!--------------------------------------------------------------------------------------------------!
!   CP2K: A general program to perform molecular dynamics simulations                              !
!   Copyright 2000-2025 CP2K developers group <https://cp2k.org>                                   !
!                                                                                                  !
!   SPDX-License-Identifier: GPL-2.0-or-later                                                      !
!--------------------------------------------------------------------------------------------------!

! **************************************************************************************************
!> \brief Define the neighbor list data types and the corresponding functionality
!> \par History
!>      - cleaned (23.07.2003,MK)
!>      - full refactoring, list iterators (20.10.2010, JGH)
!>      - add get_neighbor_list_set_p, return info for a set of neighborlists
!>                                                             (07.2014,JGH)
!> \author Matthias Krack (21.06.2000)
! **************************************************************************************************
MODULE qs_neighbor_list_types

   USE kinds,                           ONLY: dp
   USE util,                            ONLY: locate,&
                                              sort
#include "./base/base_uses.f90"

   IMPLICIT NONE

   PRIVATE

! *** Global parameters (in this module) ***

   CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'qs_neighbor_list_types'

! *** Definition of the data types for a linked list of neighbors ***

! **************************************************************************************************
   TYPE neighbor_node_type
      PRIVATE
      TYPE(neighbor_node_type), POINTER :: next_neighbor_node => NULL()
      REAL(dp), DIMENSION(3)            :: r = -1.0_dp
      INTEGER, DIMENSION(3)             :: cell = -1
      INTEGER                           :: neighbor = -1
   END TYPE neighbor_node_type

! **************************************************************************************************
   TYPE neighbor_list_type
      PRIVATE
      TYPE(neighbor_list_type), POINTER :: next_neighbor_list => NULL()
      TYPE(neighbor_node_type), POINTER :: first_neighbor_node => NULL(), &
                                           last_neighbor_node => NULL()
      INTEGER                           :: atom = -1, nnode = -1
   END TYPE neighbor_list_type

! **************************************************************************************************
   TYPE neighbor_list_set_type
      PRIVATE
      TYPE(neighbor_list_type), POINTER :: first_neighbor_list => NULL(), &
                                           last_neighbor_list => NULL()
      INTEGER                           :: nlist = -1
      LOGICAL                           :: symmetric = .FALSE.
   END TYPE neighbor_list_set_type

! **************************************************************************************************
   TYPE neighbor_list_p_type
      TYPE(neighbor_list_type), POINTER :: neighbor_list => NULL()
   END TYPE neighbor_list_p_type

! **************************************************************************************************
   TYPE neighbor_list_set_p_type
      TYPE(neighbor_list_set_type), POINTER                :: neighbor_list_set => NULL()
      INTEGER                                              :: nl_size = -1
      INTEGER                                              :: nl_start = -1
      INTEGER                                              :: nl_end = -1
      TYPE(neighbor_list_task_type), DIMENSION(:), POINTER :: nlist_task => NULL()
   END TYPE neighbor_list_set_p_type

! **************************************************************************************************
   TYPE list_search_type
      PRIVATE
      INTEGER                               :: nlist = -1
      INTEGER, DIMENSION(:), POINTER        :: atom_list => NULL()
      INTEGER, DIMENSION(:), POINTER        :: atom_index => NULL()
      TYPE(neighbor_list_p_type), &
         DIMENSION(:), POINTER              :: neighbor_list => NULL()
   END TYPE list_search_type

! **************************************************************************************************
   TYPE neighbor_list_task_type
      INTEGER                               :: iatom = -1, jatom = -1, &
                                               ikind = -1, jkind = -1, nkind = -1, &
                                               ilist = -1, nlist = -1, inode = -1, nnode = -1
      REAL(KIND=dp), DIMENSION(3)           :: r = -1.0_dp
      INTEGER, DIMENSION(3)                 :: cell = -1
      TYPE(neighbor_list_task_type), &
         POINTER                            :: next => NULL() ! Pointer for forming a linked list of tasks
   END TYPE neighbor_list_task_type

   INTERFACE nl_sub_iterate
      MODULE PROCEDURE nl_sub_iterate
      MODULE PROCEDURE nl_sub_iterate_ref
   END INTERFACE

! **************************************************************************************************
! Neighbor List Iterator
! **************************************************************************************************
   TYPE neighbor_list_iterator_type
      PRIVATE
      INTEGER                               :: ikind = -1, jkind = -1, ilist = -1, inode = -1
      INTEGER                               :: nkind = -1, nlist = -1, nnode = -1
      INTEGER                               :: iatom = -1, jatom = -1
      TYPE(neighbor_list_set_p_type), &
         DIMENSION(:), POINTER               :: nl => NULL()
      TYPE(neighbor_list_type), POINTER     :: neighbor_list => NULL()
      TYPE(neighbor_node_type), POINTER     :: neighbor_node => NULL()
      TYPE(list_search_type), &
         DIMENSION(:), POINTER               :: list_search => NULL()
   END TYPE neighbor_list_iterator_type

