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   "cell_type": "markdown",
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   "source": [
    "# Deriving an energy term\n",
    "\n",
    "All energy terms in `micromagneticmodel` are in `micromagneticmodel/energy` directory. They are all derived from `micromagneticmodel.energy.EnergyTerm` base class.\n",
    "\n",
    "For instance, let us say we want to implement an energy term with following specifications:\n",
    "\n",
    "| property | value |\n",
    "| --- | --- |\n",
    "| name | `SpecialEnergy` |\n",
    "| expression | $U\\mathbf{m}\\cdot\\mathbf{m}$ |\n",
    "| parameter | $U$ |\n",
    "| parameter properties | $U \\ge 0$, can be spatially varying |\n",
    "\n",
    "The energy term class would be:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "import micromagneticmodel as mm\n",
    "\n",
    "\n",
    "class SpecialEnergy(mm.EnergyTerm):\n",
    "    pass"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now, we can try to instantiate it"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Exception raised.\n"
     ]
    }
   ],
   "source": [
    "try:\n",
    "    se = SpecialEnergy(U=3)\n",
    "except TypeError:\n",
    "    print(\"Exception raised.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "An exception was raised because `effective_field`, `_reprlatex` and `_allowed_attributes` properties must be implemented. Therefore, an extended implementation of the class is:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [],
   "source": [
    "class SpecialEnergy(mm.EnergyTerm):\n",
    "    _reprlatex = r\"$U\\mathbf{m}\\cdot\\mathbf{m}$\"\n",
    "    _allowed_attributes = [\"U\"]\n",
    "\n",
    "    def effective_field(self, m):\n",
    "        raise NotImplementedError"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We can try to instantiate the class again:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/latex": [
       "$$U\\mathbf{m}\\cdot\\mathbf{m}$$"
      ],
      "text/plain": [
       "SpecialEnergy(U=3)"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "se = SpecialEnergy(U=3)\n",
    "se"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The energy object is created. The last thing we have to impose on the energy class is the typesystem. More precisely, we have to make sure no negative $U$ values are allowed and that `name` attribute accepts only valid Python variable names. This is done by using `ubermagutil`. Full documentation can be found [here](https://ubermagutil.readthedocs.io/en/latest/)."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [],
   "source": [
    "import discretisedfield as df\n",
    "import ubermagutil.typesystem as ts\n",
    "\n",
    "\n",
    "@ts.typesystem(U=ts.Parameter(descriptor=ts.Scalar(unsigned=True), otherwise=df.Field))\n",
    "class SpecialEnergy(mm.EnergyTerm):\n",
    "    _reprlatex = r\"$U\\mathbf{m}\\cdot\\mathbf{m}$\"\n",
    "    _allowed_attributes = [\"U\"]\n",
    "\n",
    "    def effective_field(self, m):\n",
    "        raise NotImplementedError"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "If we now attempt to pass invalid input arguments, exceptions are raised."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Exception raised.\n"
     ]
    }
   ],
   "source": [
    "try:\n",
    "    se = SpecialEnergy(U=-3, name=\"valid_name\")  # negative U\n",
    "except ValueError:\n",
    "    print(\"Exception raised.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Some of the properties and methods of the implemented energy term are:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [],
   "source": [
    "se = SpecialEnergy(U=5e-5)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "5e-05"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "se.U"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/latex": [
       "$$U\\mathbf{m}\\cdot\\mathbf{m}$$"
      ],
      "text/plain": [
       "SpecialEnergy(U=5e-05)"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "se"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "editable": true,
    "slideshow": {
     "slide_type": ""
    },
    "tags": []
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'SpecialEnergy(U=5e-05)'"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "repr(se)"
   ]
  }
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