PaddleSpeech/third_party/ctc_decoders/scorer.h

// Licensed under GNU Lesser General Public License v3 (LGPLv3) (LGPL-3) (the
// "COPYING.LESSER.3");

#ifndef SCORER_H_
#define SCORER_H_

#include <memory>
#include <string>
#include <unordered_map>
#include <vector>

#include "lm/enumerate_vocab.hh"
#include "lm/virtual_interface.hh"
#include "lm/word_index.hh"

#include "path_trie.h"

const double OOV_SCORE = -1000.0;
const std::string START_TOKEN = "<s>";
const std::string UNK_TOKEN = "<unk>";
const std::string END_TOKEN = "</s>";

// Implement a callback to retrive the dictionary of language model.
class RetriveStrEnumerateVocab : public lm::EnumerateVocab {
  public:
    RetriveStrEnumerateVocab() {}

    void Add(lm::WordIndex index, const StringPiece &str) {
        vocabulary.push_back(std::string(str.data(), str.length()));
    }

    std::vector<std::string> vocabulary;
};

/* External scorer to query score for n-gram or sentence, including language
 * model scoring and word insertion.
 *
 * Example:
 *     Scorer scorer(alpha, beta, "path_of_language_model");
 *     scorer.get_log_cond_prob({ "WORD1", "WORD2", "WORD3" });
 *     scorer.get_sent_log_prob({ "WORD1", "WORD2", "WORD3" });
 */
class Scorer {
  public:
    Scorer(double alpha,
           double beta,
           const std::string &lm_path,
           const std::vector<std::string> &vocabulary);
    ~Scorer();

    double get_log_cond_prob(const std::vector<std::string> &words);

    double get_sent_log_prob(const std::vector<std::string> &words);

    // return the max order
    size_t get_max_order() const { return max_order_; }

    // return the dictionary size of language model
    size_t get_dict_size() const { return dict_size_; }

    // retrun true if the language model is character based
    bool is_character_based() const { return is_character_based_; }

    // reset params alpha & beta
    void reset_params(float alpha, float beta);

    // make ngram for a given prefix
    std::vector<std::string> make_ngram(PathTrie *prefix);

    // trransform the labels in index to the vector of words (word based lm) or
    // the vector of characters (character based lm)
    std::vector<std::string> split_labels(const std::vector<int> &labels);

    // language model weight
    double alpha;
    // word insertion weight
    double beta;

    // pointer to the dictionary of FST
    void *dictionary;

  protected:
    // necessary setup: load language model, set char map, fill FST's dictionary
    void setup(const std::string &lm_path,
               const std::vector<std::string> &vocab_list);

    // load language model from given path
    void load_lm(const std::string &lm_path);

    // fill dictionary for FST
    void fill_dictionary(bool add_space);

    // set char map
    void set_char_map(const std::vector<std::string> &char_list);

    double get_log_prob(const std::vector<std::string> &words);

    // translate the vector in index to string
    std::string vec2str(const std::vector<int> &input);

  private:
    void *language_model_;
    bool is_character_based_;
    size_t max_order_;
    size_t dict_size_;

    int SPACE_ID_;
    std::vector<std::string> char_list_;
    std::unordered_map<std::string, int> char_map_;

    std::vector<std::string> vocabulary_;
};

#endif  // SCORER_H_