   TYPE neighbor_list_iterator_p_type
      PRIVATE
      TYPE(neighbor_list_iterator_type), POINTER :: neighbor_list_iterator => NULL()
      INTEGER                                    :: last = -1
   END TYPE neighbor_list_iterator_p_type
! **************************************************************************************************

! *** Public data types ***

   PUBLIC :: neighbor_list_p_type, &
             neighbor_list_set_type, &
             neighbor_list_set_p_type, &
             neighbor_list_task_type

! *** Public subroutines ***

   PUBLIC :: add_neighbor_list, &
             add_neighbor_node, &
             allocate_neighbor_list_set, &
             deallocate_neighbor_list_set, &
             release_neighbor_list_sets, &
             get_iterator_task, &
             get_neighbor_list_set, &
             get_neighbor_list_set_p

! *** Iterator functions and types ***

   PUBLIC :: neighbor_list_iterator_p_type, &
             neighbor_list_iterator_create, &
             neighbor_list_iterator_release, &
             neighbor_list_iterate, &
             nl_set_sub_iterator, &
             nl_sub_iterate, &
             get_iterator_info

CONTAINS

! **************************************************************************************************
!> \brief   Neighbor list iterator functions
!> \param iterator_set ...
!> \param nl ...
!> \param search ...
!> \param nthread ...
!> \date    28.07.2010
!> \author  jhu
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE neighbor_list_iterator_create(iterator_set, nl, search, nthread)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      TYPE(neighbor_list_set_p_type), DIMENSION(:), &
         POINTER                                         :: nl
      LOGICAL, INTENT(IN), OPTIONAL                      :: search
      INTEGER, INTENT(IN), OPTIONAL                      :: nthread

      INTEGER                                            :: iatom, il, ilist, mthread, nlist
      TYPE(list_search_type), DIMENSION(:), POINTER      :: list_search
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator
      TYPE(neighbor_list_type), POINTER                  :: neighbor_list

      mthread = 1
      IF (PRESENT(nthread)) mthread = nthread

      ALLOCATE (iterator_set(0:mthread - 1))

      DO il = 0, mthread - 1
         ALLOCATE (iterator_set(il)%neighbor_list_iterator)

         iterator => iterator_set(il)%neighbor_list_iterator

         iterator%nl => nl

         iterator%ikind = 0
         iterator%jkind = 0
         iterator%nkind = NINT(SQRT(REAL(SIZE(nl), dp)))

         iterator%ilist = 0
         iterator%nlist = 0
         iterator%inode = 0
         iterator%nnode = 0

         iterator%iatom = 0
         iterator%jatom = 0

         NULLIFY (iterator%neighbor_list)
         NULLIFY (iterator%neighbor_node)
         NULLIFY (iterator%list_search)
      END DO

      iterator_set(:)%last = 0

      IF (PRESENT(search)) THEN
         IF (search) THEN
            ALLOCATE (list_search(SIZE(nl)))
            DO il = 1, SIZE(nl)
               IF (ASSOCIATED(nl(il)%neighbor_list_set)) THEN
                  CALL get_neighbor_list_set(neighbor_list_set=nl(il)%neighbor_list_set, nlist=nlist)
                  list_search(il)%nlist = nlist
                  ALLOCATE (list_search(il)%atom_list(nlist))
                  ALLOCATE (list_search(il)%atom_index(nlist))
                  ALLOCATE (list_search(il)%neighbor_list(nlist))

                  NULLIFY (neighbor_list)
                  DO ilist = 1, nlist
                     IF (.NOT. ASSOCIATED(neighbor_list)) THEN
                        neighbor_list => first_list(nl(il)%neighbor_list_set)
                     ELSE
                        neighbor_list => neighbor_list%next_neighbor_list
                     END IF
                     CALL get_neighbor_list(neighbor_list=neighbor_list, atom=iatom)
                     list_search(il)%atom_list(ilist) = iatom
                     list_search(il)%neighbor_list(ilist)%neighbor_list => neighbor_list
                  END DO
                  CALL sort(list_search(il)%atom_list, nlist, list_search(il)%atom_index)

               ELSE
                  list_search(il)%nlist = -1
                  NULLIFY (list_search(il)%atom_list, list_search(il)%atom_index, list_search(il)%neighbor_list)
               END IF
            END DO
            DO il = 0, mthread - 1
               iterator => iterator_set(il)%neighbor_list_iterator
               iterator%list_search => list_search
            END DO
         END IF
      END IF

   END SUBROUTINE neighbor_list_iterator_create

! **************************************************************************************************
!> \brief ...
!> \param iterator_set ...
! **************************************************************************************************
   SUBROUTINE neighbor_list_iterator_release(iterator_set)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set

