#167: add first problem for c++ to doc

docs/source/endtoend_cpp.rst

@@ -1,6 +1,385 @@
 Step-by-step in C++
 ===================
 
-This page shows you how to setup, run and visualize your first problem in C++.
+This page shows you how to setup, build, run and visualize your first problem in C++.
 
-**finish**
+For demonstration purposes, we show how to run the **1D Sod** problem,
+but the *same* process applies to *every* other problem.
+
+.. Hint::
+   You can copy each snippet below by moving your mouse over
+   the snippet's box, and clicking the copy icon
+   appearing on the top-right corner.
+
+Step 1: Prepare
+---------------
+
+The C++ library is header-only so it does not need to be compiled and installed.
+
+.. code-block:: shell
+
+    export CXX=<path-to-your-CXX-compiler> # must support C++17
+    export MYTEST=/home/myDemoTest
+    mkdir $MYTEST && cd $MYTEST
+    git clone --recursive [email protected]:Pressio/pressio-demoapps.git
+    cd pressio-demoapps
+
+.. _step-2:
+
+Step 2: Generate the mesh
+-------------------------
+
+.. code-block:: shell
+
+   python3 $MYTEST/pressio-demoapps/meshing_scripts/create_full_mesh_for.py \
+           --problem sod1d_s7 --outdir ${MYTEST}/mesh -n 500
+
+where ``sod1d_s7`` specifies that we want Sod1d and
+a 7-point stencil, and ``500`` is the number of cells.
+The mesh files are generated inside ``${MYTEST}/mesh``.
+
+Step 3: Create Main File
+------------------------
+
+Create a ``main.cpp`` as follows:
+
+.. code-block:: shell
+
+    touch $MYTEST/main.cpp
+
+and open it in a text editor.
+
+The next sections explain the program line-by-line. To jump to the complete, uncommented code,
+see :ref:`complete-file-label`.
+
+Includes
+^^^^^^^^
+
+First, we need to include Pressio's explicit steppers. These are abstractions that
+represent "how" to take a step when applying an explicit scheme to initial value problems.
+
+You can read more about them in the `Pressio documentation <https://pressio.github.io/pressio/components/ode_steppers_explicit.html>`_.
+
+.. code-block:: cpp
+
+    #include "pressio/ode_steppers_explicit.hpp"
+
+Next, we need functions to advance our stepper object forward in time.
+
+.. code-block:: cpp
+
+    #include "pressio/ode_advancers.hpp"
+
+Finally, we include the default 1D Euler problem that we will solve (more information on
+the problem can be found `here <https://pressio.github.io/pressio-demoapps/euler_1d_sod.html>`_).
+
+.. code-block:: cpp
+
+    #include "pressiodemoapps/euler1d.hpp"
+
+Observer class to monitor and sample the simulation's state
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In order to write out the state with a specified frequency,
+we need to create an instance of the observer class.
+This can be implemented with the following code:
+
+.. code-block:: cpp
+
+    template <typename StateType>
+    class Observer
+    {
+        public:
+        Observer(const std::string & f0, int freq)
+            : myfile0_(f0,  std::ios::out | std::ios::binary),
+            sampleFreq_(freq){}
+
+        ~Observer(){
+            myfile0_.close();
+        }
+
+        template<typename TimeType>
+        void operator()(const pressio::ode::StepCount stepIn,
+                const TimeType /*currentTime, unused*/,
+                const StateType & state)
+        {
+            const auto step = stepIn.get();
+            if (step % sampleFreq_ == 0){
+		const std::size_t ext = state.size()*sizeof(double);
+		myfile0_.write(reinterpret_cast<const char*>(&state(0)), ext);
+            }
+        }
+
+        private:
+	    std::ofstream myfile0_;
+	    int sampleFreq_ = {};
+    };
+
+main()
+^^^^^^
+
+Create your main function:
+
+.. code-block:: cpp
+
+    int main() {}
+
+.. Note:: All of the subsequent code will go inside of the ``main()`` function.
+
+First, we'll create an alias to simplify our program.
