Improve bigram probability computation for decaying dicts.

Without personalization:
Total words: 1079345, Success Num: 819749, Success Percentage: 75.949%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1754, Bad Failure Percentage: 0.163%
Failures, with auto-correction (F-C): 28463, F-C Percentage: 2.637%
Max Keystrokes: 6074285, Min Keystrokes: 4649326, Keystroke Saving Percentage:23.459%

With current probability computing logic:
Total words: 1079382, Success Num: 838329, Success Percentage: 77.667%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1332, Bad Failure Percentage: 0.123%
Failures, with auto-correction (F-C): 28558, F-C Percentage: 2.646%
Max Keystrokes: 6074503, Min Keystrokes: 4474102, Keystroke Saving Percentage:26.346%
Remove isof files.

With new probability computing logic:
Total words: 1079356, Success Num: 844954, Success Percentage: 78.283%
Bad Failures, with auto-correction (typed word == expected word, output word != expected word): 1306, Bad Failure Percentage: 0.121%
Failures, with auto-correction (F-C): 27214, F-C Percentage: 2.521%
Max Keystrokes: 6074477, Min Keystrokes: 4243021, Keystroke Saving Percentage:30.150%
Remove isof files.

Bug: 16547409
Change-Id: I3d2a49c7aaa2c0f6835c52ef72d22466ee225789

6 files changed, 83 insertions, 98 deletions

diff --git a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.cpp b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.cpp
index 6ed65d921..4c4dfc578 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.cpp
@@ -35,23 +35,15 @@ const char *const HeaderPolicy::EXTENDED_REGION_SIZE_KEY = "EXTENDED_REGION_SIZE
 // count.
 const char *const HeaderPolicy::HAS_HISTORICAL_INFO_KEY = "HAS_HISTORICAL_INFO";
 const char *const HeaderPolicy::LOCALE_KEY = "locale"; // match Java declaration
-const char *const HeaderPolicy::FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP_KEY =
-        "FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP";
 const char *const HeaderPolicy::FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID_KEY =
         "FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID";
-const char *const HeaderPolicy::FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS_KEY =
-        "FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS";
 
 const char *const HeaderPolicy::MAX_UNIGRAM_COUNT_KEY = "MAX_UNIGRAM_COUNT";
 const char *const HeaderPolicy::MAX_BIGRAM_COUNT_KEY = "MAX_BIGRAM_COUNT";
 
 const int HeaderPolicy::DEFAULT_MULTIPLE_WORDS_DEMOTION_RATE = 100;
 const float HeaderPolicy::MULTIPLE_WORD_COST_MULTIPLIER_SCALE = 100.0f;
-const int HeaderPolicy::DEFAULT_FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP = 2;
 const int HeaderPolicy::DEFAULT_FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID = 3;
-// 30 days
-const int HeaderPolicy::DEFAULT_FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS =
-        30 * 24 * 60 * 60;
 