      INTEGER                                            :: il, mthread
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator

!all threads have the same search list

      iterator => iterator_set(0)%neighbor_list_iterator
      IF (ASSOCIATED(iterator%list_search)) THEN
         DO il = 1, SIZE(iterator%list_search)
            IF (iterator%list_search(il)%nlist >= 0) THEN
               DEALLOCATE (iterator%list_search(il)%atom_list)
               DEALLOCATE (iterator%list_search(il)%atom_index)
               DEALLOCATE (iterator%list_search(il)%neighbor_list)
            END IF
         END DO
         DEALLOCATE (iterator%list_search)
      END IF

      mthread = SIZE(iterator_set)
      DO il = 0, mthread - 1
         DEALLOCATE (iterator_set(il)%neighbor_list_iterator)
      END DO
      DEALLOCATE (iterator_set)

   END SUBROUTINE neighbor_list_iterator_release

! **************************************************************************************************
!> \brief ...
!> \param iterator_set ...
!> \param ikind ...
!> \param jkind ...
!> \param iatom ...
!> \param mepos ...
! **************************************************************************************************
   SUBROUTINE nl_set_sub_iterator(iterator_set, ikind, jkind, iatom, mepos)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      INTEGER, INTENT(IN)                                :: ikind, jkind, iatom
      INTEGER, INTENT(IN), OPTIONAL                      :: mepos

      INTEGER                                            :: i, ij, ilist, me, nlist, nnode
      TYPE(list_search_type), POINTER                    :: list_search
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator
      TYPE(neighbor_list_type), POINTER                  :: neighbor_list

      IF (PRESENT(mepos)) THEN
         me = mepos
      ELSE
         me = 0
      END IF

      ! Set up my thread-local iterator for the list of iatom / jkind nodes

      iterator => iterator_set(me)%neighbor_list_iterator
      ij = ikind + iterator%nkind*(jkind - 1)
      IF (ASSOCIATED(iterator%list_search)) THEN
         list_search => iterator%list_search(ij)
         nlist = list_search%nlist
         ilist = 0
         NULLIFY (neighbor_list)
         IF (nlist > 0) THEN
            i = locate(list_search%atom_list, iatom)
            i = list_search%atom_index(i)
            IF (i > 0) neighbor_list => list_search%neighbor_list(i)%neighbor_list
            ilist = i
         END IF
         IF (ASSOCIATED(neighbor_list)) THEN
            CALL get_neighbor_list(neighbor_list=neighbor_list, nnode=nnode)
         ELSE
            nnode = 0
         END IF
      ELSE
         CPABORT("")
      END IF

      iterator%ikind = ikind
      iterator%jkind = jkind

      iterator%ilist = ilist
      iterator%nlist = nlist
      iterator%inode = 0
      iterator%nnode = nnode

      iterator%iatom = iatom
      iterator%jatom = 0

      iterator%neighbor_list => neighbor_list
      NULLIFY (iterator%neighbor_node)

   END SUBROUTINE nl_set_sub_iterator

! **************************************************************************************************
!> \brief ...
!> \param iterator_set ...
!> \param mepos ...
!> \return ...
! **************************************************************************************************
   FUNCTION neighbor_list_iterate(iterator_set, mepos) RESULT(istat)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      INTEGER, OPTIONAL                                  :: mepos
      INTEGER                                            :: istat

      INTEGER                                            :: iab, last, me
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator
      TYPE(neighbor_list_set_p_type), DIMENSION(:), &
         POINTER                                         :: nl

      IF (SIZE(iterator_set) .NE. 1 .AND. .NOT. PRESENT(mepos)) &
         CPABORT("Parallel iterator calls must include 'mepos'")

      IF (PRESENT(mepos)) THEN
         me = mepos
      ELSE
         me = 0
      END IF

      istat = 0

!$OMP CRITICAL(neighbour_list_iterate_critical)
      last = iterator_set(0)%last
      IF (last /= me) THEN
         iterator_set(me)%neighbor_list_iterator = iterator_set(last)%neighbor_list_iterator
      END IF
      iterator => iterator_set(me)%neighbor_list_iterator
      nl => iterator%nl

