native/utils/token-feature-extractor.h

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef LIBTEXTCLASSIFIER_UTILS_TOKEN_FEATURE_EXTRACTOR_H_
#define LIBTEXTCLASSIFIER_UTILS_TOKEN_FEATURE_EXTRACTOR_H_

#include <memory>
#include <unordered_set>
#include <vector>

#include "annotator/types.h"
#include "utils/strings/stringpiece.h"
#include "utils/utf8/unilib.h"

namespace libtextclassifier3 {

struct TokenFeatureExtractorOptions {
  // Number of buckets used for hashing charactergrams.
  int num_buckets = 0;

  // Orders of charactergrams to extract. E.g., 2 means character bigrams, 3
  // character trigrams etc.
  std::vector<int> chargram_orders;

  // Whether to extract the token case feature.
  bool extract_case_feature = false;

  // If true, will use the unicode-aware functionality for extracting features.
  bool unicode_aware_features = false;

  // Whether to extract the selection mask feature.
  bool extract_selection_mask_feature = false;

  // Regexp features to extract.
  std::vector<std::string> regexp_features;

  // Whether to remap digits to a single number.
  bool remap_digits = false;

  // Whether to lowercase all tokens.
  bool lowercase_tokens = false;

  // Maximum length of a word.
  int max_word_length = 20;

  // List of allowed charactergrams. The extracted charactergrams are filtered
  // using this list, and charactergrams that are not present are interpreted as
  // out-of-vocabulary.
  // If no allowed_chargrams are specified, all charactergrams are allowed.
  std::unordered_set<std::string> allowed_chargrams;
};

class TokenFeatureExtractor {
 public:
  // Des not take ownership of unilib, which must refer to a valid unilib
  // instance that outlives this feature extractor.
  explicit TokenFeatureExtractor(const TokenFeatureExtractorOptions& options,
                                 const UniLib* unilib);

  // Extracts both the sparse (charactergram) and the dense features from a
  // token. is_in_span is a bool indicator whether the token is a part of the
  // selection span (true) or not (false).
  // The sparse_features output is optional. Fails and returns false if
  // dense_fatures in a nullptr.
  bool Extract(const Token& token, bool is_in_span,
               std::vector<int>* sparse_features,
               std::vector<float>* dense_features) const;

  // Extracts the sparse (charactergram) features from the token.
  std::vector<int> ExtractCharactergramFeatures(const Token& token) const;

  // Extracts the dense features from the token. is_in_span is a bool indicator
  // whether the token is a part of the selection span (true) or not (false).
  std::vector<float> ExtractDenseFeatures(const Token& token,
                                          bool is_in_span) const;

  int DenseFeaturesCount() const {
    int feature_count =
        options_.extract_case_feature + options_.extract_selection_mask_feature;
    feature_count += regex_patterns_.size();
    return feature_count;
  }

 protected:
  // Hashes given token to given number of buckets.
  int HashToken(StringPiece token) const;

  // Extracts the charactergram features from the token in a non-unicode-aware
  // way.
  std::vector<int> ExtractCharactergramFeaturesAscii(const Token& token) const;

  // Extracts the charactergram features from the token in a unicode-aware way.
  std::vector<int> ExtractCharactergramFeaturesUnicode(
      const Token& token) const;

 private:
  TokenFeatureExtractorOptions options_;
  std::vector<std::unique_ptr<UniLib::RegexPattern>> regex_patterns_;
  const UniLib& unilib_;
};

}  // namespace libtextclassifier3

#endif  // LIBTEXTCLASSIFIER_UTILS_TOKEN_FEATURE_EXTRACTOR_H_