Forget user history

Bug: 4192129

Change-Id: Ic98398e5383093a1c24373849eadc48fc4d3626f

8 files changed, 333 insertions, 100 deletions

diff --git a/java/src/com/android/inputmethod/latin/ContactsDictionary.java b/java/src/com/android/inputmethod/latin/ContactsDictionary.java
index c9b8d6eb1..cbfbd0ec8 100644
--- a/java/src/com/android/inputmethod/latin/ContactsDictionary.java
+++ b/java/src/com/android/inputmethod/latin/ContactsDictionary.java
@@ -159,7 +159,7 @@ public class ContactsDictionary extends ExpandableDictionary {
                                     super.addWord(word, null /* shortcut */,
                                             FREQUENCY_FOR_CONTACTS);
                                     if (!TextUtils.isEmpty(prevWord)) {
-                                        super.setBigram(prevWord, word,
+                                        super.setBigramAndGetFrequency(prevWord, word,
                                                 FREQUENCY_FOR_CONTACTS_BIGRAM);
                                     }
                                     prevWord = word;
diff --git a/java/src/com/android/inputmethod/latin/ExpandableDictionary.java b/java/src/com/android/inputmethod/latin/ExpandableDictionary.java
index 6c457afd2..358cd4d4d 100644
--- a/java/src/com/android/inputmethod/latin/ExpandableDictionary.java
+++ b/java/src/com/android/inputmethod/latin/ExpandableDictionary.java
@@ -21,6 +21,7 @@ import android.content.Context;
 import com.android.inputmethod.keyboard.KeyDetector;
 import com.android.inputmethod.keyboard.Keyboard;
 import com.android.inputmethod.keyboard.ProximityInfo;
+import com.android.inputmethod.latin.UserHistoryForgettingCurveUtils.ForgettingCurveParams;
 
 import java.util.ArrayList;
 import java.util.LinkedList;
@@ -80,31 +81,73 @@ public class ExpandableDictionary extends Dictionary {
         }
     }
 
-    private static class NextWord {
-        public final Node mWord;
-        private int mFrequency;
+    protected interface NextWord {
+        public Node getWordNode();
+        public int getFrequency();
+        /** FcValue is a bit set */
+        public int getFcValue();
+        public int notifyTypedAgainAndGetFrequency();
+    }
 
-        public NextWord(Node word, int frequency) {
+    private static class NextStaticWord implements NextWord {
+        public final Node mWord;
+        private final int mFrequency;
+        public NextStaticWord(Node word, int frequency) {
             mWord = word;
             mFrequency = frequency;
         }
 
+        @Override
+        public Node getWordNode() {
+            return mWord;
+        }
+
+        @Override
         public int getFrequency() {
             return mFrequency;
         }
 
-        public int setFrequency(int freq) {
-            mFrequency = freq;
+        @Override
+        public int getFcValue() {
             return mFrequency;
         }
 
-        public int addFrequency(int add) {
-            mFrequency += add;
-            if (mFrequency > BIGRAM_MAX_FREQUENCY) mFrequency = BIGRAM_MAX_FREQUENCY;
+        @Override
+        public int notifyTypedAgainAndGetFrequency() {
             return mFrequency;
         }
     }
 
+    private static class NextHistoryWord implements NextWord {
+        public final Node mWord;
+        public final ForgettingCurveParams mFcp;
+
+        public NextHistoryWord(Node word, ForgettingCurveParams fcp) {
+            mWord = word;
+            mFcp = fcp;
+        }
+
+        @Override
+        public Node getWordNode() {
+            return mWord;
+        }
+
+        @Override
+        public int getFrequency() {
+            return mFcp.getFrequency();
+        }
+
+        @Override
+        public int getFcValue() {
+            return mFcp.getFc();
+        }
+
+        @Override
+        public int notifyTypedAgainAndGetFrequency() {
+            return mFcp.notifyTypedAgainAndGetFrequency();
+        }
+    }
+
     private NodeArray mRoots;
 
     private int[][] mCodes;
@@ -183,7 +226,7 @@ public class ExpandableDictionary extends Dictionary {
             childNode.mShortcutOnly = isShortcutOnly;
             children.add(childNode);
         }
-        if (wordLength == depth + 1) {
+        if (wordLength == depth + 1 && shortcutTarget != null) {
             // Terminate this word
             childNode.mTerminal = true;
             if (isShortcutOnly) {
@@ -221,7 +264,7 @@ public class ExpandableDictionary extends Dictionary {
 