      IF (iterator%inode < iterator%nnode) THEN
         ! we can be sure that there is another node in this list
         iterator%inode = iterator%inode + 1
         iterator%neighbor_node => iterator%neighbor_node%next_neighbor_node
      ELSE
         iab = MAX(iterator%ikind + iterator%nkind*(iterator%jkind - 1), 0)
         kindloop: DO ! look for the next list with nnode /= 0
            listloop: DO
               IF (iterator%ilist >= iterator%nlist) EXIT listloop
               iterator%ilist = iterator%ilist + 1
               IF (ASSOCIATED(iterator%neighbor_list)) THEN
                  iterator%neighbor_list => iterator%neighbor_list%next_neighbor_list
               ELSE
                  iterator%neighbor_list => first_list(nl(iab)%neighbor_list_set)
               END IF
               CALL get_neighbor_list(neighbor_list=iterator%neighbor_list, atom=iterator%iatom, &
                                      nnode=iterator%nnode)
               IF (iterator%nnode > 0) EXIT kindloop
            END DO listloop
            IF (iab >= iterator%nkind**2) THEN
               istat = 1
               EXIT kindloop
            ELSE
               iab = iab + 1
               iterator%jkind = (iab - 1)/iterator%nkind + 1
               iterator%ikind = iab - iterator%nkind*(iterator%jkind - 1)
               iterator%ilist = 0
               IF (.NOT. ASSOCIATED(nl(iab)%neighbor_list_set)) THEN
                  iterator%ilist = 0
                  iterator%nlist = 0
               ELSE
                  CALL get_neighbor_list_set(neighbor_list_set= &
                                             nl(iab)%neighbor_list_set, nlist=iterator%nlist)
                  iterator%ilist = 0
               END IF
               NULLIFY (iterator%neighbor_list)
            END IF
         END DO kindloop
         IF (istat == 0) THEN
            iterator%inode = 1
            iterator%neighbor_node => first_node(iterator%neighbor_list)
         END IF
      END IF
      IF (istat == 0) THEN
         CALL get_neighbor_node(neighbor_node=iterator%neighbor_node, neighbor=iterator%jatom)
      END IF

      ! mark the last iterator updated
      iterator_set(:)%last = me
!$OMP END CRITICAL(neighbour_list_iterate_critical)

   END FUNCTION neighbor_list_iterate

! **************************************************************************************************
!> \brief ...
!> \param iterator_set ...
!> \param mepos ...
!> \return ...
! **************************************************************************************************
   FUNCTION nl_sub_iterate(iterator_set, mepos) RESULT(istat)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      INTEGER, INTENT(IN), OPTIONAL                      :: mepos
      INTEGER                                            :: istat

      INTEGER                                            :: me
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator

      ! Each thread's sub-iterator are independent, no need to synchronise with other threads

      IF (PRESENT(mepos)) THEN
         me = mepos
      ELSE
         me = 0
      END IF

      istat = 0

      iterator => iterator_set(me)%neighbor_list_iterator

      IF (ASSOCIATED(iterator%neighbor_list)) THEN
         IF (iterator%inode >= iterator%nnode) THEN
            ! end of loop
            istat = 1
         ELSEIF (iterator%inode == 0) THEN
            iterator%inode = 1
            iterator%neighbor_node => first_node(iterator%neighbor_list)
         ELSEIF (iterator%inode > 0) THEN
            ! we can be sure that there is another node in this list
            iterator%inode = iterator%inode + 1
            iterator%neighbor_node => iterator%neighbor_node%next_neighbor_node
         ELSE
            CPABORT("wrong")
         END IF
      ELSE
         ! no list available
         istat = 1
      END IF
      IF (istat == 0) THEN
         CALL get_neighbor_node(neighbor_node=iterator%neighbor_node, neighbor=iterator%jatom)
      END IF

   END FUNCTION nl_sub_iterate

! **************************************************************************************************
!> \brief wrap nl_sub_iterate s.t. external loop over kinds and calls to nl_set_sub_iterator
!> are no longer needed. This fixes first atom of iter_sub to second atom of iter_ref.
!> \param iter_sub ...
!> \param iter_ref ...
!> \param mepos ...
!> \return ...
! **************************************************************************************************
   RECURSIVE FUNCTION nl_sub_iterate_ref(iter_sub, iter_ref, mepos) RESULT(iter_stat)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iter_sub, iter_ref
      INTEGER, INTENT(IN), OPTIONAL                      :: mepos
      INTEGER                                            :: iter_stat

      INTEGER                                            :: atom_ref, kind_ref, kind_sub, me, nkind
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator

      IF (PRESENT(mepos)) THEN
         me = mepos
      ELSE
         me = 0
      END IF

      iterator => iter_sub(me)%neighbor_list_iterator
      kind_sub = iterator%jkind

      CALL get_iterator_info(iter_ref, jatom=atom_ref, jkind=kind_ref)

      IF (iterator%inode == 0) THEN
         CALL nl_set_sub_iterator(iter_sub, kind_ref, MAX(kind_sub, 1), atom_ref)
      END IF
      iter_stat = nl_sub_iterate(iter_sub)
      IF (iter_stat == 0) RETURN

      nkind = iterator%nkind

      IF (kind_sub == nkind) THEN
         CALL nl_set_sub_iterator(iter_sub, kind_ref, 1, atom_ref)
         RETURN
      ELSE
         kind_sub = kind_sub + 1
         CALL nl_set_sub_iterator(iter_sub, kind_ref, kind_sub, atom_ref)
         iter_stat = nl_sub_iterate_ref(iter_sub, iter_ref)
      END IF

