IB markers: index each save by the save count under CFL time stepping

Under cfl_dt the caller passes the save count and t_step_save is left at its
default, so t_step/t_step_save collapsed saves 1-99 onto one slot in
lustre_ib.dat. Share one f_save_index between writer and post_process reader.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sbryngelson

src/common/m_helper_basic.fpp

@@ -15,7 +15,7 @@ module m_helper_basic
 
     private
     public :: f_approx_equal, f_approx_in_array, f_is_default, f_all_default, f_is_integer, s_configure_coordinate_bounds, &
-        & s_update_cell_bounds
+        & s_update_cell_bounds, f_save_index
 
 contains
 
@@ -109,6 +109,17 @@ contains
 
     end function f_is_integer
 
+    !> Index of the save written at t_step: t_step itself under CFL-based stepping (the caller passes the save count), else
+    !! t_step/t_step_save.
+    pure integer function f_save_index(t_step, cfl_mode, t_step_save) result(idx)
+
+        integer, intent(in) :: t_step, t_step_save
+        logical, intent(in) :: cfl_mode
+
+        idx = merge(t_step, t_step/t_step_save, cfl_mode)
+
+    end function f_save_index
+
     !> Compute ghost-cell buffer size and set interior/buffered coordinate index bounds.
     subroutine s_configure_coordinate_bounds(recon_type, weno_polyn, muscl_polyn, igr_order, buff_size, idwint, idwbuff, viscous, &
         & bubbles_lagrange, m, n, p, num_dims, igr, ib, fd_number)

src/post_process/m_data_input.f90

@@ -17,6 +17,7 @@ module m_data_input
     use m_boundary_common
     use m_boundary_io
     use m_helper
+    use m_helper_basic, only: f_save_index
 
     implicit none
 
@@ -153,7 +154,7 @@ impure subroutine s_read_ib_data_files(file_loc_base, t_step)
                 p_MOK = int(p_glb + 1, MPI_OFFSET_KIND)
                 MOK = int(1._wp, MPI_OFFSET_KIND)
                 WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
-                save_index = t_step/t_step_save  ! get the number of saves done to this point
+                save_index = f_save_index(t_step, cfl_dt, t_step_save)
 
                 data_size = (m + 1)*(n + 1)*(p + 1)
                 var_MOK = int(sys_size + 1, MPI_OFFSET_KIND)

src/simulation/m_data_output.fpp

@@ -1103,7 +1103,7 @@ contains
             call s_check_mpi_file_open(ierr, file_loc)
 
             var_MOK = int(sys_size + 1, MPI_OFFSET_KIND)
-            disp = m_MOK*max(MOK, n_MOK)*max(MOK, p_MOK)*WP_MOK*(var_MOK - 1 + int(time_step/t_step_save))
+            disp = m_MOK*max(MOK, n_MOK)*max(MOK, p_MOK)*WP_MOK*(var_MOK - 1 + f_save_index(time_step, cfl_dt, t_step_save))
             if (time_step == 0) disp = 0
 
             call MPI_FILE_SET_VIEW(ifile, disp, MPI_INTEGER, MPI_IO_IB_DATA%view, 'native', mpi_info_int, ierr)