     protected final void getWordsInner(final WordComposer codes,
             final CharSequence prevWordForBigrams, final WordCallback callback,
-            @SuppressWarnings("unused") final ProximityInfo proximityInfo) {
+            final ProximityInfo proximityInfo) {
         mInputLength = codes.size();
         if (mCodes.length < mInputLength) mCodes = new int[mInputLength][];
         final int[] xCoordinates = codes.getXCoordinates();
@@ -265,13 +308,13 @@ public class ExpandableDictionary extends Dictionary {
         // Refer to addOrSetBigram() about word1.toLowerCase()
         final Node firstWord = searchWord(mRoots, word1.toLowerCase(), 0, null);
         final Node secondWord = searchWord(mRoots, word2, 0, null);
-        LinkedList<NextWord> bigram = firstWord.mNGrams;
+        LinkedList<NextWord> bigrams = firstWord.mNGrams;
         NextWord bigramNode = null;
-        if (bigram == null || bigram.size() == 0) {
+        if (bigrams == null || bigrams.size() == 0) {
             return false;
         } else {
-            for (NextWord nw : bigram) {
-                if (nw.mWord == secondWord) {
+            for (NextWord nw : bigrams) {
+                if (nw.getWordNode() == secondWord) {
                     bigramNode = nw;
                     break;
                 }
@@ -280,7 +323,7 @@ public class ExpandableDictionary extends Dictionary {
         if (bigramNode == null) {
             return false;
         }
-        return bigram.remove(bigramNode);
+        return bigrams.remove(bigramNode);
     }
 
     /**
@@ -292,6 +335,23 @@ public class ExpandableDictionary extends Dictionary {
         return (node == null) ? -1 : node.mFrequency;
     }
 
+    protected NextWord getBigramWord(String word1, String word2) {
+        // Refer to addOrSetBigram() about word1.toLowerCase()
+        final Node firstWord = searchWord(mRoots, word1.toLowerCase(), 0, null);
+        final Node secondWord = searchWord(mRoots, word2, 0, null);
+        LinkedList<NextWord> bigrams = firstWord.mNGrams;
+        if (bigrams == null || bigrams.size() == 0) {
+            return null;
+        } else {
+            for (NextWord nw : bigrams) {
+                if (nw.getWordNode() == secondWord) {
+                    return nw;
+                }
+            }
+        }
+        return null;
+    }
+
     private static int computeSkippedWordFinalFreq(int freq, int snr, int inputLength) {
         // The computation itself makes sense for >= 2, but the == 2 case returns 0
         // anyway so we may as well test against 3 instead and return the constant
@@ -445,43 +505,45 @@ public class ExpandableDictionary extends Dictionary {
         }
     }
 
-    protected int setBigram(String word1, String word2, int frequency) {
-        return addOrSetBigram(word1, word2, frequency, false);
+    public int setBigramAndGetFrequency(String word1, String word2, int frequency) {
+        return setBigramAndGetFrequency(word1, word2, frequency, null /* unused */);
     }
 
-    protected int addBigram(String word1, String word2, int frequency) {
-        return addOrSetBigram(word1, word2, frequency, true);
+    public int setBigramAndGetFrequency(String word1, String word2, ForgettingCurveParams fcp) {
+        return setBigramAndGetFrequency(word1, word2, 0 /* unused */, fcp);
     }
 
     /**
      * Adds bigrams to the in-memory trie structure that is being used to retrieve any word
      * @param frequency frequency for this bigram
      * @param addFrequency if true, it adds to current frequency, else it overwrites the old value
-     * @return returns the final frequency
+     * @return returns the final bigram frequency
      */
-    private int addOrSetBigram(String word1, String word2, int frequency, boolean addFrequency) {
+    private int setBigramAndGetFrequency(
+            String word1, String word2, int frequency, ForgettingCurveParams fcp) {
         // We don't want results to be different according to case of the looked up left hand side
         // word. We do want however to return the correct case for the right hand side.
         // So we want to squash the case of the left hand side, and preserve that of the right
         // hand side word.
         Node firstWord = searchWord(mRoots, word1.toLowerCase(), 0, null);
         Node secondWord = searchWord(mRoots, word2, 0, null);
-        LinkedList<NextWord> bigram = firstWord.mNGrams;
-        if (bigram == null || bigram.size() == 0) {
+        LinkedList<NextWord> bigrams = firstWord.mNGrams;
+        if (bigrams == null || bigrams.size() == 0) {
             firstWord.mNGrams = new LinkedList<NextWord>();
-            bigram = firstWord.mNGrams;
+            bigrams = firstWord.mNGrams;
         } else {
-            for (NextWord nw : bigram) {
-                if (nw.mWord == secondWord) {
-                    if (addFrequency) {
-                        return nw.addFrequency(frequency);
-                    } else {
-                        return nw.setFrequency(frequency);
-                    }
+            for (NextWord nw : bigrams) {
+                if (nw.getWordNode() == secondWord) {
+                    return nw.notifyTypedAgainAndGetFrequency();
                 }
             }
         }
-        firstWord.mNGrams.add(new NextWord(secondWord, frequency));
+        if (fcp != null) {
+            // history
+            firstWord.mNGrams.add(new NextHistoryWord(secondWord, fcp));
+        } else {
+            firstWord.mNGrams.add(new NextStaticWord(secondWord, frequency));
+        }
         return frequency;
     }
 