   END FUNCTION

! **************************************************************************************************
!> \brief ...
!> \param iterator_set ...
!> \param mepos ...
!> \param ikind ...
!> \param jkind ...
!> \param nkind ...
!> \param ilist ...
!> \param nlist ...
!> \param inode ...
!> \param nnode ...
!> \param iatom ...
!> \param jatom ...
!> \param r ...
!> \param cell ...
! **************************************************************************************************
   SUBROUTINE get_iterator_info(iterator_set, mepos, &
                                ikind, jkind, nkind, ilist, nlist, inode, nnode, iatom, jatom, r, cell)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      INTEGER, OPTIONAL                                  :: mepos, ikind, jkind, nkind, ilist, &
                                                            nlist, inode, nnode, iatom, jatom
      REAL(dp), DIMENSION(3), OPTIONAL                   :: r
      INTEGER, DIMENSION(3), OPTIONAL                    :: cell

      INTEGER                                            :: me
      TYPE(neighbor_list_iterator_type), POINTER         :: iterator

      IF (SIZE(iterator_set) .NE. 1 .AND. .NOT. PRESENT(mepos)) &
         CPABORT("Parallel iterator calls must include 'mepos'")

      IF (PRESENT(mepos)) THEN
         me = mepos
      ELSE
         me = 0
      END IF
      iterator => iterator_set(me)%neighbor_list_iterator

      IF (PRESENT(ikind)) ikind = iterator%ikind
      IF (PRESENT(jkind)) jkind = iterator%jkind
      IF (PRESENT(nkind)) nkind = iterator%nkind
      IF (PRESENT(ilist)) ilist = iterator%ilist
      IF (PRESENT(nlist)) nlist = iterator%nlist
      IF (PRESENT(inode)) inode = iterator%inode
      IF (PRESENT(nnode)) nnode = iterator%nnode
      IF (PRESENT(iatom)) iatom = iterator%iatom
      IF (PRESENT(jatom)) jatom = iterator%jatom
      IF (PRESENT(r)) THEN
         CALL get_neighbor_node(neighbor_node=iterator%neighbor_node, r=r)
      END IF
      IF (PRESENT(cell)) THEN
         CALL get_neighbor_node(neighbor_node=iterator%neighbor_node, cell=cell)
      END IF

   END SUBROUTINE get_iterator_info

! **************************************************************************************************
!> \brief Captures the current state of the iterator in a neighbor_list_task_type
!> \param iterator_set the iterator / array of iterators (for multiple threads)
!> \param task the task structure which is returned
!> \param mepos OpenMP thread index
! **************************************************************************************************
   SUBROUTINE get_iterator_task(iterator_set, task, mepos)
      TYPE(neighbor_list_iterator_p_type), &
         DIMENSION(:), POINTER                           :: iterator_set
      TYPE(neighbor_list_task_type), INTENT(OUT)         :: task
      INTEGER, OPTIONAL                                  :: mepos

      IF (PRESENT(mepos)) THEN
         CALL get_iterator_info(iterator_set, mepos=mepos, ikind=task%ikind, jkind=task%jkind, &
                                nkind=task%nkind, &
                                ilist=task%ilist, nlist=task%nlist, &
                                inode=task%inode, nnode=task%nnode, &
                                iatom=task%iatom, jatom=task%jatom, &
                                r=task%r, cell=task%cell)
      ELSE
         CALL get_iterator_info(iterator_set, ikind=task%ikind, jkind=task%jkind, &
                                nkind=task%nkind, &
                                ilist=task%ilist, nlist=task%nlist, &
                                inode=task%inode, nnode=task%nnode, &
                                iatom=task%iatom, jatom=task%jatom, &
                                r=task%r, cell=task%cell)
      END IF

      NULLIFY (task%next)

   END SUBROUTINE get_iterator_task

! **************************************************************************************************
!> \brief   Add a new neighbor list to a neighbor list set.
!> \param neighbor_list_set ...
!> \param atom ...
!> \param neighbor_list ...
!> \date    13.09.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE add_neighbor_list(neighbor_list_set, atom, neighbor_list)

      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set
      INTEGER, INTENT(IN)                                :: atom
      TYPE(neighbor_list_type), POINTER                  :: neighbor_list

      TYPE(neighbor_list_type), POINTER                  :: new_neighbor_list

      IF (ASSOCIATED(neighbor_list_set)) THEN

         IF (ASSOCIATED(neighbor_list_set%last_neighbor_list)) THEN

            new_neighbor_list => &
               neighbor_list_set%last_neighbor_list%next_neighbor_list

