutine aabb_tree_insert_leaf(this, leaf_node_index)
    class(aabb_tree_t), intent(inout) :: this
    integer, intent(in) :: leaf_node_index
 
    integer :: tree_node_index
 
    real(kind=rp) :: cost_left
    real(kind=rp) :: cost_right
 
    type(aabb_node_t) :: leaf_node
    type(aabb_node_t) :: tree_node
    type(aabb_node_t) :: left_node
    type(aabb_node_t) :: right_node
 
    type(aabb_t) :: combined_aabb
    real(kind=rp) :: new_parent_node_cost
    real(kind=rp) :: minimum_push_down_cost
    type(aabb_t) :: new_left_aabb
    type(aabb_t) :: new_right_aabb
    integer :: leaf_sibling_index
    type(aabb_node_t) :: leaf_sibling
    integer :: old_parent_index
    integer :: new_parent_index
    type(aabb_node_t) :: new_parent
    type(aabb_node_t) :: old_parent
 
    ! make sure were inserting a new leaf
    leaf_node = this%nodes(leaf_node_index)
 
    ! if the tree is empty then we make the root the leaf
    if (this%root_node_index .eq. aabb_null_node) then
       this%root_node_index = leaf_node_index
       leaf_node%parent_node_index = aabb_null_node
       leaf_node%left_node_index = aabb_null_node
       leaf_node%right_node_index = aabb_null_node
 
       return
    end if
 
    ! search for the best place to put the new leaf in the tree
    ! we use surface area and depth as search heuristics
    tree_node_index = this%root_node_index
    tree_node = this%get_node(tree_node_index)
    do while (.not. tree_node%is_leaf())
 
       ! because of the test in the while loop above we know we are never a
       ! leaf inside it
       left_node = this%get_left_node(tree_node_index)
       right_node = this%get_right_node(tree_node_index)
 
       ! ------------------------------------------------------------------- !
 
       combined_aabb = merge(tree_node%aabb, leaf_node%get_aabb())
 
       new_parent_node_cost = 2.0_rp * combined_aabb%get_surface_area()
       minimum_push_down_cost = 2.0_rp * ( &
       & combined_aabb%get_surface_area() &
       & - tree_node%aabb%get_surface_area()&
       & )
 
       ! use the costs to figure out whether to create a new parent here or
       ! descend
       if (left_node%is_leaf()) then
          new_left_aabb = merge(leaf_node%aabb, left_node%get_aabb())
          cost_left = new_left_aabb%get_surface_area() + minimum_push_down_cost
       else
          new_left_aabb = merge(leaf_node%aabb, left_node%get_aabb())
          cost_left = ( &
          & new_left_aabb%get_surface_area() &
          & - left_node%aabb%get_surface_area()&
          & ) + minimum_push_down_cost
       end if
 
       if (right_node%is_leaf()) then
          new_right_aabb = merge(leaf_node%aabb, right_node%aabb)
          cost_right = new_right_aabb%get_surface_area() + &
               minimum_push_down_cost
       else
          new_right_aabb = merge(leaf_node%aabb, right_node%aabb)
          cost_right = ( &
          & new_right_aabb%get_surface_area() &
          & - right_node%aabb%get_surface_area() &
          & ) + minimum_push_down_cost
       end if
 
       ! if the cost of creating a new parent node here is less than descending
       ! in either direction then we know we need to create a new parent node,
       ! errrr, here and attach the leaf to that
       if (new_parent_node_cost < cost_left .and. &
            new_parent_node_cost < cost_right) then
          exit
       end if
 
       ! otherwise descend in the cheapest direction
       if (cost_left .lt. cost_right) then
          tree_node_index = tree_node%get_left_index()
       else
          tree_node_index = tree_node%get_right_index()
       end if
 
       ! ------------------------------------------------------------------- !
       ! Update the node and continue the loop
       tree_node = this%get_node(tree_node_index)
    end do
 
    ! the leafs sibling is going to be the node we found above and we are
    ! going to create a new parent node and attach the leaf and this item
    leaf_sibling_index = tree_node_index
    leaf_sibling = this%nodes(leaf_sibling_index)
    old_parent_index = this%get_parent_index(leaf_sibling_index)
    new_parent_index = this%allocate_node()
    new_parent = this%nodes(new_parent_index)
    new_parent%parent_node_index = old_parent_index
    new_parent%aabb = merge(leaf_node%aabb, leaf_sibling%aabb)
 
    if (leaf_node .lt. leaf_sibling) then
       new_parent%left_node_index = leaf_node_index
       new_parent%right_node_index = leaf_sibling_index
    else
       new_parent%left_node_index = leaf_sibling_index
       new_parent%right_node_index = leaf_node_index
    end if
 