@@ -580,7 +642,7 @@ public class ExpandableDictionary extends Dictionary {
         Node node;
         int freq;
         for (NextWord nextWord : terminalNodes) {
-            node = nextWord.mWord;
+            node = nextWord.getWordNode();
             freq = nextWord.getFrequency();
             int index = BinaryDictionary.MAX_WORD_LENGTH;
             do {
@@ -589,8 +651,10 @@ public class ExpandableDictionary extends Dictionary {
                 node = node.mParent;
             } while (node != null);
 
-            callback.addWord(mLookedUpString, index, BinaryDictionary.MAX_WORD_LENGTH - index,
-                    freq, mDicTypeId, Dictionary.BIGRAM);
+            if (freq >= 0) {
+                callback.addWord(mLookedUpString, index, BinaryDictionary.MAX_WORD_LENGTH - index,
+                        freq, mDicTypeId, Dictionary.BIGRAM);
+            }
         }
     }
 
diff --git a/java/src/com/android/inputmethod/latin/LatinIME.java b/java/src/com/android/inputmethod/latin/LatinIME.java
index c58549497..e9117e29a 100644
--- a/java/src/com/android/inputmethod/latin/LatinIME.java
+++ b/java/src/com/android/inputmethod/latin/LatinIME.java
@@ -494,7 +494,7 @@ public class LatinIME extends InputMethodService implements KeyboardActionListen
         resetContactsDictionary(oldContactsDictionary);
 
         mUserHistoryDictionary = new UserHistoryDictionary(
-                this, localeStr, Suggest.DIC_USER_HISTORY);
+                this, localeStr, Suggest.DIC_USER_HISTORY, mPrefs);
         mSuggest.setUserHistoryDictionary(mUserHistoryDictionary);
     }
 
@@ -745,7 +745,6 @@ public class LatinIME extends InputMethodService implements KeyboardActionListen
 
         KeyboardView inputView = mKeyboardSwitcher.getKeyboardView();
         if (inputView != null) inputView.closing();
-        if (mUserHistoryDictionary != null) mUserHistoryDictionary.flushPendingWrites();
     }
 
     private void onFinishInputViewInternal(boolean finishingInput) {
diff --git a/java/src/com/android/inputmethod/latin/Settings.java b/java/src/com/android/inputmethod/latin/Settings.java
index 74c4aea0c..08f3e8456 100644
--- a/java/src/com/android/inputmethod/latin/Settings.java
+++ b/java/src/com/android/inputmethod/latin/Settings.java
@@ -58,6 +58,8 @@ public class Settings extends InputMethodSettingsFragment
     public static final String PREF_SHOW_SUGGESTIONS_SETTING = "show_suggestions_setting";
     public static final String PREF_MISC_SETTINGS = "misc_settings";
     public static final String PREF_USABILITY_STUDY_MODE = "usability_study_mode";
+    public static final String PREF_LAST_USER_DICTIONARY_WRITE_TIME =
+            "last_user_dictionary_write_time";
     public static final String PREF_ADVANCED_SETTINGS = "pref_advanced_settings";
     public static final String PREF_SUPPRESS_LANGUAGE_SWITCH_KEY =
             "pref_suppress_language_switch_key";
@@ -244,7 +246,6 @@ public class Settings extends InputMethodSettingsFragment
         refreshEnablingsOfKeypressSoundAndVibrationSettings(prefs, res);
     }
 
