void ng_short_alpha(ng_t *ng, double alpha, int N, int i)
{
if (ng->four_byte_alphas)
ng->bo_weight4[N][i] = (four_byte_t)alpha;
else {
ng->bo_weight[N][i] = short_alpha(alpha,
ng->alpha_array,
&(ng->size_of_alpha_array),
65535 - ng->out_of_range_alphas,
ng->min_alpha,
ng->max_alpha);
}
}
void compute_back_off(ng_t *ng,int n, int verbosity) {
int *current_pos;
int *end_pos;
id__t *sought_ngram;
int current_table;
int ng_count;
int i;
double sum_cond_prob;
double sum_bo_prob;
double discounted_ngcount;
double cond_prob;
double bo_prob;
double discount_mass;
double leftout_bo_prob;
double alpha;
int bo_case;
sum_cond_prob = 0.0;
sum_bo_prob = 0.0;
/* For the sake of warning-free compilation... */
discounted_ngcount = 0.0;
current_pos = (int *)rr_calloc(n+1,sizeof(int));
sought_ngram = (id__t *) rr_calloc(n+1,sizeof(id__t));
end_pos = (int *)rr_calloc(n+1,sizeof(int));
/* Process the tree so that we get all the n-grams out in the right
order. */
for (current_pos[0]=ng->first_id;
current_pos[0]<=ng->vocab_size;
current_pos[0]++) {
if (return_count(ng->four_byte_counts,
ng->count_table[0],
ng->marg_counts,
ng->marg_counts4,
current_pos[0]) > 0) {
current_table = 1;
if (current_pos[0] == ng->vocab_size) {
end_pos[1] = ng->num_kgrams[1]-1;
}
else {
end_pos[1] = get_full_index(ng->ind[0][current_pos[0]+1],
ng->ptr_table[0],
ng->ptr_table_size[0],
current_pos[0]+1)-1;
}
while (current_table > 0) {
if (current_table == n) {
if (current_pos[n] <= end_pos[n]){
ng_count = return_count(ng->four_byte_counts,
ng->count_table[n],
ng->count[n],
ng->count4[n],
current_pos[n]);
switch (ng->discounting_method) {
case GOOD_TURING:
if (ng_count <= ng->disc_range[n]) {
discounted_ngcount = ng->gt_disc_ratio[n][ng_count] * ng_count;
}
else {
discounted_ngcount = ng_count;
}
break;
case LINEAR:
discounted_ngcount = ng->lin_disc_ratio[n] * ng_count;
break;
case ABSOLUTE:
discounted_ngcount = ng_count - ng->abs_disc_const[n];
break;
case WITTEN_BELL:
if (n==1) {
discounted_ngcount = ((double)
return_count(ng->four_byte_counts,
ng->count_table[0],
ng->marg_counts,
ng->marg_counts4,
current_pos[0]) * ng_count)
/ (return_count(ng->four_byte_counts,
ng->count_table[0],
ng->marg_counts,
ng->marg_counts4,
current_pos[0]) +
num_of_types(0,current_pos[0],ng));
}
else {
discounted_ngcount = ((double)
return_count(ng->four_byte_counts,
ng->count_table[n-1],
ng->count[n-1],
ng->count4[n-1],
current_pos[n-1])* ng_count)
/ (return_count(ng->four_byte_counts,
ng->count_table[n-1],
ng->count[n-1],
ng->count4[n-1],
current_pos[n-1]) +
num_of_types(n-1,current_pos[n-1],ng));
}
break;
}
if (n==1) {
cond_prob = ((double) discounted_ngcount /
return_count(ng->four_byte_counts,
ng->count_table[0],
ng->marg_counts,
ng->marg_counts4,
current_pos[0]));
}
else {
cond_prob = ((double) discounted_ngcount /
return_count(ng->four_byte_counts,
ng->count_table[n-1],
ng->count[n-1],
ng->count4[n-1],
current_pos[n-1]));
}
sum_cond_prob += cond_prob;
/* Fill up sought ngram array with correct stuff */
for (i=1;i<=n;i++) {
sought_ngram[i-1] = ng->word_id[i][current_pos[i]];
}
bo_ng_prob(n-1,sought_ngram,ng,verbosity,&bo_prob,&bo_case);
sum_bo_prob += bo_prob;
current_pos[n]++;
}
else {
discount_mass = 1.0 - sum_cond_prob;
if (discount_mass < 1e-10) {
discount_mass = 0.0;
pc_message(verbosity,2,"Warning : Back off weight for %s(id %d) ",
ng->vocab[current_pos[0]],current_pos[0]);
for (i=1;i<=n-1;i++) {
pc_message(verbosity,2,"%s(id %d) ",ng->vocab[ng->word_id[i][current_pos[i]]],ng->word_id[i][current_pos[i]]);
}
pc_message(verbosity,2,
"is set to 0 (sum of probs = %f).\nMay cause problems with zero probabilities.\n",sum_cond_prob);
}
leftout_bo_prob = 1.0 - sum_bo_prob;
if (leftout_bo_prob < 1e-10) {
leftout_bo_prob = 0.0;
}
if (leftout_bo_prob > 0.0) {
alpha = discount_mass / leftout_bo_prob;
}
else {
alpha = 0.0; /* Will not be used. Should happen very rarely. */
pc_message(verbosity,2,"Warning : Back off weight for %s(id %d) ",
ng->vocab[current_pos[0]],current_pos[0]);
for (i=1;i<=n-1;i++) {
pc_message(verbosity,2,"%s(id %d) ",ng->vocab[ng->word_id[i][current_pos[i]]],ng->word_id[i][current_pos[i]]);
}
pc_message(verbosity,2,
"is set to 0.\nMay cause problems with zero probabilities.\n");
}
if (ng->four_byte_alphas) {
ng->bo_weight4[n-1][current_pos[n-1]] = alpha;
}
else {
ng->bo_weight[n-1][current_pos[n-1]] =
short_alpha(alpha,
ng->alpha_array,
&(ng->size_of_alpha_array),
65535 - ng->out_of_range_alphas,
ng->min_alpha,
ng->max_alpha);
}
/* Finished current (n-1)-gram */
sum_cond_prob = 0.0;
sum_bo_prob = 0.0;
current_table--;
if (current_table > 0) {
current_pos[current_table]++;
}
}
}
else {
if (current_pos[current_table] <= end_pos[current_table]) {
current_table++;
if (current_pos[current_table-1] == ng->num_kgrams[current_table-1]-1) {
end_pos[current_table] = ng->num_kgrams[current_table]-1;
}
else {
end_pos[current_table] = get_full_index(ng->ind[current_table-1][current_pos[current_table-1]+1],ng->ptr_table[current_table-1],ng->ptr_table_size[current_table-1],current_pos[current_table-1]+1)-1;
}
}
else {
current_table--;
if (current_table > 0) {
current_pos[current_table]++;
}
}
}
}
}
/* Now deal with zeroton unigrams */
else {
if (n == 1) {
if (ng->four_byte_alphas) {
ng->bo_weight4[0][current_pos[0]] = 1.0;
}
else {
ng->bo_weight[0][current_pos[0]] =
short_alpha(1.0,
ng->alpha_array,
&(ng->size_of_alpha_array),
65535 - ng->out_of_range_alphas,
ng->min_alpha,
ng->max_alpha);
}
}
}
}
free(end_pos);
free(current_pos);
free(sought_ngram);
}