+
+.. code-block:: cpp
+
+    namespace pda = pressiodemoapps;
+
+Then we load the mesh that we created in :ref:`step-2`.
+
+.. code-block:: cpp
+
+    const auto meshObj = pda::load_cellcentered_uniform_mesh_eigen(".");
+
+.. Note:: ``"."`` (above) assumes that ``mesh/`` is located in the same directory as ``main.cpp`` (this should be your ``$MYTEST`` directory).
+
+For this demonstration, we will use First-Order Inviscid Flux Reconstruction.
+
+.. code-block:: cpp
+
+    constexpr auto order = pda::InviscidFluxReconstruction::FirstOrder;
+
+Now we create the simulation problem for Euler 1D equations and initialize the state.
+
+.. code-block:: cpp
+
+    auto appObj = pda::create_problem_eigen(meshObj, pda::Euler1d::Sod, order);
+    using app_t = decltype(appObj);
+    using state_t = typename app_t::state_type;
+    state_t state = appObj.initialCondition();
+
+We use built-in time stepping with Runge-Kutta4 and instantiate our Observer class
+such that the state is observed and saved to file every step.
+
+.. code-block:: cpp
+
+    auto stepperObj = pressio::ode::create_rk4_stepper(appObj);
+    const int observeEveryNSteps = 1;
+    Observer<state_t> Obs("sod1d_solution.bin", observeEveryNSteps);
+
+Then we set our simulation parameters, where ``dt`` is the time step size and
+the total number of steps is given by ``Nsteps``.
+
+.. code-block:: cpp
+
+    const auto dt = 0.001;
+    const auto Nsteps = pressio::ode::StepCount(100);
+
+Then we advance the simulation forward ``Nsteps``.
+
+.. code-block:: cpp
+
+    pressio::ode::advance_n_steps(stepperObj, state, 0., dt, Nsteps, Obs);
+
+Finally, we return ``0`` if the process succeeds.
+
+.. code-block:: cpp
+
+    return 0;
+
+.. _complete-file-label:
+
+Complete File
+^^^^^^^^^^^^^
+
+The full, uncommented file is:
+
+.. code-block:: cpp
+
+    #include "pressio/ode_steppers_explicit.hpp"
+    #include "pressio/ode_advancers.hpp"
+    #include "pressiodemoapps/euler1d.hpp"
+
+    template <typename StateType>
+    class Observer
+    {
+        public:
+        Observer(const std::string & f0, int freq)
+            : myfile0_(f0,  std::ios::out | std::ios::binary),
+            sampleFreq_(freq){}
+
+        ~Observer(){
+            myfile0_.close();
+        }
+
+        template<typename TimeType>
+        void operator()(const pressio::ode::StepCount stepIn,
+                const TimeType /*currentTime, unused*/,
+                const StateType & state)
+        {
+            const auto step = stepIn.get();
+            if (step % sampleFreq_ == 0){
+            const std::size_t ext = state.size()*sizeof(double);
+            myfile0_.write(reinterpret_cast<const char*>(&state(0)), ext);
+            }
+        }
+
+        private:
+        std::ofstream myfile0_;
+        int sampleFreq_ = {};
+    };
+
+    int main()
+    {
+        namespace pda = pressiodemoapps;
+        const auto meshObj = pda::load_cellcentered_uniform_mesh_eigen(".");
+
+        constexpr auto order = pda::InviscidFluxReconstruction::FirstOrder;
+
+        auto appObj = pda::create_problem_eigen(meshObj, pda::Euler1d::Sod, order);
+        using app_t = decltype(appObj);
+        using state_t = typename app_t::state_type;
+        state_t state = appObj.initialCondition();
+
+        auto stepperObj = pressio::ode::create_rk4_stepper(appObj);
+	const int observeEveryNSteps = 1;
+	Observer<state_t> Obs("sod1d_solution.bin", observeEveryNSteps);
+
+        const auto dt = 0.001;
+        const auto Nsteps = pressio::ode::StepCount(100);
+        pressio::ode::advance_n_steps(stepperObj, state, 0., dt, Nsteps, Obs);
+
+        return 0;
+    }
+
+Step 4: Compile and Run
+-----------------------
+
+You can compile ``main.cpp`` either on the command line or using ``cmake``.
+
+Command line
+^^^^^^^^^^^^
+
+.. code-block:: shell
+
+    $CXX \
+    -I $MYTEST/pressio-demoapps/include \