-    @SuppressWarnings("unused")
     @Override
     public void onResume() {
         super.onResume();
diff --git a/java/src/com/android/inputmethod/latin/SettingsValues.java b/java/src/com/android/inputmethod/latin/SettingsValues.java
index 932920a60..4aae6a85e 100644
--- a/java/src/com/android/inputmethod/latin/SettingsValues.java
+++ b/java/src/com/android/inputmethod/latin/SettingsValues.java
@@ -28,6 +28,8 @@ import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
 
 /**
  * When you call the constructor of this class, you may want to change the current system locale by
@@ -351,4 +353,23 @@ public class SettingsValues {
         // TODO: use mUsabilityStudyMode instead of reading it again here
         return prefs.getBoolean(Settings.PREF_USABILITY_STUDY_MODE, true);
     }
+
+    public static long getLastUserHistoryWriteTime(
+            final SharedPreferences prefs, final String locale) {
+        final String str = prefs.getString(Settings.PREF_LAST_USER_DICTIONARY_WRITE_TIME, "");
+        final HashMap<String, Long> map = Utils.localeAndTimeStrToHashMap(str);
+        if (map.containsKey(locale)) {
+            return map.get(locale);
+        }
+        return 0;
+    }
+
+    public static void setLastUserHistoryWriteTime(
+            final SharedPreferences prefs, final String locale) {
+        final String oldStr = prefs.getString(Settings.PREF_LAST_USER_DICTIONARY_WRITE_TIME, "");
+        final HashMap<String, Long> map = Utils.localeAndTimeStrToHashMap(oldStr);
+        map.put(locale, System.currentTimeMillis());
+        final String newStr = Utils.localeAndTimeHashMapToStr(map);
+        prefs.edit().putString(Settings.PREF_LAST_USER_DICTIONARY_WRITE_TIME, newStr).apply();
+    }
 }
diff --git a/java/src/com/android/inputmethod/latin/UserHistoryDictionary.java b/java/src/com/android/inputmethod/latin/UserHistoryDictionary.java
index efafacc8a..fa3d1be11 100644
--- a/java/src/com/android/inputmethod/latin/UserHistoryDictionary.java
+++ b/java/src/com/android/inputmethod/latin/UserHistoryDictionary.java
@@ -18,6 +18,7 @@ package com.android.inputmethod.latin;
 
 import android.content.ContentValues;
 import android.content.Context;
+import android.content.SharedPreferences;
 import android.database.Cursor;
 import android.database.sqlite.SQLiteDatabase;
 import android.database.sqlite.SQLiteOpenHelper;
@@ -26,6 +27,8 @@ import android.os.AsyncTask;
 import android.provider.BaseColumns;
 import android.util.Log;
 
+import com.android.inputmethod.latin.UserHistoryForgettingCurveUtils.ForgettingCurveParams;
+
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -40,9 +43,6 @@ public class UserHistoryDictionary extends ExpandableDictionary {
     /** Any pair being typed or picked */
     private static final int FREQUENCY_FOR_TYPED = 2;
 
-    /** Maximum frequency for all pairs */
-    private static final int FREQUENCY_MAX = 127;
-
     /** Maximum number of pairs. Pruning will start when databases goes above this number. */
     private static int sMaxHistoryBigrams = 10000;
 
@@ -81,6 +81,7 @@ public class UserHistoryDictionary extends ExpandableDictionary {
     private HashSet<Bigram> mPendingWrites = new HashSet<Bigram>();
     private final Object mPendingWritesLock = new Object();
     private static volatile boolean sUpdatingDB = false;
+    private final SharedPreferences mPrefs;
 
     private final static HashMap<String, String> sDictProjectionMap;
 
@@ -101,12 +102,10 @@ public class UserHistoryDictionary extends ExpandableDictionary {
     private static class Bigram {
         public final String mWord1;
         public final String mWord2;
-        public final int mFrequency;
 
-        Bigram(String word1, String word2, int frequency) {
+        Bigram(String word1, String word2) {
             this.mWord1 = word1;
             this.mWord2 = word2;
-            this.mFrequency = frequency;
         }
 
         @Override
@@ -137,7 +136,8 @@ public class UserHistoryDictionary extends ExpandableDictionary {
         sDeleteHistoryBigrams = deleteHistoryBigram;
     }
 
