1 files changed, 234 insertions, 0 deletions
diff --git a/native/jni/src/dictionary/utils/forgetting_curve_utils.cpp b/native/jni/src/dictionary/utils/forgetting_curve_utils.cpp
new file mode 100644
index 000000000..d79ed911b
--- /dev/null
+++ b/native/jni/src/dictionary/utils/forgetting_curve_utils.cpp
@@ -0,0 +1,234 @@
+/*
+ * Copyright (C) 2013, 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.
+ */
+
+#include "dictionary/utils/forgetting_curve_utils.h"
+
+#include <algorithm>
+#include <cmath>
+#include <stdlib.h>
+
+#include "dictionary/header/header_policy.h"
+#include "dictionary/utils/probability_utils.h"
+#include "utils/time_keeper.h"
+
+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 = 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::ENTRY_COUNT_HARD_LIMIT_WEIGHT = 1.2;
+
+const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityTable;
+
+// TODO: Revise the logic to decide the initial probability depending on the given probability.
+/* static */ const HistoricalInfo ForgettingCurveUtils::createUpdatedHistoricalInfo(
+        const HistoricalInfo *const originalHistoricalInfo, const int newProbability,
+        const HistoricalInfo *const newHistoricalInfo, const HeaderPolicy *const headerPolicy) {
+    const int timestamp = newHistoricalInfo->getTimestamp();
+    if (newProbability != NOT_A_PROBABILITY && originalHistoricalInfo->getLevel() == 0) {
+        // Add entry as a valid word.
+        const int level = clampToVisibleEntryLevelRange(newHistoricalInfo->getLevel());
+        const int count = clampToValidCountRange(newHistoricalInfo->getCount(), headerPolicy);
+        return HistoricalInfo(timestamp, level, count);
+    } else if (!originalHistoricalInfo->isValid()
+            || originalHistoricalInfo->getLevel() < newHistoricalInfo->getLevel()
+            || (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());
+        return HistoricalInfo(timestamp, level, clampToValidCountRange(count, headerPolicy));
+    } else {
+        const int updatedCount = originalHistoricalInfo->getCount() + 1;
+        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 {
+                // Raise the level.
+                return HistoricalInfo(timestamp,
+                        originalHistoricalInfo->getLevel() + 1, 0 /* count */);
+            }
+        } else {
+            return HistoricalInfo(timestamp, originalHistoricalInfo->getLevel(), updatedCount);
+        }
+    }
+}
+
+/* static */ int ForgettingCurveUtils::decodeProbability(
+        const HistoricalInfo *const historicalInfo, const HeaderPolicy *const headerPolicy) {
+    const int elapsedTimeStepCount = getElapsedTimeStepCount(historicalInfo->getTimestamp(),
+            DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS);
+    return sProbabilityTable.getProbability(
+            headerPolicy->getForgettingCurveProbabilityValuesTableId(),
+            clampToValidLevelRange(historicalInfo->getLevel()),
+            clampToValidTimeStepCountRange(elapsedTimeStepCount));
+}
+
+/* static */ bool ForgettingCurveUtils::needsToKeep(const HistoricalInfo *const historicalInfo,
+        const HeaderPolicy *const headerPolicy) {
+    return historicalInfo->getLevel() > 0
+            || getElapsedTimeStepCount(historicalInfo->getTimestamp(),
+                    DURATION_TO_LOWER_THE_LEVEL_IN_SECONDS)
+                            < DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
+}
+
+/* static */ const HistoricalInfo ForgettingCurveUtils::createHistoricalInfoToSave(
+        const HistoricalInfo *const originalHistoricalInfo,
+        const HeaderPolicy *const headerPolicy) {
+    if (originalHistoricalInfo->getTimestamp() == NOT_A_TIMESTAMP) {
+        return HistoricalInfo();
+    }
+    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;
+    }
+    // Lower the level.
+    const int maxLevelDownAmonut = elapsedTimeStep / (MAX_ELAPSED_TIME_STEP_COUNT + 1);
+    const int levelDownAmount = (maxLevelDownAmonut >= originalHistoricalInfo->getLevel()) ?
+            originalHistoricalInfo->getLevel() : maxLevelDownAmonut;
+    const int adjustedTimestampInSeconds = originalHistoricalInfo->getTimestamp() +
+            levelDownAmount * durationToLevelDownInSeconds;
+    return HistoricalInfo(adjustedTimestampInSeconds,
+            originalHistoricalInfo->getLevel() - levelDownAmount, 0 /* count */);
+}
+
+/* static */ bool ForgettingCurveUtils::needsToDecay(const bool mindsBlockByDecay,
+        const EntryCounts &entryCounts, const HeaderPolicy *const headerPolicy) {
+    const EntryCounts &maxNgramCounts = headerPolicy->getMaxNgramCounts();
+    for (const auto ngramType : AllNgramTypes::ASCENDING) {
+        if (entryCounts.getNgramCount(ngramType)
+                >= getEntryCountHardLimit(maxNgramCounts.getNgramCount(ngramType))) {
+            // Unigram count exceeds the limit.