 const int HeaderPolicy::DEFAULT_MAX_UNIGRAM_COUNT = 10000;
 const int HeaderPolicy::DEFAULT_MAX_BIGRAM_COUNT = 10000;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
index daf40d4f9..bc8eaded3 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/header/header_policy.h
@@ -53,15 +53,9 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
                       EXTENDED_REGION_SIZE_KEY, 0 /* defaultValue */)),
               mHasHistoricalInfoOfWords(HeaderReadWriteUtils::readBoolAttributeValue(
                       &mAttributeMap, HAS_HISTORICAL_INFO_KEY, false /* defaultValue */)),
-              mForgettingCurveOccurrencesToLevelUp(HeaderReadWriteUtils::readIntAttributeValue(
-                      &mAttributeMap, FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP_KEY,
-                      DEFAULT_FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP)),
               mForgettingCurveProbabilityValuesTableId(HeaderReadWriteUtils::readIntAttributeValue(
                       &mAttributeMap, FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID_KEY,
                       DEFAULT_FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID)),
-              mForgettingCurveDurationToLevelDown(HeaderReadWriteUtils::readIntAttributeValue(
-                      &mAttributeMap, FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS_KEY,
-                      DEFAULT_FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS)),
               mMaxUnigramCount(HeaderReadWriteUtils::readIntAttributeValue(
                       &mAttributeMap, MAX_UNIGRAM_COUNT_KEY, DEFAULT_MAX_UNIGRAM_COUNT)),
               mMaxBigramCount(HeaderReadWriteUtils::readIntAttributeValue(
@@ -86,15 +80,9 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
               mUnigramCount(0), mBigramCount(0), mExtendedRegionSize(0),
               mHasHistoricalInfoOfWords(HeaderReadWriteUtils::readBoolAttributeValue(
                       &mAttributeMap, HAS_HISTORICAL_INFO_KEY, false /* defaultValue */)),
-              mForgettingCurveOccurrencesToLevelUp(HeaderReadWriteUtils::readIntAttributeValue(
-                      &mAttributeMap, FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP_KEY,
-                      DEFAULT_FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP)),
               mForgettingCurveProbabilityValuesTableId(HeaderReadWriteUtils::readIntAttributeValue(
                       &mAttributeMap, FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID_KEY,
                       DEFAULT_FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID)),
-              mForgettingCurveDurationToLevelDown(HeaderReadWriteUtils::readIntAttributeValue(
-                      &mAttributeMap, FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS_KEY,
-                      DEFAULT_FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS)),
               mMaxUnigramCount(HeaderReadWriteUtils::readIntAttributeValue(
                       &mAttributeMap, MAX_UNIGRAM_COUNT_KEY, DEFAULT_MAX_UNIGRAM_COUNT)),
               mMaxBigramCount(HeaderReadWriteUtils::readIntAttributeValue(
@@ -113,12 +101,8 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
               mUnigramCount(headerPolicy->mUnigramCount), mBigramCount(headerPolicy->mBigramCount),
               mExtendedRegionSize(headerPolicy->mExtendedRegionSize),
               mHasHistoricalInfoOfWords(headerPolicy->mHasHistoricalInfoOfWords),
-              mForgettingCurveOccurrencesToLevelUp(
-                      headerPolicy->mForgettingCurveOccurrencesToLevelUp),
               mForgettingCurveProbabilityValuesTableId(
                       headerPolicy->mForgettingCurveProbabilityValuesTableId),
-              mForgettingCurveDurationToLevelDown(
-                      headerPolicy->mForgettingCurveDurationToLevelDown),
               mMaxUnigramCount(headerPolicy->mMaxUnigramCount),
               mMaxBigramCount(headerPolicy->mMaxBigramCount),
               mCodePointTable(headerPolicy->mCodePointTable) {}
@@ -130,8 +114,7 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
               mRequiresGermanUmlautProcessing(false), mIsDecayingDict(false),
               mDate(0), mLastDecayedTime(0), mUnigramCount(0), mBigramCount(0),
               mExtendedRegionSize(0), mHasHistoricalInfoOfWords(false),
-              mForgettingCurveOccurrencesToLevelUp(0), mForgettingCurveProbabilityValuesTableId(0),
-              mForgettingCurveDurationToLevelDown(0), mMaxUnigramCount(0), mMaxBigramCount(0),
+              mForgettingCurveProbabilityValuesTableId(0), mMaxUnigramCount(0), mMaxBigramCount(0),
               mCodePointTable(nullptr) {}
 
     ~HeaderPolicy() {}
@@ -217,18 +200,10 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
         return &mAttributeMap;
     }
 
-    AK_FORCE_INLINE int getForgettingCurveOccurrencesToLevelUp() const {
-        return mForgettingCurveOccurrencesToLevelUp;
-    }
-
     AK_FORCE_INLINE int getForgettingCurveProbabilityValuesTableId() const {
         return mForgettingCurveProbabilityValuesTableId;
     }
 
-    AK_FORCE_INLINE int getForgettingCurveDurationToLevelDown() const {
-        return mForgettingCurveDurationToLevelDown;
-    }
-
     AK_FORCE_INLINE int getMaxUnigramCount() const {
         return mMaxUnigramCount;
     }
@@ -280,9 +255,7 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
     static const char *const MAX_BIGRAM_COUNT_KEY;
     static const int DEFAULT_MULTIPLE_WORDS_DEMOTION_RATE;
     static const float MULTIPLE_WORD_COST_MULTIPLIER_SCALE;
-    static const int DEFAULT_FORGETTING_CURVE_OCCURRENCES_TO_LEVEL_UP;
     static const int DEFAULT_FORGETTING_CURVE_PROBABILITY_VALUES_TABLE_ID;
-    static const int DEFAULT_FORGETTING_CURVE_DURATION_TO_LEVEL_DOWN_IN_SECONDS;
     static const int DEFAULT_MAX_UNIGRAM_COUNT;
     static const int DEFAULT_MAX_BIGRAM_COUNT;
 
