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      1 /*
      2  * Copyright (C) 2014, The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *     http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #ifndef LATINIME_DYNAMIC_LANGUAGE_MODEL_PROBABILITY_UTILS_H
     18 #define LATINIME_DYNAMIC_LANGUAGE_MODEL_PROBABILITY_UTILS_H
     19 
     20 #include <algorithm>
     21 
     22 #include "defines.h"
     23 #include "dictionary/property/historical_info.h"
     24 #include "utils/ngram_utils.h"
     25 #include "utils/time_keeper.h"
     26 
     27 namespace latinime {
     28 
     29 class DynamicLanguageModelProbabilityUtils {
     30  public:
     31     static float computeRawProbabilityFromCounts(const int count, const int contextCount,
     32             const NgramType ngramType) {
     33         const int minCount = ASSUMED_MIN_COUNTS[static_cast<int>(ngramType)];
     34         return static_cast<float>(count) / static_cast<float>(std::max(contextCount, minCount));
     35     }
     36 
     37     static float backoff(const int ngramProbability, const NgramType ngramType) {
     38         const int probability =
     39                 ngramProbability + ENCODED_BACKOFF_WEIGHTS[static_cast<int>(ngramType)];
     40         return std::min(std::max(probability, NOT_A_PROBABILITY), MAX_PROBABILITY);
     41     }
     42 
     43     static int getDecayedProbability(const int probability, const HistoricalInfo historicalInfo) {
     44         const int elapsedTime = TimeKeeper::peekCurrentTime() - historicalInfo.getTimestamp();
     45         if (elapsedTime < 0) {
     46             AKLOGE("The elapsed time is negatime value. Timestamp overflow?");
     47             return NOT_A_PROBABILITY;
     48         }
     49         // TODO: Improve this logic.
     50         // We don't modify probability depending on the elapsed time.
     51         return probability;
     52     }
     53 
     54     static int shouldRemoveEntryDuringGC(const HistoricalInfo historicalInfo) {
     55         // TODO: Improve this logic.
     56         const int elapsedTime = TimeKeeper::peekCurrentTime() - historicalInfo.getTimestamp();
     57         return elapsedTime > DURATION_TO_DISCARD_ENTRY_IN_SECONDS;
     58     }
     59 
     60     static int getPriorityToPreventFromEviction(const HistoricalInfo historicalInfo) {
     61         // TODO: Improve this logic.
     62         // More recently input entries get higher priority.
     63         return historicalInfo.getTimestamp();
     64     }
     65 
     66 private:
     67     DISALLOW_IMPLICIT_CONSTRUCTORS(DynamicLanguageModelProbabilityUtils);
     68 
     69     static_assert(MAX_PREV_WORD_COUNT_FOR_N_GRAM <= 3, "Max supported Ngram is Quadgram.");
     70 
     71     static const int ASSUMED_MIN_COUNTS[];
     72     static const int ENCODED_BACKOFF_WEIGHTS[];
     73     static const int DURATION_TO_DISCARD_ENTRY_IN_SECONDS;
     74 };
     75 
     76 } // namespace latinime
     77 #endif /* LATINIME_DYNAMIC_LANGUAGE_MODEL_PROBABILITY_UTILS_H */
     78