PR #3279 - use a tree structure to store timesteps

native_client/ctcdecode/ctc_beam_search_decoder.cpp

@@ -37,6 +37,8 @@ DecoderState::init(const Alphabet& alphabet,
   prefix_root_.reset(root);
   prefixes_.push_back(root);
   
+  timestep_tree_root_=std::make_shared<TimestepTreeNode>(nullptr, 0);
+
   if (ext_scorer && (bool)(ext_scorer_->dictionary)) {
     // no need for std::make_shared<>() since Copy() does 'new' behind the doors
     auto dict_ptr = std::shared_ptr<PathTrie::FstType>(ext_scorer->dictionary->Copy(true));
@@ -96,7 +98,7 @@ DecoderState::next(const double *probs,
         if (full_beam && log_prob_c + prefix->score < min_cutoff) {
           break;
         }
-        assert(prefix->is_empty() || !prefix->timesteps.empty());
+        assert(prefix->is_empty() || prefix->timesteps!=nullptr);
 
         // blank
         if (c == blank_id_) {
@@ -167,7 +169,7 @@ DecoderState::next(const double *probs,
           if (prefix_new->log_prob_nb_cur < log_p) {
             // record data needed to update timesteps
             // the actual update will be done if nothing better is found
-            prefix_new->previous_timesteps = &prefix->timesteps;
+            prefix_new->previous_timesteps = prefix->timesteps;
             prefix_new->new_timestep = abs_time_step_;
           }
           prefix_new->log_prob_nb_cur =
@@ -179,6 +181,11 @@ DecoderState::next(const double *probs,
     // update log probs
     prefixes_.clear();
     prefix_root_->iterate_to_vec(prefixes_);
+    if (abs_time_step_ == 0) {
+      for (PathTrie* prefix:prefixes_) {
+        prefix->timesteps = timestep_tree_root_;
+      }
+    }
 
     // only preserve top beam_size prefixes
     if (prefixes_.size() > beam_size_) {
@@ -234,7 +241,7 @@ DecoderState::decode(size_t num_results) const
   for (PathTrie* prefix : prefixes_copy) {
     Output output;
     prefix->get_path_vec(output.tokens);
-    output.timesteps  = prefix->timesteps;
+    output.timesteps  = get_history(prefix->timesteps);
     output.confidence = scores[prefix];
     outputs.push_back(output);
     if(outputs.size()>=num_returned) break;

native_client/ctcdecode/ctc_beam_search_decoder.h

@@ -21,6 +21,7 @@ class DecoderState {
   std::shared_ptr<Scorer> ext_scorer_;
   std::vector<PathTrie*> prefixes_;
   std::unique_ptr<PathTrie> prefix_root_;
+  std::shared_ptr<TimestepTreeNode> timestep_tree_root_;
 
 public:
   DecoderState() = default;

native_client/ctcdecode/path_trie.cpp

@@ -172,8 +172,16 @@ void PathTrie::iterate_to_vec(std::vector<PathTrie*>& output) {
     score = log_sum_exp(log_prob_b_prev, log_prob_nb_prev);
 
     if (previous_timesteps != nullptr) {
-      timesteps = *previous_timesteps;
+      timesteps = nullptr;
-      timesteps.push_back(new_timestep);
+      for (auto const& child : previous_timesteps->children) {
+        if (child->data == new_timestep) {
+            timesteps=child;
+            break;
+        }
+      }
+      if (timesteps == nullptr){
+          timesteps = add_child(previous_timesteps, new_timestep);
+      }
     }
     previous_timesteps=nullptr;
 
@@ -230,7 +238,7 @@ void PathTrie::print(const Alphabet& a) {
     }
   }
   printf("\ntimesteps:\t ");
-  for (unsigned int timestep : timesteps) {
+  for (unsigned int timestep : get_history(timesteps)) {
     printf("%d ", timestep);
   }
   printf("\n");

native_client/ctcdecode/path_trie.h

@@ -10,6 +10,27 @@
 #include "fst/fstlib.h"
 #include "alphabet.h"
 
+/* Tree structure with parent and children information
+ * It is used to store the timesteps data for the PathTrie below
+ */
+template<class DataT>
+struct TreeNode{
+	std::shared_ptr<TreeNode<DataT>> parent;
+    std::vector<std::shared_ptr<TreeNode<DataT>>> children;
+
+    DataT data;
+
+    TreeNode(std::shared_ptr<TreeNode<DataT>> const& parent_, DataT const& data_): parent{parent_}, data{data_} {}
+};
+
+template<class NodeDataT, class ChildDataT>
+std::shared_ptr<TreeNode<NodeDataT>> add_child(std::shared_ptr<TreeNode<NodeDataT>> const& node, ChildDataT&& data_);
+
+template<class DataT>
+std::vector<DataT> get_history(TreeNode<DataT>*);
+
+using TimestepTreeNode=TreeNode<unsigned int>;
+
 /* Trie tree for prefix storing and manipulating, with a dictionary in
  * finite-state transducer for spelling correction.
  */
@@ -63,10 +84,10 @@ public:
   float score;
   float approx_ctc;
   unsigned int character;
-  std::vector<unsigned int> timesteps;
+  std::shared_ptr<TimestepTreeNode> timesteps;
 
   // timestep temporary storage for each decoding step. 
-  std::vector<unsigned int>* previous_timesteps=nullptr; 
+  std::shared_ptr<TimestepTreeNode> previous_timesteps=nullptr; 
   unsigned int new_timestep;
 
   PathTrie* parent;
@@ -84,4 +105,26 @@ private:
   std::shared_ptr<fst::SortedMatcher<FstType>> matcher_;
 };
 
+// TreeNode implementation
+template<class NodeDataT, class ChildDataT>
+std::shared_ptr<TreeNode<NodeDataT>> add_child(std::shared_ptr<TreeNode<NodeDataT>> const& node, ChildDataT&& data_){
+    node->children.push_back(std::make_shared<TreeNode<NodeDataT>>(node, std::forward<ChildDataT>(data_)));
+    return node->children.back();
+}
+
+template<class DataT>
+void get_history_helper(std::shared_ptr<TreeNode<DataT>> const& tree_node, std::vector<DataT>* output){
+    if(tree_node==nullptr) return;
+	assert(tree_node->parent != tree_node);
+    get_history_helper(tree_node->parent, output);
+    output->push_back(tree_node->data);
+}
+template<class DataT>
+std::vector<DataT> get_history(std::shared_ptr<TreeNode<DataT>> const& tree_node){
+    std::vector<DataT> output;
+    get_history_helper(tree_node, &output);
+    return output;
+}
+
+
 #endif  // PATH_TRIE_H

native_client/ctcdecode/scorer.cpp

@@ -293,7 +293,6 @@ std::vector<std::string> Scorer::make_ngram(PathTrie* prefix)
     }
 
     std::vector<unsigned int> prefix_vec;
-    std::vector<unsigned int> prefix_steps = current_node->timesteps;
 
     if (is_utf8_mode_) {
       new_node = current_node->get_prev_grapheme(prefix_vec, alphabet_);