change probs' computation into log scale & add best path decoder

deploy/ctc_beam_search_decoder.cpp

@@ -3,8 +3,11 @@
 #include <algorithm>
 #include <utility>
 #include <cmath>
+#include <limits>
 #include "ctc_beam_search_decoder.h"
 
+typedef float log_prob_type;
+
 template <typename T1, typename T2>
 bool pair_comp_first_rev(const std::pair<T1, T2> a, const std::pair<T1, T2> b)
 {
@@ -17,6 +20,65 @@ bool pair_comp_second_rev(const std::pair<T1, T2> a, const std::pair<T1, T2> b)
     return a.second > b.second;
 }
 
+template <typename T>
+T log_sum_exp(T x, T y)
+{
+    static T num_min = -std::numeric_limits<T>::max();
+    if (x <= -num_min) return y;
+    if (y <= -num_min) return x;
+    T xmax = std::max(x, y);
+    return std::log(std::exp(x-xmax) + std::exp(y-xmax)) + xmax;
+}
+
+std::string ctc_best_path_decoder(std::vector<std::vector<double> > probs_seq,
+                                  std::vector<std::string> vocabulary) {
+    // dimension check
+    int num_time_steps = probs_seq.size();
+    for (int i=0; i<num_time_steps; i++) {
+        if (probs_seq[i].size() != vocabulary.size()+1) {
+            std::cout<<"The shape of probs_seq does not match"
+                     <<" with the shape of the vocabulary!"<<std::endl;
+            exit(1);
+        }
+    }
+
+    int blank_id = vocabulary.size();
+
+    std::vector<int> max_idx_vec;
+    double max_prob = 0.0;
+    int max_idx = 0;
+    for (int i=0; i<num_time_steps; i++) {
+        for (int j=0; j<probs_seq[i].size(); j++) {
+            if (max_prob < probs_seq[i][j]) {
+                max_idx = j;
+                max_prob = probs_seq[i][j];
+            }
+        }
+        max_idx_vec.push_back(max_idx);
+        std::cout<<max_idx<<",";
+        max_prob = 0.0;
+        max_idx = 0;
+    }
+    std::cout<<std::endl;
+
+    std::vector<int> idx_vec;
+    for (int i=0; i<max_idx_vec.size(); i++) {
+        std::cout<<max_idx_vec[i]<<",";
+        if ((i == 0) || ((i>0) && max_idx_vec[i]!=max_idx_vec[i-1])) {
+            std::cout<<max_idx_vec[i]<<",";
+            idx_vec.push_back(max_idx_vec[i]);
+        }
+    }
+
+    std::string best_path_result;
+    for (int i=0; i<idx_vec.size(); i++) {
+        if (idx_vec[i] != blank_id) {
+            best_path_result += vocabulary[i];
+        }
+    }
+    return best_path_result;
+}
+
 std::vector<std::pair<double, std::string> >
     ctc_beam_search_decoder(std::vector<std::vector<double> > probs_seq,
                             int beam_size,
@@ -52,106 +114,147 @@ std::vector<std::pair<double, std::string> >
 
     // initialize
     // two sets containing selected and candidate prefixes respectively
-    std::map<std::string, double> prefix_set_prev, prefix_set_next;
+    std::map<std::string, log_prob_type> prefix_set_prev, prefix_set_next;
     // probability of prefixes ending with blank and non-blank
-    std::map<std::string, double> probs_b_prev, probs_nb_prev;
-    std::map<std::string, double> probs_b_cur, probs_nb_cur;
-    prefix_set_prev["\t"] = 1.0;
-    probs_b_prev["\t"] = 1.0;
-    probs_nb_prev["\t"] = 0.0;
+    std::map<std::string, log_prob_type> log_probs_b_prev, log_probs_nb_prev;
+    std::map<std::string, log_prob_type> log_probs_b_cur, log_probs_nb_cur;
+
+    static log_prob_type NUM_MAX = std::numeric_limits<log_prob_type>::max();
+    prefix_set_prev["\t"] = 0.0;
+    log_probs_b_prev["\t"] = 0.0;
+    log_probs_nb_prev["\t"] = -NUM_MAX;
 
     for (int time_step=0; time_step<num_time_steps; time_step++) {
         prefix_set_next.clear();
-        probs_b_cur.clear();
-        probs_nb_cur.clear();
+        log_probs_b_cur.clear();
+        log_probs_nb_cur.clear();
         std::vector<double> prob = probs_seq[time_step];
 
