sample transition(sample& init_sample)
{
// Initialize the algorithm
this->_sample_stepsize();
nuts_util util;
this->seed(init_sample.cont_params(), init_sample.disc_params());
this->_hamiltonian.sample_p(this->_z, this->_rand_int);
this->_hamiltonian.init(this->_z);
ps_point z_plus(static_cast<ps_point>(this->_z));
ps_point z_minus(z_plus);
ps_point z_sample(z_plus);
ps_point z_propose(z_plus);
int n_cont = init_sample.cont_params().size();
Eigen::VectorXd rho_init = this->_z.p;
Eigen::VectorXd rho_plus = Eigen::VectorXd::Zero(n_cont);
Eigen::VectorXd rho_minus = Eigen::VectorXd::Zero(n_cont);
util.H0 = this->_hamiltonian.H(this->_z);
// Sample the slice variable
util.log_u = std::log(this->_rand_uniform());
// Build a balanced binary tree until the NUTS criterion fails
util.criterion = true;
int n_valid = 0;
this->_depth = 0;
while (util.criterion && (this->_depth <= this->_max_depth) ) {
util.n_tree = 0;
util.sum_prob = 0;
// Randomly sample a direction in time
ps_point* z = 0;
Eigen::VectorXd* rho = 0;
if (this->_rand_uniform() > 0.5)
{
z = &z_plus;
rho = &rho_plus;
util.sign = 1;
}
else
{
z = &z_minus;
rho = &rho_minus;
util.sign = -1;
}
// And build a new subtree in that direction
this->_z.copy_base(*z);
int n_valid_subtree = build_tree(_depth, *rho, 0, z_propose, util);
*z = static_cast<ps_point>(this->_z);
// Metropolis-Hastings sample the fresh subtree
if (!util.criterion)
break;
double subtree_prob = 0;
if (n_valid) {
subtree_prob = static_cast<double>(n_valid_subtree) /
static_cast<double>(n_valid);
} else {
subtree_prob = n_valid_subtree ? 1 : 0;
}
if (this->_rand_uniform() < subtree_prob)
z_sample = z_propose;
n_valid += n_valid_subtree;
// Check validity of completed tree
this->_z.copy_base(z_plus);
Eigen::VectorXd delta_rho = rho_minus + rho_init + rho_plus;
util.criterion = _compute_criterion(z_minus, this->_z, delta_rho);
++(this->_depth);
}
--(this->_depth); // Correct for increment at end of loop
this->_z.copy_base(z_sample);
double accept_prob = util.sum_prob / static_cast<double>(util.n_tree);
return sample(this->_z.q, this->_z.r, - this->_hamiltonian.V(this->_z), accept_prob);
}
sample transition(sample& init_sample) {
this->seed(init_sample.cont_params(), init_sample.disc_params());
return sample(this->_z.q, this->_z.r, - this->_hamiltonian.V(this->_z), 0);
}