-    public UserHistoryDictionary(final Context context, final String locale, final int dicTypeId) {
+    public UserHistoryDictionary(final Context context, final String locale, final int dicTypeId,
+            SharedPreferences sp) {
         super(context, dicTypeId);
         mLocale = locale;
         if (sOpenHelper == null) {
@@ -146,11 +146,13 @@ public class UserHistoryDictionary extends ExpandableDictionary {
         if (mLocale != null && mLocale.length() > 1) {
             loadDictionary();
         }
+        mPrefs = sp;
     }
 
     @Override
     public void close() {
         flushPendingWrites();
+        SettingsValues.setLastUserHistoryWriteTime(mPrefs, mLocale);
         // Don't close the database as locale changes will require it to be reopened anyway
         // Also, the database is written to somewhat frequently, so it needs to be kept alive
         // throughout the life of the process.
@@ -176,25 +178,20 @@ public class UserHistoryDictionary extends ExpandableDictionary {
      * The second word may not be null (a NullPointerException would be thrown).
      */
     public int addToUserHistory(final String word1, String word2) {
-        super.addWord(word2, null /* shortcut */, FREQUENCY_FOR_TYPED);
+        super.addWord(word2, null /* the "shortcut" parameter is null */, FREQUENCY_FOR_TYPED);
         // Do not insert a word as a bigram of itself
         if (word2.equals(word1)) {
             return 0;
         }
-
-        int freq;
+        final int freq;
         if (null == word1) {
             freq = FREQUENCY_FOR_TYPED;
         } else {
-            freq = super.addBigram(word1, word2, FREQUENCY_FOR_TYPED);
+            freq = super.setBigramAndGetFrequency(word1, word2, new ForgettingCurveParams());
         }
-        if (freq > FREQUENCY_MAX) freq = FREQUENCY_MAX;
         synchronized (mPendingWritesLock) {
-            if (freq == FREQUENCY_FOR_TYPED || mPendingWrites.isEmpty()) {
-                mPendingWrites.add(new Bigram(word1, word2, freq));
-            } else {
-                Bigram bi = new Bigram(word1, word2, freq);
-                mPendingWrites.remove(bi);
+            final Bigram bi = new Bigram(word1, word2);
+            if (!mPendingWrites.contains(bi)) {
                 mPendingWrites.add(bi);
             }
         }
@@ -203,7 +200,7 @@ public class UserHistoryDictionary extends ExpandableDictionary {
     }
 
     public boolean cancelAddingUserHistory(String word1, String word2) {
-        final Bigram bi = new Bigram(word1, word2, 0);
+        final Bigram bi = new Bigram(word1, word2);
         if (mPendingWrites.contains(bi)) {
             mPendingWrites.remove(bi);
             return super.removeBigram(word1, word2);
@@ -214,12 +211,12 @@ public class UserHistoryDictionary extends ExpandableDictionary {
     /**
      * Schedules a background thread to write any pending words to the database.
      */
-    public void flushPendingWrites() {
+    private void flushPendingWrites() {
         synchronized (mPendingWritesLock) {
             // Nothing pending? Return
             if (mPendingWrites.isEmpty()) return;
             // Create a background thread to write the pending entries
-            new UpdateDbTask(sOpenHelper, mPendingWrites, mLocale).execute();
+            new UpdateDbTask(sOpenHelper, mPendingWrites, mLocale, this).execute();
             // Create a new map for writing new entries into while the old one is written to db
             mPendingWrites = new HashSet<Bigram>();
         }
@@ -240,25 +237,30 @@ public class UserHistoryDictionary extends ExpandableDictionary {
 