@@ -300,9 +273,7 @@ class HeaderPolicy : public DictionaryHeaderStructurePolicy {
     const int mBigramCount;
     const int mExtendedRegionSize;
     const bool mHasHistoricalInfoOfWords;
-    const int mForgettingCurveOccurrencesToLevelUp;
     const int mForgettingCurveProbabilityValuesTableId;
-    const int mForgettingCurveDurationToLevelDown;
     const int mMaxUnigramCount;
     const int mMaxBigramCount;
     const int *const mCodePointTable;
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.cpp
index 11f7b305f..e0c3af8df 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.cpp
@@ -146,18 +146,15 @@ const WordAttributes Ver4PatriciaTriePolicy::getWordAttributes(const int probabi
 
 int Ver4PatriciaTriePolicy::getProbability(const int unigramProbability,
         const int bigramProbability) const {
-    if (mHeaderPolicy->isDecayingDict()) {
-        // Both probabilities are encoded. Decode them and get probability.
-        return ForgettingCurveUtils::getProbability(unigramProbability, bigramProbability);
-    } else {
-        if (unigramProbability == NOT_A_PROBABILITY) {
-            return NOT_A_PROBABILITY;
-        } else if (bigramProbability == NOT_A_PROBABILITY) {
-            return ProbabilityUtils::backoff(unigramProbability);
-        } else {
-            return bigramProbability;
-        }
+    // In the v4 format, bigramProbability is a conditional probability.
+    const int bigramConditionalProbability = bigramProbability;
+    if (unigramProbability == NOT_A_PROBABILITY) {
+        return NOT_A_PROBABILITY;
     }
+    if (bigramConditionalProbability == NOT_A_PROBABILITY) {
+        return ProbabilityUtils::backoff(unigramProbability);
+    }
+    return bigramConditionalProbability;
 }
 
 int Ver4PatriciaTriePolicy::getProbabilityOfWord(const WordIdArrayView prevWordIds,
@@ -170,37 +167,66 @@ int Ver4PatriciaTriePolicy::getProbabilityOfWord(const WordIdArrayView prevWordI
     if (ptNodeParams.isDeleted() || ptNodeParams.isBlacklisted() || ptNodeParams.isNotAWord()) {
         return NOT_A_PROBABILITY;
     }
-    if (!prevWordIds.empty()) {
-        const int bigramsPosition = getBigramsPositionOfPtNode(
-                getTerminalPtNodePosFromWordId(prevWordIds[0]));
-        BinaryDictionaryBigramsIterator bigramsIt(&mBigramPolicy, bigramsPosition);
-        while (bigramsIt.hasNext()) {
-            bigramsIt.next();
-            if (bigramsIt.getBigramPos() == ptNodePos
-                    && bigramsIt.getProbability() != NOT_A_PROBABILITY) {
-                return getProbability(ptNodeParams.getProbability(), bigramsIt.getProbability());
-            }
-        }
+    if (prevWordIds.empty()) {
+        return getProbability(ptNodeParams.getProbability(), NOT_A_PROBABILITY);
+    }
+    if (prevWordIds[0] == NOT_A_WORD_ID) {
         return NOT_A_PROBABILITY;
     }
-    return getProbability(ptNodeParams.getProbability(), NOT_A_PROBABILITY);
+    const PtNodeParams prevWordPtNodeParams =
+            mNodeReader.fetchPtNodeParamsInBufferFromPtNodePos(prevWordIds[0]);
+    if (prevWordPtNodeParams.isDeleted()) {
+        return getProbability(ptNodeParams.getProbability(), NOT_A_PROBABILITY);
+    }
+    const int bigramsPosition = mBuffers->getBigramDictContent()->getBigramListHeadPos(
+            prevWordPtNodeParams.getTerminalId());
+    BinaryDictionaryBigramsIterator bigramsIt(&mBigramPolicy, bigramsPosition);
+    while (bigramsIt.hasNext()) {
+        bigramsIt.next();
+        if (bigramsIt.getBigramPos() == ptNodePos
+                && bigramsIt.getProbability() != NOT_A_PROBABILITY) {
+            const int bigramConditionalProbability = getBigramConditionalProbability(
+                    prevWordPtNodeParams.getProbability(), bigramsIt.getProbability());
+            return getProbability(ptNodeParams.getProbability(), bigramConditionalProbability);
+        }
+    }
+    return NOT_A_PROBABILITY;
 }
 
