Added section using pySDC as integrator within libs

Author: brownbaerchen

pySDC/playgrounds/PinT_Workshop_2025/2_Coupling_pySDC_to_libraries.ipynb

@@ -760,6 +760,34 @@
    "id": "7bc27a1f-87a5-456d-b07f-d1cff17f554a",
    "metadata": {},
    "source": [
+    "## Using pySDC as an integrator within another library\n",
+    "Once you have coupled a datatype to pySDC, you can implement pySDC as an integrator for libraries building on the same datatype.\n",
+    "For instance, we have implemented this for [Gusto](https://www.firedrakeproject.org/gusto/), which builds on Firedrake to assemble dynamical cores as used in numerical weather prediction.\n",
+    "The way you go about this is to look at the interface for other integrators in the library you want to couple to.\n",
+    "Chances are an integrator accepts some initial conditions and returns the solution at the end of the interval.\n",
+    "In this case, you need to assemble a pySDC controller to do what you want and then, in the library you want to couple to, pass the solution between pySDC and the library.\n",
+    "In code, this looks approximataly like so:\n",
+    "\n",
+    "```\n",
+    "class pySDC_integrator(my_library_integrator):\n",
+    "\n",
+    "    def __init__(self, description):\n",
+    "        super().__init__()\n",
+    "        self.controller = controller_nonMPI(description, ...)\n",
+    "\n",
+    "    def integrate(u0, dt, t):\n",
+    "        P = self.controller.MS[0].levels[0].prob\n",
+    "        _u = P.u_init(u0)\n",
+    "        u_end, _ = self.controller.run(u0=_u, t0=t, Tend=t+dt)\n",
+    "        return u_end\n",
+    "```\n",
+    "\n",
+    "Of course, you need to take care that `dt` is divisible by the pySDC time step and so on.\n",
+    "Also, you need to implement the coupling in a problem class.\n",
+    "That is to say, you need an interface that can be called by pySDC and which calls the respective functions for evaluating and inverting $f$ in the library you want to couple to.\n",
+    "Since the library may not be designed to be used like this, this may cause some issues.\n",
+    "But if the library you want to couple to is well coded, it should be feasible to use pySDC as an integrator within this library.\n",
+    "\n",
     "## Summary\n",
     "In this notebook you saw how to couple pySDC with another library, in this case Firedrake.\n",
     "The main part of this is to write a datatype that connects the interfaces of pySDC and the library.\n",