Adding unit testing walkthrough.

Author: mwaxmonsky

docs/conversion/walkthrough.md

@@ -690,7 +690,164 @@ Bug gets extremely lucky (or maybe he's just really skilled) and there are no di
 Bug commits his changes to his fork/branch of the three repos: CAM, CAM-SIMA, atmospheric_physics.
 
 ### Unit testing
-Bug adds PfUnit tests for his newly-created scheme per the [atmospheric_physics development workflow](../atmospheric_physics/development_workflow.md#5-unit-testing).
+
+Lets say you added a utility function in `physics_tendency_updaters.F90
+` to apply the pressure tendency of the atmosphere:
+
+```fortran
+subroutine apply_tendency_of_air_pressure_run(nz, p_tend, pressure, dPdt_total,   &
+    dt, errcode, errmsg)
+  ! Dummy arguments
+  integer,            intent(in)    :: nz              ! Num vertical  layers
+  real(kind_phys),    intent(in)    :: p_tend(:,:)     ! pressure tendency
+  real(kind_phys),    intent(inout) :: pressure(:,:)       ! air pressure
+  real(kind_phys),    intent(inout) :: dPdt_total(:,:) ! total temp. tendency
+  real(kind_phys),    intent(in)    :: dt              ! physics time step
+  integer,            intent(out)   :: errcode
+  character(len=512), intent(out)   :: errmsg
+
+  ! Local variable
+  integer :: klev
+
+  errcode = 0
+  errmsg = ''
+
+  do klev = 1, nz
+      pressure(:, klev) = pressure(:, klev) + (p_tend(:, klev) * dt)
+      dPdt_total(:, klev) = dPdt_total(:, klev) + p_tend(:, klev)
+  end do
+
+end subroutine apply_tendency_of_air_pressure_run
+```
+
+Then you would need to check that the file you've modified is being built by the tests.  You can check this in `test/unit-test/CMakeLists.txt` and see that we have:
+
+```cmake
+set(UTILITIES_SRC
+    ../../schemes/utilities/state_converters.F90
+    ../../schemes/utilities/static_energy.F90
+    ../../schemes/utilities/physics_tendency_updaters.F90
+    include/ccpp_kinds.F90
+)
+```
+
+Since the file is being built, we don't have to add to this list.
+
+Next we look for `test/unit-test/tests/utilities/CMakeLists.txt` to see if there is a matching test file:
+
+```sh
+$ ls cat test/unit-test/tests/utilities/
+CMakeLists.txt
+test_state_converters.pf
+$
+$ cat test/unit-test/tests/utilities/CMakeLists.txt
+add_pfunit_ctest(utilities_tests
+  TEST_SOURCES test_state_converters.pf
+  LINK_LIBRARIES utilities
+)
+$
+```
+
+Because there is no matching file or test, we need to create it:
+
+```sh
+$ cat > test/unit-test/tests/utilities/test_physics_tendency_updaters.pf << EOF
+@test
+subroutine test_pressure_tendency_update()
+  use funit
+  use physics_tendency_updaters
+
+end subroutine test_pressure_tendency_update
+EOF
+```
+
+And add the ability to build/run it from the test harness by updating `test/unit-test/tests/utilities/CMakeLists.txt`:
+
+```
+add_pfunit_ctest(utilities_tests
+  TEST_SOURCES test_state_converters.pf test_physics_tendency_updaters.pf
+  LINK_LIBRARIES utilities
+```
+where `test_physics_tendency_updaters.pf` has been added to the `TEST_SOURCES` list.
+
+Now, to actually test the function, we can try something like:
+
+```fortran
+@test
+subroutine test_pressure_tendency_update()
+  use funit
+  use ccpp_kinds, only: kind_phys
+  use physics_tendency_updaters
+
+  # Declare and setup test data with known results
+
+  integer, parameter :: ncol = 3            ! Num columns
+  integer, parameter :: nz = 3              ! Num vertical layers
+  integer, parameter :: errcode = 0
+  character(len=512), parameter :: errmsg = ''
+
+  real(kind_phys) :: p_tend(ncol, nz)    ! pressure tendency
+  real(kind_phys) :: pressure(ncol, nz)  ! air pressure
+  real(kind_phys) :: dPdt_total(nol, nz) ! total temp. tendency
+  real(kind_phys) :: dt                  ! physics time step
+
+  pressure = 1
+  p_tend = 1
+  dPdt_total = 0
+  dt = 1
+
+  call apply_tendency_of_air_pressure_run(nz, p_tend, pressure, &
+                                          dPdt_total, dt,       &
+                                          errcode, errmsg)
+
+  @assertEqual(2, pressure)
+  @assertEqual(1, dPdt_total)
+  @assertEqual(0, errcode)
+  @assertEqual('', errmsg)
+end subroutine test_pressure_tendency_update
+```
+
+The values should be scientifically relevant to test a valid physics case or be set up to test edge cases that the subroutine must support.
+
+Notice that we are only testing `intent(out)` and `intent(inout)` as these are the only values that can change.
+
+Assuming you have built the test framework according to the [instructions](../atmospheric_physics/development_workflow.md#5-unit-testing), you can now run:
+
+```bash
+$ cmake \
+  -DCMAKE_PREFIX_PATH=<PATH_TO_PFUNIT>/build/installed \
+  -DATMOSPHERIC_PHYSICS_ENABLE_CODE_COVERAGE=ON \
+  -B./build \
+  -S./test/unit-test
+$ cd build
+$ make
+$ ctest -V --output-on-failure
+```
+
+After which you should see output similar to:
+
+```
+1:  Start: <test_state_converters_suite.test_temp_to_potential_temp>
+1: .   end: <test_state_converters_suite.test_temp_to_potential_temp>
+1:
+1: Time:         0.000 seconds
+1:
+1:  OK
+1:  (1 test)
+1:
+2:  Start: <test_physics_tendency_updaters_suite.test_pressure_tendency_update>
+2: .   end: <test_physics_tendency_updaters_suite.test_pressure_tendency_update>
+2:
+2: Time:         0.000 seconds
+2:
+2:  OK
+2:  (1 test)
+2/2 Test #1: utilities_tests ..................   Passed    0.00 sec
+
+100% tests passed, 0 tests failed out of 1
+```
+
+If 100% of the tests pass, you may proceed to the next section.
 
 ### Pull Requests
 - He opens a PR into atmospheric_physics (target: `development` branch), goes through the review process, updates the NamesNotInDictionary.txt file, and then commits the PR when approvals are received. He then opens a PR from `development` to `main` and makes a tag (incrementing the minor version) when that is merged.