            IF (.NOT. ASSOCIATED(new_neighbor_list)) THEN

!         *** Allocate a new neighbor list ***

               ALLOCATE (new_neighbor_list)

               NULLIFY (new_neighbor_list%next_neighbor_list)
               NULLIFY (new_neighbor_list%first_neighbor_node)

!         *** Link the new neighbor list to the neighbor list set ***

               neighbor_list_set%last_neighbor_list%next_neighbor_list => new_neighbor_list

            END IF

         ELSE

            new_neighbor_list => neighbor_list_set%first_neighbor_list

            IF (.NOT. ASSOCIATED(new_neighbor_list)) THEN

!         *** Allocate a new first neighbor list ***

               ALLOCATE (new_neighbor_list)

               NULLIFY (new_neighbor_list%next_neighbor_list)
               NULLIFY (new_neighbor_list%first_neighbor_node)

!         *** Link the new first neighbor list to the neighbor list set ***

               neighbor_list_set%first_neighbor_list => new_neighbor_list

            END IF

         END IF

!     *** Store the data set of the new neighbor list ***

         NULLIFY (new_neighbor_list%last_neighbor_node)
         new_neighbor_list%atom = atom
         new_neighbor_list%nnode = 0

!     *** Update the pointer to the last neighbor ***
!     *** list of the neighbor list set           ***

         neighbor_list_set%last_neighbor_list => new_neighbor_list

!     *** Increment the neighbor list counter ***

         neighbor_list_set%nlist = neighbor_list_set%nlist + 1

!     *** Return a pointer to the new neighbor list ***

         neighbor_list => new_neighbor_list

      ELSE

         CPABORT("The requested neighbor list set is not associated")

      END IF

   END SUBROUTINE add_neighbor_list

! **************************************************************************************************
!> \brief   Add a new neighbor list node to a neighbor list.
!> \param neighbor_list ...
!> \param neighbor ...
!> \param cell ...
!> \param r ...
!> \param exclusion_list ...
!> \param nkind ...
!> \date    23.06.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE add_neighbor_node(neighbor_list, neighbor, cell, r, exclusion_list, nkind)

      TYPE(neighbor_list_type), POINTER                  :: neighbor_list
      INTEGER, INTENT(IN)                                :: neighbor
      INTEGER, DIMENSION(3), INTENT(IN)                  :: cell
      REAL(dp), DIMENSION(3), INTENT(IN)                 :: r
      INTEGER, DIMENSION(:), OPTIONAL, POINTER           :: exclusion_list
      INTEGER, INTENT(IN), OPTIONAL                      :: nkind

      INTEGER                                            :: iatom, my_nkind
      TYPE(neighbor_node_type), POINTER                  :: new_neighbor_node

      IF (ASSOCIATED(neighbor_list)) THEN

!     *** Check for exclusions ***

         IF (PRESENT(exclusion_list)) THEN
            IF (ASSOCIATED(exclusion_list)) THEN
               DO iatom = 1, SIZE(exclusion_list)
                  IF (exclusion_list(iatom) == 0) EXIT
                  IF (exclusion_list(iatom) == neighbor) RETURN
               END DO
            END IF
         END IF

         my_nkind = 0
         IF (PRESENT(nkind)) my_nkind = nkind

         IF (ASSOCIATED(neighbor_list%last_neighbor_node)) THEN

            new_neighbor_node => neighbor_list%last_neighbor_node%next_neighbor_node

            IF (.NOT. ASSOCIATED(new_neighbor_node)) THEN

!         *** Allocate a new neighbor node ***

               ALLOCATE (new_neighbor_node)

               NULLIFY (new_neighbor_node%next_neighbor_node)

!         *** Link the new neighbor node to the neighbor list ***

               neighbor_list%last_neighbor_node%next_neighbor_node => new_neighbor_node

            END IF

         ELSE

            new_neighbor_node => neighbor_list%first_neighbor_node

            IF (.NOT. ASSOCIATED(new_neighbor_node)) THEN

!         *** Allocate a new first neighbor node ***

               ALLOCATE (new_neighbor_node)

               NULLIFY (new_neighbor_node%next_neighbor_node)

!         *** Link the new first neighbor node to the neighbor list ***

               neighbor_list%first_neighbor_node => new_neighbor_node

            END IF

         END IF

!     *** Store the data set of the new neighbor ***

         new_neighbor_node%neighbor = neighbor
         new_neighbor_node%cell(:) = cell(:)
         new_neighbor_node%r(:) = r(:)

!     *** Update the pointer to the last neighbor node of the neighbor list ***

         neighbor_list%last_neighbor_node => new_neighbor_node

!     *** Increment the neighbor node counter ***

         neighbor_list%nnode = neighbor_list%nnode + 1

      ELSE

         CPABORT("The requested neighbor list is not associated")