 void Ver4PatriciaTriePolicy::iterateNgramEntries(const WordIdArrayView prevWordIds,
         NgramListener *const listener) const {
-    if (prevWordIds.empty()) {
+    if (prevWordIds.firstOrDefault(NOT_A_DICT_POS) == NOT_A_DICT_POS) {
+        return;
+    }
+    const PtNodeParams prevWordPtNodeParams =
+            mNodeReader.fetchPtNodeParamsInBufferFromPtNodePos(prevWordIds[0]);
+    if (prevWordPtNodeParams.isDeleted()) {
         return;
     }
-    const int bigramsPosition = getBigramsPositionOfPtNode(
-            getTerminalPtNodePosFromWordId(prevWordIds[0]));
+    const int bigramsPosition = mBuffers->getBigramDictContent()->getBigramListHeadPos(
+            prevWordPtNodeParams.getTerminalId());
     BinaryDictionaryBigramsIterator bigramsIt(&mBigramPolicy, bigramsPosition);
     while (bigramsIt.hasNext()) {
         bigramsIt.next();
-        listener->onVisitEntry(bigramsIt.getProbability(),
+        const int bigramConditionalProbability = getBigramConditionalProbability(
+                prevWordPtNodeParams.getProbability(), bigramsIt.getProbability());
+        listener->onVisitEntry(bigramConditionalProbability,
                 getWordIdFromTerminalPtNodePos(bigramsIt.getBigramPos()));
     }
 }
 
+int Ver4PatriciaTriePolicy::getBigramConditionalProbability(const int prevWordUnigramProbability,
+        const int bigramProbability) const {
+    if (mHeaderPolicy->hasHistoricalInfoOfWords()) {
+        // Calculate conditional probability.
+        return std::min(MAX_PROBABILITY - prevWordUnigramProbability + bigramProbability,
+                MAX_PROBABILITY);
+    } else {
+        // bigramProbability is a conditional probability.
+        return bigramProbability;
+    }
+}
+
 BinaryDictionaryShortcutIterator Ver4PatriciaTriePolicy::getShortcutIterator(
         const int wordId) const {
     const int shortcutPos = getShortcutPositionOfPtNode(getTerminalPtNodePosFromWordId(wordId));
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.h b/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.h
index 995d7764f..b82563e61 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/backward/v402/ver4_patricia_trie_policy.h
@@ -174,6 +174,8 @@ class Ver4PatriciaTriePolicy : public DictionaryStructureWithBufferPolicy {
     int getTerminalPtNodePosFromWordId(const int wordId) const;
     const WordAttributes getWordAttributes(const int probability,
             const PtNodeParams &ptNodeParams) const;
+    int getBigramConditionalProbability(const int prevWordUnigramProbability,
+            const int bigramProbability) const;
 };
 } // namespace v402
 } // namespace backward
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
index af4bc186a..e5ef2abf8 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
@@ -29,10 +29,14 @@ namespace latinime {
 const int ForgettingCurveUtils::MULTIPLIER_TWO_IN_PROBABILITY_SCALE = 8;
 const int ForgettingCurveUtils::DECAY_INTERVAL_SECONDS = 2 * 60 * 60;
 