         std::vector<std::pair<int, double> > prob_idx;
         for (int i=0; i<prob.size(); i++) {
             prob_idx.push_back(std::pair<int, double>(i, prob[i]));
         }
+
         // pruning of vacobulary
+        int cutoff_len = prob.size();
         if (cutoff_prob < 1.0) {
-            std::sort(prob_idx.begin(), prob_idx.end(),
+            std::sort(prob_idx.begin(),
+                      prob_idx.end(),
                       pair_comp_second_rev<int, double>);
-            float cum_prob = 0.0;
-            int cutoff_len = 0;
+            double cum_prob = 0.0;
+            cutoff_len = 0;
             for (int i=0; i<prob_idx.size(); i++) {
                 cum_prob += prob_idx[i].second;
                 cutoff_len += 1;
                 if (cum_prob >= cutoff_prob) break;
             }
             prob_idx = std::vector<std::pair<int, double> >( prob_idx.begin(),
-                                                  prob_idx.begin() + cutoff_len);
+                            prob_idx.begin() + cutoff_len);
         }
+
+        std::vector<std::pair<int, log_prob_type> > log_prob_idx;
+        for (int i=0; i<cutoff_len; i++) {
+            log_prob_idx.push_back(std::pair<int, log_prob_type>
+                        (prob_idx[i].first, log(prob_idx[i].second)));
+        }
+
         // extend prefix
-        for (std::map<std::string, double>::iterator it = prefix_set_prev.begin();
+        for (std::map<std::string, log_prob_type>::iterator
+             it = prefix_set_prev.begin();
             it != prefix_set_prev.end(); it++) {
             std::string l = it->first;
             if( prefix_set_next.find(l) == prefix_set_next.end()) {
-                probs_b_cur[l] = probs_nb_cur[l] = 0.0;
+                log_probs_b_cur[l] = log_probs_nb_cur[l] = -NUM_MAX;
             }
 
-            for (int index=0; index<prob_idx.size(); index++) {
-                int c = prob_idx[index].first;
-                double prob_c = prob_idx[index].second;
+            for (int index=0; index<log_prob_idx.size(); index++) {
+                int c = log_prob_idx[index].first;
+                log_prob_type log_prob_c = log_prob_idx[index].second;
+                log_prob_type log_probs_prev;
                 if (c == blank_id) {
-                    probs_b_cur[l] += prob_c * (probs_b_prev[l] + probs_nb_prev[l]);
+                    log_probs_prev = log_sum_exp(log_probs_b_prev[l],
+                                                 log_probs_nb_prev[l]);
+                    log_probs_b_cur[l] = log_sum_exp(log_probs_b_cur[l],
+                                                     log_prob_c+log_probs_prev);
                 } else {
                     std::string last_char = l.substr(l.size()-1, 1);
                     std::string new_char = vocabulary[c];
                     std::string l_plus = l + new_char;
 
                     if( prefix_set_next.find(l_plus) == prefix_set_next.end()) {
-                        probs_b_cur[l_plus] = probs_nb_cur[l_plus] = 0.0;
+                        log_probs_b_cur[l_plus] = -NUM_MAX;
+                        log_probs_nb_cur[l_plus] = -NUM_MAX;
                     }
                     if (last_char == new_char) {
-                        probs_nb_cur[l_plus] += prob_c * probs_b_prev[l];
-                        probs_nb_cur[l] += prob_c * probs_nb_prev[l];
+                        log_probs_nb_cur[l_plus] = log_sum_exp(
+                                                log_probs_nb_cur[l_plus],
+                                                log_prob_c+log_probs_b_prev[l]
+                                            );
+                        log_probs_nb_cur[l] = log_sum_exp(
+                                                log_probs_nb_cur[l],
+                                                log_prob_c+log_probs_nb_prev[l]
+                                            );
                     } else if (new_char == " ") {
-                        double score = 1.0;
+                        float score = 0.0;
                         if (ext_scorer != NULL && l.size() > 1) {
-                            score = ext_scorer->get_score(l.substr(1));
+                            score = ext_scorer->get_score(l.substr(1), true);
                         }
-                        probs_nb_cur[l_plus] += score * prob_c * (
-                            probs_b_prev[l] + probs_nb_prev[l]);
+                        log_probs_prev = log_sum_exp(log_probs_b_prev[l],
+                                                     log_probs_nb_prev[l]);
+                        log_probs_nb_cur[l_plus] = log_sum_exp(
+                                            log_probs_nb_cur[l_plus],
+                                            score + log_prob_c + log_probs_prev
+                                        );
                     } else {
-                        probs_nb_cur[l_plus] += prob_c * (
-                            probs_b_prev[l] + probs_nb_prev[l]);
+                        log_probs_prev = log_sum_exp(log_probs_b_prev[l],
+                                                     log_probs_nb_prev[l]);
+                        log_probs_nb_cur[l_plus] = log_sum_exp(
+                                                    log_probs_nb_cur[l_plus],
+                                                    log_prob_c+log_probs_prev
+                                                );
                     }
-                    prefix_set_next[l_plus] = probs_nb_cur[l_plus] + probs_b_cur[l_plus];
+                    prefix_set_next[l_plus] = log_sum_exp(
+                                                log_probs_nb_cur[l_plus],
+                                                log_probs_b_cur[l_plus]
+                                            );
                 }
             }
 