      END IF

   END SUBROUTINE add_neighbor_node

! **************************************************************************************************
!> \brief   Allocate and initialize a set of neighbor lists.
!> \param neighbor_list_set ...
!> \param symmetric ...
!> \date    23.06.2000
!> \author MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE allocate_neighbor_list_set(neighbor_list_set, symmetric)

      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set
      LOGICAL, INTENT(IN)                                :: symmetric

!   *** Deallocate the old neighbor list set ***

      IF (ASSOCIATED(neighbor_list_set)) THEN
         CALL deallocate_neighbor_list_set(neighbor_list_set)
      END IF

!   *** Allocate a set of neighbor lists ***

      ALLOCATE (neighbor_list_set)

      NULLIFY (neighbor_list_set%first_neighbor_list)

!   *** Initialize the pointers to the first neighbor list ***

      CALL init_neighbor_list_set(neighbor_list_set, symmetric)

   END SUBROUTINE allocate_neighbor_list_set

! **************************************************************************************************
!> \brief   Deallocate a neighbor list.
!> \param neighbor_list ...
!> \date    20.09.2002
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE deallocate_neighbor_list(neighbor_list)

      TYPE(neighbor_list_type), POINTER                  :: neighbor_list

      TYPE(neighbor_node_type), POINTER                  :: neighbor_node, next_neighbor_node

      IF (ASSOCIATED(neighbor_list)) THEN

         neighbor_node => neighbor_list%first_neighbor_node

         DO WHILE (ASSOCIATED(neighbor_node))
            next_neighbor_node => neighbor_node%next_neighbor_node
            DEALLOCATE (neighbor_node)
            neighbor_node => next_neighbor_node
         END DO

         DEALLOCATE (neighbor_list)

      END IF

   END SUBROUTINE deallocate_neighbor_list

! **************************************************************************************************
!> \brief   Deallocate a neighbor list set.
!> \param neighbor_list_set ...
!> \date    03.11.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE deallocate_neighbor_list_set(neighbor_list_set)
      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set

      TYPE(neighbor_list_type), POINTER                  :: neighbor_list, next_neighbor_list

      IF (ASSOCIATED(neighbor_list_set)) THEN

         neighbor_list => neighbor_list_set%first_neighbor_list

         DO WHILE (ASSOCIATED(neighbor_list))
            next_neighbor_list => neighbor_list%next_neighbor_list
            CALL deallocate_neighbor_list(neighbor_list)
            neighbor_list => next_neighbor_list
         END DO

         DEALLOCATE (neighbor_list_set)

      END IF

   END SUBROUTINE deallocate_neighbor_list_set

! **************************************************************************************************
!> \brief   Return a pointer to the first neighbor list of a neighbor list set.
!> \param neighbor_list_set ...
!> \return ...
!> \date    13.09.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   FUNCTION first_list(neighbor_list_set) RESULT(first_neighbor_list)

      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set
      TYPE(neighbor_list_type), POINTER                  :: first_neighbor_list

      first_neighbor_list => neighbor_list_set%first_neighbor_list

   END FUNCTION first_list

! **************************************************************************************************
!> \brief   Return a pointer to the first neighbor node of a neighbor list.
!> \param neighbor_list ...
!> \return ...
!> \date    23.06.2000,
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   FUNCTION first_node(neighbor_list) RESULT(first_neighbor_node)

      TYPE(neighbor_list_type), POINTER                  :: neighbor_list
      TYPE(neighbor_node_type), POINTER                  :: first_neighbor_node

      first_neighbor_node => neighbor_list%first_neighbor_node

   END FUNCTION first_node

! **************************************************************************************************
!> \brief   Return the requested data of a neighbor list.
!> \param neighbor_list ...
!> \param atom ...
!> \param nnode ...
!> \date    13.09.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE get_neighbor_list(neighbor_list, atom, nnode)

      TYPE(neighbor_list_type), POINTER                  :: neighbor_list
      INTEGER, INTENT(OUT), OPTIONAL                     :: atom, nnode

      IF (ASSOCIATED(neighbor_list)) THEN

         IF (PRESENT(atom)) atom = neighbor_list%atom
         IF (PRESENT(nnode)) nnode = neighbor_list%nnode

      ELSE

         CPABORT("The requested neighbor list is not associated")

      END IF

   END SUBROUTINE get_neighbor_list

! **************************************************************************************************
!> \brief   Return the components of a neighbor list set.
!> \param neighbor_list_set ...
!> \param nlist ...
!> \param symmetric ...
!> \date    10.11.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE get_neighbor_list_set(neighbor_list_set, nlist, symmetric)

      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set
      INTEGER, INTENT(OUT), OPTIONAL                     :: nlist
      LOGICAL, INTENT(OUT), OPTIONAL                     :: symmetric

      IF (ASSOCIATED(neighbor_list_set)) THEN

         IF (PRESENT(nlist)) nlist = neighbor_list_set%nlist
         IF (PRESENT(symmetric)) symmetric = neighbor_list_set%symmetric