-const int ForgettingCurveUtils::MAX_LEVEL = 3;
-const int ForgettingCurveUtils::MIN_VISIBLE_LEVEL = 1;
-const int ForgettingCurveUtils::MAX_ELAPSED_TIME_STEP_COUNT = 15;
-const int ForgettingCurveUtils::DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD = 14;
+const int ForgettingCurveUtils::MAX_LEVEL = 15;
+const int ForgettingCurveUtils::MIN_VISIBLE_LEVEL = 2;
+const int ForgettingCurveUtils::MAX_ELAPSED_TIME_STEP_COUNT = 31;
+const int ForgettingCurveUtils::DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD = 30;
+const int ForgettingCurveUtils::OCCURRENCES_TO_RAISE_THE_LEVEL = 1;
+// TODO: Evaluate whether this should be 7.5 days.
+// 15 days
+const int ForgettingCurveUtils::DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS = 15 * 24 * 60 * 60;
 
 const float ForgettingCurveUtils::UNIGRAM_COUNT_HARD_LIMIT_WEIGHT = 1.2;
 const float ForgettingCurveUtils::BIGRAM_COUNT_HARD_LIMIT_WEIGHT = 1.2;
@@ -54,19 +58,23 @@ const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityT
             || (originalHistoricalInfo->getLevel() == newHistoricalInfo->getLevel()
                     && originalHistoricalInfo->getCount() < newHistoricalInfo->getCount())) {
         // Initial information.
+        int count = newHistoricalInfo->getCount();
+        if (count >= OCCURRENCES_TO_RAISE_THE_LEVEL) {
+            const int level = clampToValidLevelRange(newHistoricalInfo->getLevel() + 1);
+            return HistoricalInfo(timestamp, level, 0 /* count */);
+        }
         const int level = clampToValidLevelRange(newHistoricalInfo->getLevel());
-        const int count = clampToValidCountRange(newHistoricalInfo->getCount(), headerPolicy);
-        return HistoricalInfo(timestamp, level, count);
+        return HistoricalInfo(timestamp, level, clampToValidCountRange(count, headerPolicy));
     } else {
         const int updatedCount = originalHistoricalInfo->getCount() + 1;
-        if (updatedCount >= headerPolicy->getForgettingCurveOccurrencesToLevelUp()) {
+        if (updatedCount >= OCCURRENCES_TO_RAISE_THE_LEVEL) {
             // The count exceeds the max value the level can be incremented.
             if (originalHistoricalInfo->getLevel() >= MAX_LEVEL) {
                 // The level is already max.
                 return HistoricalInfo(timestamp,
                         originalHistoricalInfo->getLevel(), originalHistoricalInfo->getCount());
             } else {
-                // Level up.
+                // Raise the level.
                 return HistoricalInfo(timestamp,
                         originalHistoricalInfo->getLevel() + 1, 0 /* count */);
             }
@@ -79,31 +87,18 @@ const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityT
 /* static */ int ForgettingCurveUtils::decodeProbability(
         const HistoricalInfo *const historicalInfo, const HeaderPolicy *const headerPolicy) {
     const int elapsedTimeStepCount = getElapsedTimeStepCount(historicalInfo->getTimestamp(),
-            headerPolicy->getForgettingCurveDurationToLevelDown());
+            DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS);
     return sProbabilityTable.getProbability(
             headerPolicy->getForgettingCurveProbabilityValuesTableId(),
             clampToValidLevelRange(historicalInfo->getLevel()),
             clampToValidTimeStepCountRange(elapsedTimeStepCount));
 }
 
-/* static */ int ForgettingCurveUtils::getProbability(const int unigramProbability,
-        const int bigramProbability) {
-    if (unigramProbability == NOT_A_PROBABILITY) {
-        return NOT_A_PROBABILITY;
-    } else if (bigramProbability == NOT_A_PROBABILITY) {
-        return std::min(backoff(unigramProbability), MAX_PROBABILITY);
-    } else {
-        // TODO: Investigate better way to handle bigram probability.
-        return std::min(std::max(unigramProbability,
-                bigramProbability + MULTIPLIER_TWO_IN_PROBABILITY_SCALE), MAX_PROBABILITY);
-    }
-}
-
 /* static */ bool ForgettingCurveUtils::needsToKeep(const HistoricalInfo *const historicalInfo,
         const HeaderPolicy *const headerPolicy) {
     return historicalInfo->getLevel() > 0
             || getElapsedTimeStepCount(historicalInfo->getTimestamp(),
-                    headerPolicy->getForgettingCurveDurationToLevelDown())
+                    DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS)
                             < DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
 }
 
