Added optional flag to time_interp_external to speedup initialization

 - time_interp_external_nml:check_uniform_times=.true. will enable
   this option which performs a time axis check for uniformly spaced
   time levels prior to attempting to read the entire axis. In some
   cases, this will significantly reduce initialization time when
   reading large input files via data_override for example.

time_interp/time_interp_external2.F90

@@ -80,6 +80,9 @@ module time_interp_external2_mod
   integer, private :: num_io_buffers = 2 ! set -1 to read all records from disk into memory
   logical, private :: module_initialized = .false.
   logical, private :: debug_this_module = .false.
+  logical, private :: check_uniform_times=.false. !< If true, check for uniform time axis
+                                          !! in all files in order to speedup initialization for
+                                          !! large files.
 
   public init_external_field, time_interp_external, time_interp_external_init, &
        time_interp_external_exit, get_external_field_size, get_time_axis, get_external_field_missing
@@ -183,7 +186,7 @@ subroutine time_interp_external_init()
       integer :: io_status, logunit, ierr
 
       namelist /time_interp_external_nml/ num_io_buffers, debug_this_module, &
-                                          max_fields, max_files
+                                          max_fields, max_files, check_uniform_times
 
       ! open and read namelist
 
@@ -285,11 +288,10 @@ function init_external_field(file,fieldname,domain,desired_units,&
       logical, optional                       :: ongrid !< Optional flag indicating if the data is ongrid
 
       logical :: ongrid_local !< Flag indicating if the data is ongrid
-
       integer :: init_external_field
 
-      real(r8_kind) :: slope, intercept
-      integer :: ndim,ntime,i,j
+      real(r8_kind) :: slope, intercept, dtime
+      integer :: ndim,ntime,i,j,n
       integer :: iscomp,iecomp,jscomp,jecomp,isglobal,ieglobal,jsglobal,jeglobal
       integer :: isdata,iedata,jsdata,jedata, dxsize, dysize,dxsize_max,dysize_max
       logical :: verb, transpose_xy,use_comp_domain1
@@ -323,7 +325,6 @@ function init_external_field(file,fieldname,domain,desired_units,&
       if (debug_this_module) verb = .true.
       numwindows = 1
       if(present(nwindows)) numwindows = nwindows
-
       units = 'same'
       if (PRESENT(desired_units)) then
           units = desired_units
@@ -384,11 +385,37 @@ function init_external_field(file,fieldname,domain,desired_units,&
       allocate(pes(mpp_npes()))
       call mpp_get_current_pelist(pes)
       allocate(tstamp(ntime),tstart(ntime),tend(ntime),tavg(ntime))
-
-      !< Only root reads the unlimited dimension and broadcasts it to the other ranks
-      if (mpp_root_pe() .eq. mpp_pe()) call read_data(fileobj, timename, tstamp)
-      call mpp_broadcast(tstamp, size(tstamp), mpp_root_pe(), pelist=pes)
-      deallocate(pes)
+      if (check_uniform_times .and. ntime .gt. 4) then
+         if (mpp_root_pe() .eq. mpp_pe()) &
+           call read_data(fileobj,timename,tstamp,corner=(/1/),edge_lengths=(/3/))
+         call mpp_broadcast(tstamp(1:3), 3, mpp_root_pe(), pelist=pes)
+         if((tstamp(3)-tstamp(2)).eq.(tstamp(2)-tstamp(1))) then
+           if (mpp_root_pe() .eq. mpp_pe()) &
+                call read_data(fileobj,timename,tstamp(ntime),unlim_dim_level=ntime)
+           call mpp_broadcast(tstamp(ntime:ntime), 1, mpp_root_pe(), pelist=pes)
+           dtime=tstamp(2)-tstamp(1)
+           do n=2,ntime-1
+             tstamp(n)=tstamp(n-1)+dtime
+           enddo
+           if (abs(tstamp(ntime)-tstamp(ntime-1)-dtime).gt.0.01*dtime) then
+             call mpp_error(WARNING,"init_external_field:"//&
+                  " Uniform time check failed. Reverting to slow initialization ")
+             if (mpp_root_pe() .eq. mpp_pe()) &
+                  call read_data(fileobj,timename,tstamp)
+             call mpp_broadcast(tstamp, size(tstamp), mpp_root_pe(), pelist=pes)
+           endif
+           deallocate(pes)
+         else
+           if (mpp_root_pe() .eq. mpp_pe()) call read_data(fileobj, timename, tstamp)
+           call mpp_broadcast(tstamp, size(tstamp), mpp_root_pe(), pelist=pes)
+           deallocate(pes)
+         endif
+      else
+        !< Only root reads the unlimited dimension and broadcasts it to the other ranks
+        if (mpp_root_pe() .eq. mpp_pe()) call read_data(fileobj, timename, tstamp)
+        call mpp_broadcast(tstamp, size(tstamp), mpp_root_pe(), pelist=pes)
+        deallocate(pes)
+      endif
 
       transpose_xy = .false.
       isdata=1; iedata=1; jsdata=1; jedata=1