int64_radix_sort Module Subroutine

   pure module subroutine int64_radix_sort(array, work, reverse)
      integer(kind=int64), dimension(:), intent(inout) :: array
      integer(kind=int64), dimension(:), intent(inout), target, optional :: work
      logical, intent(in), optional :: reverse
      integer(kind=int_index) :: i, N, start, middle, end
      integer(kind=int64), dimension(:), pointer :: buffer
      integer(kind=int64) :: item
      logical :: use_internal_buffer
      N = size(array, kind=int_index)
      if (present(work)) then
         if (size(work, kind=int_index) < N) then
            error stop "int64_radix_sort: work array is too small."
         end if
         use_internal_buffer = .false.
         buffer => work
      else
         use_internal_buffer = .true.
         allocate (buffer(N))
      end if
      call radix_sort_u64_helper(N, array, buffer)
      if (array(1) >= 0 .and. array(N) < 0) then
         start = 1
         end = N
         middle = (1 + N)/2
         do while (.true.)
            if (array(middle) >= 0) then
               start = middle + 1
            else
               end = middle
            end if
            middle = (start + end)/2
            if (start == end) exit
         end do
         buffer(1:(N - middle + 1)) = array(middle:N)
         buffer(N - middle + 2:N) = array(1:middle - 1)
         array(:) = buffer(:)
      end if
      if (pic_optional(reverse, .false.)) then
         do i = 1, N/2
            item = array(i)
            array(i) = array(N - i + 1)
            array(N - i + 1) = item
         end do
      end if
      if (use_internal_buffer) then
         deallocate (buffer)
      end if
   end subroutine int64_radix_sort