#
# Convert a PyBaMM expression tree to a CasADi expression tree
#
from __future__ import annotations
import pybamm
import casadi
import numpy as np
from scipy import special
[docs]
class CasadiConverter:
def __init__(self, casadi_symbols=None):
self._casadi_symbols = casadi_symbols or {}
pybamm.citations.register("Andersson2019")
[docs]
def convert(
self,
symbol: pybamm.Symbol,
t: casadi.MX,
y: casadi.MX,
y_dot: casadi.MX,
inputs: dict | None,
) -> casadi.MX:
"""
This function recurses down the tree, converting the PyBaMM expression tree to
a CasADi expression tree
Parameters
----------
symbol : :class:`pybamm.Symbol`
The symbol to convert
t : :class:`casadi.MX`
A casadi symbol representing time
y : :class:`casadi.MX`
A casadi symbol representing state vectors
y_dot : :class:`casadi.MX`
A casadi symbol representing time derivatives of state vectors
inputs : dict
A dictionary of casadi symbols representing parameters
Returns
-------
:class:`casadi.MX`
The converted symbol
"""
try:
return self._casadi_symbols[symbol]
except KeyError:
# Change inputs to empty dictionary if it's None
inputs = inputs or {}
casadi_symbol = self._convert(symbol, t, y, y_dot, inputs)
self._casadi_symbols[symbol] = casadi_symbol
return casadi_symbol
def _convert(self, symbol, t, y, y_dot, inputs):
"""See :meth:`CasadiConverter.convert()`."""
if isinstance(
symbol,
(
pybamm.Scalar,
pybamm.Array,
pybamm.Time,
pybamm.InputParameter,
),
):
return casadi.MX(symbol.evaluate(t, y, y_dot, inputs))
elif isinstance(symbol, pybamm.StateVector):
if y is None:
raise ValueError("Must provide a 'y' for converting state vectors")
return casadi.vertcat(*[y[y_slice] for y_slice in symbol.y_slices])
elif isinstance(symbol, pybamm.StateVectorDot):
if y_dot is None:
raise ValueError("Must provide a 'y_dot' for converting state vectors")
return casadi.vertcat(*[y_dot[y_slice] for y_slice in symbol.y_slices])
elif isinstance(symbol, pybamm.BinaryOperator):
left, right = symbol.children
# process children
converted_left = self.convert(left, t, y, y_dot, inputs)
converted_right = self.convert(right, t, y, y_dot, inputs)
if isinstance(symbol, pybamm.Modulo):
return casadi.fmod(converted_left, converted_right)
if isinstance(symbol, pybamm.Minimum):
return casadi.fmin(converted_left, converted_right)
if isinstance(symbol, pybamm.Maximum):
return casadi.fmax(converted_left, converted_right)
# _binary_evaluate defined in derived classes for specific rules
return symbol._binary_evaluate(converted_left, converted_right)
elif isinstance(symbol, pybamm.UnaryOperator):
converted_child = self.convert(symbol.child, t, y, y_dot, inputs)
if isinstance(symbol, pybamm.AbsoluteValue):
return casadi.fabs(converted_child)
if isinstance(symbol, pybamm.Floor):
return casadi.floor(converted_child)
if isinstance(symbol, pybamm.Ceiling):
return casadi.ceil(converted_child)
return symbol._unary_evaluate(converted_child)
elif isinstance(symbol, pybamm.Function):
converted_children = [
self.convert(child, t, y, y_dot, inputs) for child in symbol.children
]
# Special functions
if symbol.function == np.min:
return casadi.mmin(*converted_children)
elif symbol.function == np.max:
return casadi.mmax(*converted_children)
elif symbol.function == np.abs:
return casadi.fabs(*converted_children)
elif symbol.function == np.sqrt:
return casadi.sqrt(*converted_children)
elif symbol.function == np.sin:
return casadi.sin(*converted_children)
elif symbol.function == np.arcsinh:
return casadi.arcsinh(*converted_children)
elif symbol.function == np.arccosh:
return casadi.arccosh(*converted_children)
elif symbol.function == np.tanh:
return casadi.tanh(*converted_children)
elif symbol.function == np.cosh:
return casadi.cosh(*converted_children)
elif symbol.function == np.sinh:
return casadi.sinh(*converted_children)
elif symbol.function == np.cos:
return casadi.cos(*converted_children)
elif symbol.function == np.exp:
return casadi.exp(*converted_children)
elif symbol.function == np.log:
return casadi.log(*converted_children)
elif symbol.function == np.sign:
return casadi.sign(*converted_children)
elif symbol.function == special.erf:
return casadi.erf(*converted_children)
elif isinstance(symbol, pybamm.Interpolant):
if symbol.interpolator == "linear":
solver = "linear"
elif symbol.interpolator == "cubic":
solver = "bspline"
elif symbol.interpolator == "pchip":
raise NotImplementedError(
"The interpolator 'pchip' is not supported by CasAdi. "
"Use 'linear' or 'cubic' instead. "
"Alternatively, set 'model.convert_to_format = 'python'' "
"and use a non-CasADi solver. "
)
else: # pragma: no cover
raise NotImplementedError(
f"Unknown interpolator: {symbol.interpolator}"
)
if len(converted_children) == 1:
return casadi.interpolant(
"LUT", solver, symbol.x, symbol.y.flatten()
)(*converted_children)
elif len(converted_children) in [2, 3]:
LUT = casadi.interpolant(
"LUT", solver, symbol.x, symbol.y.ravel(order="F")
)
res = LUT(casadi.hcat(converted_children).T).T
return res
else: # pragma: no cover
raise ValueError(
f"Invalid converted_children count: {len(converted_children)}"
)
elif symbol.function.__name__.startswith("elementwise_grad_of_"):
differentiating_child_idx = int(symbol.function.__name__[-1])
# Create dummy symbolic variables in order to differentiate using CasADi
dummy_vars = [
casadi.MX.sym("y_" + str(i)) for i in range(len(converted_children))
]
func_diff = casadi.gradient(
symbol.differentiated_function(*dummy_vars),
dummy_vars[differentiating_child_idx],
)
# Create function and evaluate it using the children
casadi_func_diff = casadi.Function("func_diff", dummy_vars, [func_diff])
return casadi_func_diff(*converted_children)
# Other functions
else:
return symbol._function_evaluate(converted_children)
elif isinstance(symbol, pybamm.Concatenation):
converted_children = [
self.convert(child, t, y, y_dot, inputs) for child in symbol.children
]
if isinstance(symbol, (pybamm.NumpyConcatenation, pybamm.SparseStack)):
return casadi.vertcat(*converted_children)
# DomainConcatenation specifies a particular ordering for the concatenation,
# which we must follow
elif isinstance(symbol, pybamm.DomainConcatenation):
slice_starts = []
all_child_vectors = []
for i in range(symbol.secondary_dimensions_npts):
child_vectors = []
for child_var, slices in zip(
converted_children, symbol._children_slices
):
for child_dom, child_slice in slices.items():
slice_starts.append(symbol._slices[child_dom][i].start)
child_vectors.append(
child_var[child_slice[i].start : child_slice[i].stop]
)
all_child_vectors.extend(
[v for _, v in sorted(zip(slice_starts, child_vectors))]
)
return casadi.vertcat(*all_child_vectors)
else:
raise TypeError(
f"""
Cannot convert symbol of type '{type(symbol)}' to CasADi. Symbols must all be
'linear algebra' at this stage.
"""
)
