Function implicitRK(Function& f, const std::string& impl, const Dictionary& impl_options,
const MX& tf, int order, const std::string& scheme, int ne) {
casadi_assert_message(ne>=1, "Parameter ne (number of elements must be at least 1), "
"but got " << ne << ".");
casadi_assert_message(order==4, "Only RK order 4 is supported now.");
casadi_assert_message(f.getNumInputs()==DAE_NUM_IN && f.getNumOutputs()==DAE_NUM_OUT,
"Supplied function must adhere to dae scheme.");
casadi_assert_message(f.output(DAE_QUAD).isEmpty(),
"Supplied function cannot have quadrature states.");
// Obtain collocation points
std::vector<double> tau_root = collocationPoints(order, "legendre");
// Retrieve collocation interpolating matrices
std::vector < std::vector <double> > C;
std::vector < double > D;
collocationInterpolators(tau_root, C, D);
// Retrieve problem dimensions
int nx = f.input(DAE_X).size();
int nz = f.input(DAE_Z).size();
int np = f.input(DAE_P).size();
//Variables for one finite element
MX X = MX::sym("X", nx);
MX P = MX::sym("P", np);
MX V = MX::sym("V", order*(nx+nz)); // Unknowns
MX X0 = X;
// Components of the unknowns that correspond to states at collocation points
std::vector<MX> Xc;Xc.reserve(order);
Xc.push_back(X0);
// Components of the unknowns that correspond to algebraic states at collocation points
std::vector<MX> Zc;Zc.reserve(order);
// Splitting the unknowns
std::vector<int> splitPositions = range(0, order*nx, nx);
if (nz>0) {
std::vector<int> Zc_pos = range(order*nx, order*nx+(order+1)*nz, nz);
splitPositions.insert(splitPositions.end(), Zc_pos.begin(), Zc_pos.end());
} else {
splitPositions.push_back(order*nx);
}
std::vector<MX> Vs = vertsplit(V, splitPositions);
// Extracting unknowns from Z
for (int i=0;i<order;++i) {
Xc.push_back(X0+Vs[i]);
}
if (nz>0) {
for (int i=0;i<order;++i) {
Zc.push_back(Vs[order+i]);
}
}
// Get the collocation Equations (that define V)
std::vector<MX> V_eq;
// Local start time
MX t0_l=MX::sym("t0");
MX h = MX::sym("h");
for (int j=1;j<order+1;++j) {
// Expression for the state derivative at the collocation point
MX xp_j = 0;
for (int r=0;r<order+1;++r) {
xp_j+= C[j][r]*Xc[r];
}
// Append collocation equations & algebraic constraints
std::vector<MX> f_out;
MX t_l = t0_l+tau_root[j]*h;
if (nz>0) {
f_out = f.call(daeIn("t", t_l, "x", Xc[j], "p", P, "z", Zc[j-1]));
} else {
f_out = f.call(daeIn("t", t_l, "x", Xc[j], "p", P));
}
V_eq.push_back(h*f_out[DAE_ODE]-xp_j);
V_eq.push_back(f_out[DAE_ALG]);
}
// Root-finding function, implicitly defines V as a function of X0 and P
std::vector<MX> vfcn_inputs;
vfcn_inputs.push_back(V);
vfcn_inputs.push_back(X);
vfcn_inputs.push_back(P);
vfcn_inputs.push_back(t0_l);
vfcn_inputs.push_back(h);
Function vfcn = MXFunction(vfcn_inputs, vertcat(V_eq));
vfcn.init();
try {
// Attempt to convert to SXFunction to decrease overhead
vfcn = SXFunction(vfcn);
vfcn.init();
} catch(CasadiException & e) {
//
}
// Create a implicit function instance to solve the system of equations
ImplicitFunction ifcn(impl, vfcn, Function(), LinearSolver());
ifcn.setOption(impl_options);
ifcn.init();
// Get an expression for the state at the end of the finite element
std::vector<MX> ifcn_call_in(5);
ifcn_call_in[0] = MX::zeros(V.sparsity());
std::copy(vfcn_inputs.begin()+1, vfcn_inputs.end(), ifcn_call_in.begin()+1);
std::vector<MX> ifcn_call_out = ifcn.call(ifcn_call_in, true);
Vs = vertsplit(ifcn_call_out[0], splitPositions);
MX XF = 0;
for (int i=0;i<order+1;++i) {
XF += D[i]*(i==0? X : X + Vs[i-1]);
}
// Get the discrete time dynamics
ifcn_call_in.erase(ifcn_call_in.begin());
MXFunction F = MXFunction(ifcn_call_in, XF);
F.init();
// Loop over all finite elements
MX h_ = tf/ne;
MX t0_ = 0;
for (int i=0;i<ne;++i) {
std::vector<MX> F_in;
F_in.push_back(X);
F_in.push_back(P);
F_in.push_back(t0_);
F_in.push_back(h_);
t0_+= h_;
std::vector<MX> F_out = F.call(F_in);
X = F_out[0];
}
// Create a ruturn function with Integrator signature
MXFunction ret = MXFunction(integratorIn("x0", X0, "p", P), integratorOut("xf", X));
ret.init();
return ret;
}
Function simpleIRK(Function f, int N, int order, const std::string& scheme,
const std::string& solver,
const Dict& solver_options) {
// Consistency check
casadi_assert_message(N>=1, "Parameter N (number of steps) must be at least 1, but got "
<< N << ".");
casadi_assert_message(f.n_in()==2, "Function must have two inputs: x and p");
casadi_assert_message(f.n_out()==1, "Function must have one outputs: dot(x)");
// Obtain collocation points
std::vector<double> tau_root = collocation_points(order, scheme);
tau_root.insert(tau_root.begin(), 0);
// Retrieve collocation interpolating matrices
std::vector < std::vector <double> > C;
std::vector < double > D;
collocationInterpolators(tau_root, C, D);
// Inputs of constructed function
MX x0 = MX::sym("x0", f.sparsity_in(0));
MX p = MX::sym("p", f.sparsity_in(1));
MX h = MX::sym("h");
// Time step
MX dt = h/N;
// Implicitly defined variables
MX v = MX::sym("v", repmat(x0.sparsity(), order));
std::vector<MX> x = vertsplit(v, x0.size1());
x.insert(x.begin(), x0);
// Collect the equations that implicitly define v
std::vector<MX> V_eq, f_in(2), f_out;
for (int j=1; j<order+1; ++j) {
// Expression for the state derivative at the collocation point
MX xp_j = 0;
for (int r=0; r<=order; ++r) xp_j+= C[j][r]*x[r];
// Collocation equations
f_in[0] = x[j];
f_in[1] = p;
f_out = f(f_in);
V_eq.push_back(dt*f_out.at(0)-xp_j);
}
// Root-finding function
Function rfp("rfp", {v, x0, p, h}, {vertcat(V_eq)});
// Create a implicit function instance to solve the system of equations
Function ifcn = rootfinder("ifcn", solver, rfp, solver_options);
// Get state at end time
MX xf = x0;
for (int k=0; k<N; ++k) {
std::vector<MX> ifcn_out = ifcn({repmat(xf, order), xf, p, h});
x = vertsplit(ifcn_out[0], x0.size1());
// State at end of step
xf = D[0]*x0;
for (int i=1; i<=order; ++i) {
xf += D[i]*x[i-1];
}
}
// Form discrete-time dynamics
return Function("F", {x0, p, h}, {xf}, {"x0", "p", "h"}, {"xf"});
}
