Modified forcing template to read time slices only once

Until now, when running with a CF forcing file, CISM would read the appropriate
forcing time slice every model timestep, even if the forcing has not changed
since the previous model time step. This can slow down the code considerably
when repeatedly reading large forcing files.

With this commit, CISM reads in a new forcing time slice only if the previous
time slice is no longer current.

The new logic appears in subroutine glide_read_forcing, an autogenerated file
that uses ncdf_template.F90.in.  This commit modifies the template.

This commit is BFB; it simply avoids reading in the same data multiple times.

libglimmer/ncdf_template.F90.in

@@ -398,14 +398,15 @@ contains
     ! Read data from forcing files
     use glimmer_log
     use glide_types
+    use parallel, only: main_task
 
     implicit none
     type(DATATYPE) :: data
     type(glide_global_type), intent(inout) :: model
 
     ! Locals
     type(glimmer_nc_input), pointer :: ic
-    integer :: t
+    integer :: t, t_prev
     real(dp) :: eps ! a tolerance to use for stepwise constant forcing
 
     ! Make eps a fraction of the time step.
@@ -415,21 +416,41 @@ contains
     ic=>model%funits%frc_first
     do while(associated(ic))
 
-       !print *, 'possible forcing times', ic%times
+!       if (main_task) print *, 'possible forcing times', ic%times
 
        ic%nc%just_processed = .true. ! until we find an acceptable time, set this to true which will prevent the file from being read.
+
+!       if (main_task) print*, 'In glide_read_forcing, model time + eps =', model%numerics%time + eps
+
+       ! Find the time index associated with the previous model time step
+       t_prev = 0
+       do t = ic%nt, 1, -1  ! look through the time array backwards
+          if ( ic%times(t) <= model%numerics%time - model%numerics%tinc + eps) then
+             t_prev = t
+!             if (main_task) print*, 'Previous time index =', t_prev
+             exit
+          end if
+       enddo
+
        ! Find the current time in the file
        do t = ic%nt, 1, -1  ! look through the time array backwards
           if ( ic%times(t) <= model%numerics%time + eps) then
-            ! use the largest time that is smaller or equal to the current time (stepwise forcing)
-
-            ! Set the desired time to be read
-            ic%current_time = t
-            ic%nc%just_processed = .false.  ! set this to false so file will be read.
-            !print *, 'time, forcing index, forcing time', model%numerics%time, ic%current_time, ic%times(ic%current_time)
-            exit  ! once we find the time, exit the loop
-          endif
-          end do  ! if we get to end of loop without exiting, then this file will not be read at this time.
+             ! use the largest time that is smaller or equal to the current time (stepwise forcing)
+!             if (main_task) print*, 'Largest time less than model time: t, times(t):', t, ic%times(t)
+
+             ! If this time index (t) is larger than the previous index (t_prev), then read a new time slice.
+             ! Otherwise, we already have the current slice, and there is nothing new to read.
+             if (t > t_prev) then
+                ! Set the desired time to be read
+                ic%current_time = t
+                ic%nc%just_processed = .false.  ! set this to false so file will be read.
+!                if (main_task) print*, 'Read new forcing slice: t, times(t) =', t, ic%times(t)
+             endif ! t > t_prev
+
+             exit  ! once we find the time, exit the loop
+          end if   ! ic%times(t) <= model%numerics%time + eps
+
+       end do  ! if we get to end of loop without exiting, then this file will not be read at this time
 
        ! move on to the next forcing file
        ic=>ic%next