+    -I $MYTEST/pressio-demoapps/tpls/eigen3 \
+    -I $MYTEST/pressio-demoapps/tests_cpp/pressio/include \
+    main.cpp -o main
+
+..
+   CMake
+   ^^^^^
+
+   Create a ``CMakeLists.txt`` file at the same level as ``main.cpp``:
+
+   .. code-block:: shell
+
+       touch CMakeLists.txt
+
+   Then copy/paste the following into your ``CMakeLists.txt`` file.
+
+   .. code-block:: cmake
+
+       cmake_minimum_required(VERSION 3.10)
+       project(PressioCppDemo)
+
+       set(CMAKE_CXX_STANDARD 17)
+       set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+       include_directories("${MYTEST}/pressio-demoapps/include")
+       include_directories("${MYTEST}/pressio-demoapps/tpls/eigen3")
+       include_directories("${MYTEST}/pressio-demoapps/tests_cpp/pressio/include")
+
+       add_executable(main main.cpp)
+
+..
+   From your ``${MYTEST}`` directory, run:
+
+   .. code-block:: shell
+
+       cmake .
+       make
+
+Running
+^^^^^^^
+
+Once you have compiled your code, you will have a new ``main`` executable in ``$MYTEST``.
+To execute it, simply run:
+
+.. code-block:: shell
+
+    ./main
+
+Step 5: Visualize Results
+-------------------------
+
+Running ``main`` will produce a binary file ``sod1d_solution.bin`` that holds the state
+at every timestep.
+
+To visualize the results, you can use the following steps.
+
+1. Create a Python file, ``plot.py``, in the ``$MYTEST`` directory
+   and fill its content with:
+
+.. code-block:: python
+
+    #!/usr/bin/env python3
+
+    import matplotlib.pyplot as plt
+    from matplotlib import cm
+    import numpy as np
+    from numpy import linalg as LA
+    import re
+
+    def extractN(ns):
+	reg = re.compile(r''+ns+'.+')
+	file1 = open('mesh/info.dat', 'r')
+	strings = re.search(reg, file1.read())
+	file1.close()
+	assert(strings)
+	return int(strings.group().split()[1])
+
+    ##########################
+    if __name__== "__main__":
+    ##########################
+        nx = extractN('nx')
+        print(nx)
+        fomTotDofs = nx*3
+
+        x = np.loadtxt('mesh/coordinates.dat', dtype=float)[:,1]
+
+        data = np.fromfile("sod1d_solution.bin")
+        nt = int(np.size(data)/fomTotDofs)
+        print("fomTest: nt = ", nt)
+        data = np.reshape(data, (nt, fomTotDofs))
+
+        fig = plt.figure(1)
+        density_t0 = np.reshape(data[0,:], (nx, 3))[:,0]
+        density_thalf = np.reshape(data[int(nt/2),:], (nx, 3))[:,0]
+        density_tfinal = np.reshape(data[nt-1,:], (nx, 3))[:,0]
+        plt.plot(x, density_t0, '-r', label='density at t=0')
+        plt.plot(x, density_thalf, '-g', label='density at t=T/2')
+        plt.plot(x, density_tfinal, '-b', label='density at t=T')
+
+        plt.xlabel("x", fontsize=12)
+        plt.ylabel("Solution", fontsize=12)
+        plt.legend()
+        fig.savefig("solution.png", format="png", bbox_inches='tight', dpi=450)
+        plt.show()
+
+.. note:: You may have to specify the paths to ``info.data`` and ``coordinates.dat`` (which are in the the ``mesh`` directory that you generated in :ref:`step-2`).
+
+3. Run the script from your ``$MYTEST`` directory.
+
+.. note:: You may need to install some packages, like ``numpy`` or ``matplotlib``, into your Python environment.
+
+.. code-block:: shell
+
+    python visualize_state.py
+
+
+This should display the following figure:
+
+.. image:: ../../figures/doc_sod1d_endtoend_cpp.png
+  :width: 65 %
+  :align: center
+  :alt: Alternative text

docs/source/endtoend_py.rst

@@ -20,7 +20,7 @@ Let's make a directory to run things and export the C++ compiler:
 
 .. code-block:: shell
 
-  export CXX=<path-to-your-CXX-compiler> #must support C++14
+  export CXX=<path-to-your-CXX-compiler> #must support C++17
   export MYTEST=/home/myDemoTest
   mkdir $MYTEST && cd $MYTEST
   git clone --recursive [email protected]:Pressio/pressio-demoapps.git