+            return true;
+        }
+    }
+    if (mindsBlockByDecay) {
+        return false;
+    }
+    if (headerPolicy->getLastDecayedTime() + DECAY_INTERVAL_SECONDS
+            < TimeKeeper::peekCurrentTime()) {
+        // Time to decay.
+        return true;
+    }
+    return false;
+}
+
+// See comments in ProbabilityUtils::backoff().
+/* static */ int ForgettingCurveUtils::backoff(const int unigramProbability) {
+    // See TODO comments in ForgettingCurveUtils::getProbability().
+    return unigramProbability;
+}
+
+/* static */ int ForgettingCurveUtils::getElapsedTimeStepCount(const int timestamp,
+        const int durationToLevelDownInSeconds) {
+    const int elapsedTimeInSeconds = TimeKeeper::peekCurrentTime() - timestamp;
+    const int timeStepDurationInSeconds =
+            durationToLevelDownInSeconds / (MAX_ELAPSED_TIME_STEP_COUNT + 1);
+    return elapsedTimeInSeconds / timeStepDurationInSeconds;
+}
+
+/* static */ int ForgettingCurveUtils::clampToVisibleEntryLevelRange(const int level) {
+    return std::min(std::max(level, MIN_VISIBLE_LEVEL), MAX_LEVEL);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidCountRange(const int count,
+        const HeaderPolicy *const headerPolicy) {
+    return std::min(std::max(count, 0), OCCURRENCES_TO_RAISE_THE_LEVEL - 1);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidLevelRange(const int level) {
+    return std::min(std::max(level, 0), MAX_LEVEL);
+}
+
+/* static */ int ForgettingCurveUtils::clampToValidTimeStepCountRange(const int timeStepCount) {
+    return std::min(std::max(timeStepCount, 0), MAX_ELAPSED_TIME_STEP_COUNT);
+}
+
+const int ForgettingCurveUtils::ProbabilityTable::PROBABILITY_TABLE_COUNT = 4;
+const int ForgettingCurveUtils::ProbabilityTable::WEAK_PROBABILITY_TABLE_ID = 0;
+const int ForgettingCurveUtils::ProbabilityTable::MODEST_PROBABILITY_TABLE_ID = 1;
+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 = 8;
+const int ForgettingCurveUtils::ProbabilityTable::STRONG_BASE_PROBABILITY = 9;
+const int ForgettingCurveUtils::ProbabilityTable::AGGRESSIVE_BASE_PROBABILITY = 10;
+
+
+ForgettingCurveUtils::ProbabilityTable::ProbabilityTable() : mTables() {
+    mTables.resize(PROBABILITY_TABLE_COUNT);
+    for (int tableId = 0; tableId < PROBABILITY_TABLE_COUNT; ++tableId) {
+        mTables[tableId].resize(MAX_LEVEL + 1);
+        for (int level = 0; level <= MAX_LEVEL; ++level) {
+            mTables[tableId][level].resize(MAX_ELAPSED_TIME_STEP_COUNT + 1);
+            const float initialProbability = getBaseProbabilityForLevel(tableId, level);
+            const float endProbability = getBaseProbabilityForLevel(tableId, level - 1);
+            for (int timeStepCount = 0; timeStepCount <= MAX_ELAPSED_TIME_STEP_COUNT;
+                    ++timeStepCount) {
+                if (level < MIN_VISIBLE_LEVEL) {
+                    mTables[tableId][level][timeStepCount] = NOT_A_PROBABILITY;
+                    continue;
+                }
+                const float probability = initialProbability
+                        * powf(initialProbability / endProbability,
+                                -1.0f * static_cast<float>(timeStepCount)
+                                        / static_cast<float>(MAX_ELAPSED_TIME_STEP_COUNT + 1));
+                mTables[tableId][level][timeStepCount] =
+                        std::min(std::max(static_cast<int>(probability), 1), MAX_PROBABILITY);
+            }
+        }
+    }
+}
+
+/* static */ int ForgettingCurveUtils::ProbabilityTable::getBaseProbabilityForLevel(
+        const int tableId, const int level) {
+    if (tableId == WEAK_PROBABILITY_TABLE_ID) {
+        // Max probability is 127.
+        return static_cast<float>(WEAK_MAX_PROBABILITY / (1 << (MAX_LEVEL - level)));
+    } else if (tableId == MODEST_PROBABILITY_TABLE_ID) {
+        // Max probability is 128.
+        return static_cast<float>(MODEST_BASE_PROBABILITY * (level + 1));
+    } else if (tableId == STRONG_PROBABILITY_TABLE_ID) {
+        // Max probability is 140.
+        return static_cast<float>(STRONG_BASE_PROBABILITY * (level + 1));
+    } else if (tableId == AGGRESSIVE_PROBABILITY_TABLE_ID) {
+        // Max probability is 160.
+        return static_cast<float>(AGGRESSIVE_BASE_PROBABILITY * (level + 1));
+    } else {
+        return NOT_A_PROBABILITY;
+    }
+}
+
+} // namespace latinime