    leaf_node%parent_node_index = new_parent_index
    leaf_sibling%parent_node_index = new_parent_index
 
    if (old_parent_index .eq. aabb_null_node) then
       ! the old parent was the root and so this is now the root
       this%root_node_index = new_parent_index
    else
       ! the old parent was not the root and so we need to patch the left or
       ! right index to point to the new node
       old_parent = this%nodes(old_parent_index)
       if (old_parent%left_node_index .eq. leaf_sibling_index) then
          old_parent%left_node_index = new_parent_index
       else
          old_parent%right_node_index = new_parent_index
       end if
       this%nodes(old_parent_index) = old_parent
    end if
 
    this%nodes(leaf_node_index) = leaf_node
    this%nodes(leaf_sibling_index) = leaf_sibling
    this%nodes(new_parent_index) = new_parent
 
    ! finally we need to walk back up the tree fixing heights and areas
    tree_node_index = leaf_node%parent_node_index
 
    call this%fix_upwards_tree(tree_node_index)
 
  end subroutine aabb_tree_insert_leaf
 
  function aabb_tree_valid_tree(this) result(valid)
    class(aabb_tree_t), intent(in) :: this
    logical :: valid
 
    type(stack_i4_t) :: simple_stack
    integer :: current_index
    integer :: root_index, left_index, right_index
 
    valid = .true.
    if (this%root_node_index .eq. aabb_null_node) then
       valid = .false.
    end if
 
    root_index = this%get_root_index()
 
    call simple_stack%init(this%node_capacity)
    call simple_stack%push(root_index)
 
    do while (.not. simple_stack%is_empty())
       current_index = simple_stack%pop()
       if (current_index == aabb_null_node) cycle
 
       valid = valid .and. this%nodes(current_index)%is_valid()
 
       if (.not. this%nodes(current_index)%is_leaf()) then
          left_index = this%get_left_index(current_index)
          right_index = this%get_right_index(current_index)
 
          call simple_stack%push(left_index)
          call simple_stack%push(right_index)
       end if
    end do
  end function aabb_tree_valid_tree
 
  subroutine aabb_tree_fix_upwards_tree(this, tree_start_index)
    class(aabb_tree_t), intent(inout) :: this
    integer, intent(in) :: tree_start_index
 
    type(aabb_node_t) :: left_node
    type(aabb_node_t) :: right_node
    integer :: tree_node_index
 
    tree_node_index = tree_start_index
    do while (tree_node_index .ne. aabb_null_node)
       left_node = this%get_left_node(tree_node_index)
       right_node = this%get_right_node(tree_node_index)
 
       this%nodes(tree_node_index)%aabb = merge(left_node%aabb, right_node%aabb)
 
       tree_node_index = this%get_parent_index(tree_node_index)
    end do
  end subroutine aabb_tree_fix_upwards_tree
 
  subroutine aabb_tree_print(this)
    class(aabb_tree_t), intent(inout) :: this
    type(stack_i4_t) :: simple_stack
 
    integer :: current_index
    integer :: root_index, left_index, right_index
 
    root_index = this%get_root_index()
    call simple_stack%init(this%node_capacity)
    call simple_stack%push(root_index)
 
    do while (.not. simple_stack%is_empty())
       current_index = simple_stack%pop()
       if (current_index .eq. aabb_null_node) cycle
 
       left_index = this%get_left_index(current_index)
       right_index = this%get_right_index(current_index)
 
       call simple_stack%push(this%nodes(current_index)%left_node_index)
       call simple_stack%push(this%nodes(current_index)%right_node_index)
 
       write(*, *) "i = ", current_index
       write(*, *) "  Parent  : ", this%get_parent_index(current_index)
       write(*, *) "  Children: ", this%get_left_index(current_index), &
            this%get_right_index(current_index)
 
       write(*, *) "  object_index = ", this%nodes(current_index)%object_index
    end do
 
  end subroutine aabb_tree_print
 
  subroutine aabb_tree_resize_node_pool(this, new_capacity)
    class(aabb_tree_t), intent(inout) :: this
    integer, intent(in) :: new_capacity
 
    type(aabb_node_t), dimension(:), allocatable :: temp
    integer :: i
 
    allocate(temp(new_capacity))
    temp(:this%node_capacity) = this%nodes(:this%node_capacity)
 
    do i = this%allocated_node_count, new_capacity
       temp(i)%next_node_index = i + 1
    end do
    temp(new_capacity)%next_node_index = aabb_null_node
 
    call move_alloc(temp, this%nodes)
 
    this%node_capacity = new_capacity
    this%next_free_node_index = this%allocated_node_count + 1
 
  end subroutine aabb_tree_resize_node_pool
 
end module aabb_tree