      ELSE

         CPABORT("The requested neighbor list set is not associated")

      END IF

   END SUBROUTINE get_neighbor_list_set

! **************************************************************************************************
!> \brief   Return the components of the first neighbor list set.
!> \param neighbor_list_sets ...
!> \param nlist ...
!> \param symmetric ...
!> \date    07.2014
!> \author  JGH
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE get_neighbor_list_set_p(neighbor_list_sets, nlist, symmetric)

      TYPE(neighbor_list_set_p_type), DIMENSION(:), &
         POINTER                                         :: neighbor_list_sets
      INTEGER, INTENT(OUT), OPTIONAL                     :: nlist
      LOGICAL, INTENT(OUT), OPTIONAL                     :: symmetric

      INTEGER                                            :: i
      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set

      IF (ASSOCIATED(neighbor_list_sets)) THEN

         NULLIFY (neighbor_list_set)
         DO i = 1, SIZE(neighbor_list_sets)
            neighbor_list_set => neighbor_list_sets(i)%neighbor_list_set
            IF (ASSOCIATED(neighbor_list_set)) EXIT
         END DO

         IF (ASSOCIATED(neighbor_list_set)) THEN
            IF (PRESENT(nlist)) nlist = neighbor_list_set%nlist
            IF (PRESENT(symmetric)) symmetric = neighbor_list_set%symmetric
         ELSE
            CALL cp_abort(__LOCATION__, "No neighbor list set is associated. "// &
                          "Did you specify *all* required basis-sets, eg. for ADMM?")
         END IF

      ELSE

         CPABORT("The requested neighbor list sets are not associated")

      END IF

   END SUBROUTINE get_neighbor_list_set_p

! **************************************************************************************************
!> \brief   Return the requested data of a neighbor node.
!> \param neighbor_node ...
!> \param neighbor ...
!> \param cell ...
!> \param r ...
!> \date    23.06.2000
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE get_neighbor_node(neighbor_node, neighbor, cell, r)

      TYPE(neighbor_node_type), POINTER                  :: neighbor_node
      INTEGER, INTENT(OUT), OPTIONAL                     :: neighbor
      INTEGER, DIMENSION(3), INTENT(OUT), OPTIONAL       :: cell
      REAL(dp), DIMENSION(3), INTENT(OUT), OPTIONAL      :: r

      IF (ASSOCIATED(neighbor_node)) THEN

         IF (PRESENT(neighbor)) neighbor = neighbor_node%neighbor
         IF (PRESENT(r)) r(:) = neighbor_node%r(:)
         IF (PRESENT(cell)) cell(:) = neighbor_node%cell(:)

      ELSE

         CPABORT("The requested neighbor node is not associated")

      END IF

   END SUBROUTINE get_neighbor_node

! **************************************************************************************************
!> \brief Initialize a neighbor list set. Nothing is (de)allocated here.
!>         This routine is also used to prepare a neighbor list set for
!>         overwriting.
!> \param neighbor_list_set ...
!> \param symmetric ...
!> \date  20.09.2002
!> \author  MK
!> \version 1.0
! **************************************************************************************************
   SUBROUTINE init_neighbor_list_set(neighbor_list_set, symmetric)

      TYPE(neighbor_list_set_type), POINTER              :: neighbor_list_set
      LOGICAL, INTENT(IN)                                :: symmetric

      IF (ASSOCIATED(neighbor_list_set)) THEN

         ! *** Initialize the pointers to the last neighbor list ***
         NULLIFY (neighbor_list_set%last_neighbor_list)

         ! *** Initialize the neighbor list counter ***
         neighbor_list_set%nlist = 0

         ! *** Initialize the neighbor list build properties
         neighbor_list_set%symmetric = symmetric

      ELSE

         CPABORT("The requested neighbor list set is not associated")

      END IF

   END SUBROUTINE init_neighbor_list_set

! **************************************************************************************************
!> \brief releases an array of neighbor_list_sets
!> \param nlists ...
!> \author Ole Schuett
! **************************************************************************************************
   SUBROUTINE release_neighbor_list_sets(nlists)
      TYPE(neighbor_list_set_p_type), DIMENSION(:), &
         POINTER                                         :: nlists

      INTEGER                                            :: i

      IF (ASSOCIATED(nlists)) THEN
         DO i = 1, SIZE(nlists)
            CALL deallocate_neighbor_list_set(nlists(i)%neighbor_list_set)
         END DO
         IF (ASSOCIATED(nlists(1)%nlist_task)) THEN
            DEALLOCATE (nlists(1)%nlist_task)
         END IF
         DEALLOCATE (nlists)
      END IF
   END SUBROUTINE release_neighbor_list_sets

END MODULE qs_neighbor_list_types