-            prefix_set_next[l] = probs_b_cur[l] + probs_nb_cur[l];
+            prefix_set_next[l] = log_sum_exp(log_probs_b_cur[l],
+                                             log_probs_nb_cur[l]);
         }
 
-        probs_b_prev = probs_b_cur;
-        probs_nb_prev = probs_nb_cur;
-        std::vector<std::pair<std::string, double> >
+        log_probs_b_prev = log_probs_b_cur;
+        log_probs_nb_prev = log_probs_nb_cur;
+        std::vector<std::pair<std::string, log_prob_type> >
                   prefix_vec_next(prefix_set_next.begin(),
                                   prefix_set_next.end());
         std::sort(prefix_vec_next.begin(),
                   prefix_vec_next.end(),
-                  pair_comp_second_rev<std::string, double>);
-        int k = beam_size<prefix_vec_next.size() ? beam_size:prefix_vec_next.size();
-        prefix_set_prev = std::map<std::string, double>
-                  (prefix_vec_next.begin(), prefix_vec_next.begin()+k);
+                  pair_comp_second_rev<std::string, log_prob_type>);
+        int num_prefixes_next = prefix_vec_next.size();
+        int k = beam_size<num_prefixes_next ? beam_size : num_prefixes_next;
+        prefix_set_prev = std::map<std::string, log_prob_type> (
+                                                   prefix_vec_next.begin(),
+                                                   prefix_vec_next.begin() + k
+                                                );
     }
 
     // post processing
     std::vector<std::pair<double, std::string> > beam_result;
-    for (std::map<std::string, double>::iterator it = prefix_set_prev.begin();
-         it != prefix_set_prev.end(); it++) {
-        if (it->second > 0.0 && it->first.size() > 1) {
-            double prob = it->second;
+    for (std::map<std::string, log_prob_type>::iterator
+         it = prefix_set_prev.begin(); it != prefix_set_prev.end(); it++) {
+        if (it->second > -NUM_MAX && it->first.size() > 1) {
+            log_prob_type log_prob = it->second;
             std::string sentence = it->first.substr(1);
             // scoring the last word
             if (ext_scorer != NULL && sentence[sentence.size()-1] != ' ') {
-                prob = prob * ext_scorer->get_score(sentence);
+                log_prob = log_prob + ext_scorer->get_score(sentence, true);
+            }
+            if (log_prob > -NUM_MAX) {
+                std::pair<double, std::string> cur_result(log_prob, sentence);
+                beam_result.push_back(cur_result);
             }
-            double log_prob = log(prob);
-            beam_result.push_back(std::pair<double, std::string>(log_prob, sentence));
         }
     }
     // sort the result and return

deploy/ctc_beam_search_decoder.h

@@ -31,5 +31,9 @@ std::vector<std::pair<double, std::string> >
                             Scorer *ext_scorer=NULL,
                             bool nproc=false
                             );
+/* CTC Best Path Decoder
+ */
+std::string ctc_best_path_decoder(std::vector<std::vector<double> > probs_seq,
+                                     std::vector<std::string> vocabulary);
 
 #endif // CTC_BEAM_SEARCH_DECODER_H_

deploy/scorer.cpp

@@ -89,10 +89,15 @@ void Scorer::reset_params(float alpha, float beta) {
     this->_beta = beta;
 }
 
-double Scorer::get_score(std::string sentence) {
+double Scorer::get_score(std::string sentence, bool log) {
     double lm_score = language_model_score(sentence);
     int word_cnt = word_count(sentence);
 
-    double final_score = pow(10, _alpha*lm_score) * pow(word_cnt, _beta);
+    double final_score = 0.0;
+    if (log == false) {
+        final_score = pow(10, _alpha*lm_score) * pow(word_cnt, _beta);
+    } else {
+        final_score = _alpha*lm_score*std::log(10) + _beta*std::log(word_cnt);
+    }
     return final_score;
 }

deploy/scorer.h

@@ -30,7 +30,7 @@ public:
     // reset params alpha & beta
     void reset_params(float alpha, float beta);
     // get the final score
-    double get_score(std::string);
+    double get_score(std::string, bool log=false);
 };
 
 #endif //SCORER_H_

deploy/swig_decoder.py

+"""Contains various CTC decoders in SWIG."""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from swig_ctc_beam_search_decoder import ctc_beam_search_decoder as beam_search_decoder
+from swig_ctc_beam_search_decoder import ctc_best_path_decoder as best_path__decoder
+
+
+def ctc_best_path_decoder(probs_seq, vocabulary):
+    best_path__decoder(probs_seq.to_list(), vocabulary)
+
+
+def ctc_beam_search_decoder(
+        probs_seq,
+        beam_size,
+        vocabulary,
+        blank_id,
+        cutoff_prob=1.0,
+        ext_scoring_func=None, ):
+    beam_search_decoder(probs_seq.to_list(), beam_size, vocabulary, blank_id,
+                        cutoff_prob, ext_scoring_func)