@@ -113,14 +108,14 @@ const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityT
     if (originalHistoricalInfo->getTimestamp() == NOT_A_TIMESTAMP) {
         return HistoricalInfo();
     }
-    const int durationToLevelDownInSeconds = headerPolicy->getForgettingCurveDurationToLevelDown();
+    const int durationToLevelDownInSeconds = DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS;
     const int elapsedTimeStep = getElapsedTimeStepCount(
             originalHistoricalInfo->getTimestamp(), durationToLevelDownInSeconds);
     if (elapsedTimeStep <= MAX_ELAPSED_TIME_STEP_COUNT) {
         // No need to update historical info.
         return *originalHistoricalInfo;
     }
-    // Level down.
+    // Lower the level.
     const int maxLevelDownAmonut = elapsedTimeStep / (MAX_ELAPSED_TIME_STEP_COUNT + 1);
     const int levelDownAmount = (maxLevelDownAmonut >= originalHistoricalInfo->getLevel()) ?
             originalHistoricalInfo->getLevel() : maxLevelDownAmonut;
@@ -170,7 +165,7 @@ const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityT
 
 /* static */ int ForgettingCurveUtils::clampToValidCountRange(const int count,
         const HeaderPolicy *const headerPolicy) {
-    return std::min(std::max(count, 0), headerPolicy->getForgettingCurveOccurrencesToLevelUp() - 1);
+    return std::min(std::max(count, 0), OCCURRENCES_TO_RAISE_THE_LEVEL - 1);
 }
 
 /* static */ int ForgettingCurveUtils::clampToValidLevelRange(const int level) {
@@ -187,9 +182,9 @@ const int ForgettingCurveUtils::ProbabilityTable::MODEST_PROBABILITY_TABLE_ID =
 const int ForgettingCurveUtils::ProbabilityTable::STRONG_PROBABILITY_TABLE_ID = 2;
 const int ForgettingCurveUtils::ProbabilityTable::AGGRESSIVE_PROBABILITY_TABLE_ID = 3;
 const int ForgettingCurveUtils::ProbabilityTable::WEAK_MAX_PROBABILITY = 127;
-const int ForgettingCurveUtils::ProbabilityTable::MODEST_BASE_PROBABILITY = 32;
-const int ForgettingCurveUtils::ProbabilityTable::STRONG_BASE_PROBABILITY = 35;
-const int ForgettingCurveUtils::ProbabilityTable::AGGRESSIVE_BASE_PROBABILITY = 40;
+const int ForgettingCurveUtils::ProbabilityTable::MODEST_BASE_PROBABILITY = 8;
+const int ForgettingCurveUtils::ProbabilityTable::STRONG_BASE_PROBABILITY = 9;
+const int ForgettingCurveUtils::ProbabilityTable::AGGRESSIVE_BASE_PROBABILITY = 10;
 
 
 ForgettingCurveUtils::ProbabilityTable::ProbabilityTable() : mTables() {
@@ -202,7 +197,7 @@ ForgettingCurveUtils::ProbabilityTable::ProbabilityTable() : mTables() {
             const float endProbability = getBaseProbabilityForLevel(tableId, level - 1);
             for (int timeStepCount = 0; timeStepCount <= MAX_ELAPSED_TIME_STEP_COUNT;
                     ++timeStepCount) {
-                if (level == 0) {
+                if (level < MIN_VISIBLE_LEVEL) {
                     mTables[tableId][level][timeStepCount] = NOT_A_PROBABILITY;
                     continue;
                 }
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
index 10abb405a..ccbc4a98d 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
@@ -39,9 +39,6 @@ class ForgettingCurveUtils {
     static int decodeProbability(const HistoricalInfo *const historicalInfo,
             const HeaderPolicy *const headerPolicy);
 
-    static int getProbability(const int encodedUnigramProbability,
-            const int encodedBigramProbability);
-
     static bool needsToKeep(const HistoricalInfo *const historicalInfo,
             const HeaderPolicy *const headerPolicy);
 
@@ -101,6 +98,8 @@ class ForgettingCurveUtils {
     static const int MIN_VISIBLE_LEVEL;
     static const int MAX_ELAPSED_TIME_STEP_COUNT;
     static const int DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
+    static const int OCCURRENCES_TO_RAISE_THE_LEVEL;
+    static const int DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS;
 
     static const float UNIGRAM_COUNT_HARD_LIMIT_WEIGHT;
     static const float BIGRAM_COUNT_HARD_LIMIT_WEIGHT;