template <class GF_q, class real> void sum_prod_alg_abstract<GF_q, real>::compute_probs(
array1vd_t& ro)
{
//ensure the output vector has the right length
ro.init(this->length_n);
//initialise some helper variables
int num_of_elements = GF_q::elements();
real a_n = real(0.0);
int size_of_M_n = 0;
int pos_m;
for (int loop_n = 0; loop_n < this->length_n; loop_n++)
{
ro(loop_n) = this->received_probs(loop_n);
size_of_M_n = this->M_n(loop_n).size();
for (int loop_e = 0; loop_e < num_of_elements; loop_e++)
{
for (int loop_m = 0; loop_m < size_of_M_n; loop_m++)
{
pos_m = this->M_n(loop_n)(loop_m) - 1;//we count from 0
ro(loop_n)(loop_e) *= this->marginal_probs(pos_m, loop_n).r_mxn(
loop_e);
}
//Use appropriate clipping method
perform_clipping(ro(loop_n)(loop_e));
}
//Note the following step is not strictly necessary apart from making the result
//look neater - however it only adds a small overhead
//normalise the result so that q_n_0+q_n_1=1
a_n = ro(loop_n).sum();
assertalways(a_n!=real(0.0));
ro(loop_n) /= a_n;
}
}
void sum_prod_alg_trad<GF_q, real>::spa_init(const array1vd_t& recvd_probs)
{
//initialise the marginal prob values
int num_of_elements = GF_q::elements();
real tmp_prob = real(0.0);
real alpha = real(0.0);
//ensure we don't have zero probabilities
//and normalise the probs at the same time
this->received_probs.init(recvd_probs.size());
for (int loop_n = 0; loop_n < this->length_n; loop_n++)
{
this->received_probs(loop_n).init(num_of_elements);
alpha = real(0.0);
for (int loop_e = 0; loop_e < num_of_elements; loop_e++)
{
tmp_prob = recvd_probs(loop_n)(loop_e);
//Clipping HACK
perform_clipping(tmp_prob);
this->received_probs(loop_n)(loop_e) = tmp_prob;
alpha += tmp_prob;
}
assertalways(alpha!=real(0.0));
this->received_probs(loop_n) /= alpha;
}
//this uses the description of the algorithm as given by
//MacKay in Information Theory, Inference and Learning Algorithms(2003)
//on page 560 - chapter 47.3
//some helper variables
int pos = 0;
int non_zeros = 0;
//simply set q_mxn(0)=P_n(0)=P(x_n=0) and q_mxn(1)=P_n(1)=P(x_n=1)
for (int loop_m = 0; loop_m < this->dim_m; loop_m++)
{
non_zeros = this->N_m(loop_m).size();
for (int loop_n = 0; loop_n < non_zeros; loop_n++)
{
pos = this->N_m(loop_m)(loop_n) - 1;//we count from zero;
this->marginal_probs(loop_m, pos).q_mxn = this->received_probs(pos);
this->marginal_probs(loop_m, pos).r_mxn.init(num_of_elements);
this->marginal_probs(loop_m, pos).r_mxn = 0.0;
}
}
#if DEBUG>=2
libbase::trace << " Memory Usage:\n ";
libbase::trace << this->marginal_probs.size()
* sizeof(sum_prod_alg_abstract<GF_q,real>::marginals) / double(1 << 20)
<< " MB" << std::endl;
libbase::trace << std::endl << "The marginal matrix is given by:" << std::endl;
this->print_marginal_probs(libbase::trace);
#endif
}