     @Override
     public void loadDictionaryAsync() {
+        final long last = SettingsValues.getLastUserHistoryWriteTime(mPrefs, mLocale);
+        final long now = System.currentTimeMillis();
         // Load the words that correspond to the current input locale
         final Cursor cursor = query(MAIN_COLUMN_LOCALE + "=?", new String[] { mLocale });
         if (null == cursor) return;
         try {
             if (cursor.moveToFirst()) {
-                int word1Index = cursor.getColumnIndex(MAIN_COLUMN_WORD1);
-                int word2Index = cursor.getColumnIndex(MAIN_COLUMN_WORD2);
-                int frequencyIndex = cursor.getColumnIndex(FREQ_COLUMN_FREQUENCY);
+                final int word1Index = cursor.getColumnIndex(MAIN_COLUMN_WORD1);
+                final int word2Index = cursor.getColumnIndex(MAIN_COLUMN_WORD2);
+                final int frequencyIndex = cursor.getColumnIndex(FREQ_COLUMN_FREQUENCY);
                 while (!cursor.isAfterLast()) {
-                    String word1 = cursor.getString(word1Index);
-                    String word2 = cursor.getString(word2Index);
-                    int frequency = cursor.getInt(frequencyIndex);
+                    final String word1 = cursor.getString(word1Index);
+                    final String word2 = cursor.getString(word2Index);
+                    final int frequency = cursor.getInt(frequencyIndex);
                     // Safeguard against adding really long words. Stack may overflow due
                     // to recursive lookup
                     if (null == word1) {
                         super.addWord(word2, null /* shortcut */, frequency);
                     } else if (word1.length() < BinaryDictionary.MAX_WORD_LENGTH
                             && word2.length() < BinaryDictionary.MAX_WORD_LENGTH) {
-                        super.setBigram(word1, word2, frequency);
+                        super.setBigramAndGetFrequency(
+                                word1, word2, new ForgettingCurveParams(frequency, now, last));
+                        // TODO: optimize
+                        mPendingWrites.add(new Bigram(word1, word2));
                     }
                     cursor.moveToNext();
                 }
@@ -340,12 +342,14 @@ public class UserHistoryDictionary extends ExpandableDictionary {
         private final HashSet<Bigram> mMap;
         private final DatabaseHelper mDbHelper;
         private final String mLocale;
+        private final UserHistoryDictionary mUserHistoryDictionary;
 
         public UpdateDbTask(DatabaseHelper openHelper, HashSet<Bigram> pendingWrites,
-                String locale) {
+                String locale, UserHistoryDictionary dict) {
             mMap = pendingWrites;
             mLocale = locale;
             mDbHelper = openHelper;
+            mUserHistoryDictionary = dict;
         }
 
         /** Prune any old data if the database is getting too big. */
@@ -357,7 +361,8 @@ public class UserHistoryDictionary extends ExpandableDictionary {
                 int totalRowCount = c.getCount();
                 // prune out old data if we have too much data
                 if (totalRowCount > sMaxHistoryBigrams) {
-                    int numDeleteRows = (totalRowCount - sMaxHistoryBigrams) + sDeleteHistoryBigrams;
+                    int numDeleteRows = (totalRowCount - sMaxHistoryBigrams)
+                            + sDeleteHistoryBigrams;
                     int pairIdColumnId = c.getColumnIndex(FREQ_COLUMN_PAIR_ID);
                     c.moveToFirst();
                     int count = 0;
@@ -397,43 +402,70 @@ public class UserHistoryDictionary extends ExpandableDictionary {
             }
             db.execSQL("PRAGMA foreign_keys = ON;");
             // Write all the entries to the db
-            Iterator<Bigram> iterator = mMap.iterator();
+            final Iterator<Bigram> iterator = mMap.iterator();
             while (iterator.hasNext()) {
                 // TODO: this process of making a text search for each pair each time
                 // is terribly inefficient. Optimize this.
-                Bigram bi = iterator.next();
+                final Bigram bi = iterator.next();
 
                 // find pair id
-                final Cursor c;
-                if (null != bi.mWord1) {
-                    c = db.query(MAIN_TABLE_NAME, new String[] { MAIN_COLUMN_ID },
-                            MAIN_COLUMN_WORD1 + "=? AND " + MAIN_COLUMN_WORD2 + "=? AND "
-                            + MAIN_COLUMN_LOCALE + "=?",
-                            new String[] { bi.mWord1, bi.mWord2, mLocale }, null, null, null);
-                } else {
-                    c = db.query(MAIN_TABLE_NAME, new String[] { MAIN_COLUMN_ID },
-                            MAIN_COLUMN_WORD1 + " IS NULL AND " + MAIN_COLUMN_WORD2 + "=? AND "
-                            + MAIN_COLUMN_LOCALE + "=?",
-                            new String[] { bi.mWord2, mLocale }, null, null, null);
-                }
+                Cursor c = null;
+                try {
+                    if (null != bi.mWord1) {
+                        c = db.query(MAIN_TABLE_NAME, new String[] { MAIN_COLUMN_ID },
+                                MAIN_COLUMN_WORD1 + "=? AND " + MAIN_COLUMN_WORD2 + "=? AND "
+                                        + MAIN_COLUMN_LOCALE + "=?",
+                                        new String[] { bi.mWord1, bi.mWord2, mLocale }, null, null,
+                                        null);
+                    } else {
+                        c = db.query(MAIN_TABLE_NAME, new String[] { MAIN_COLUMN_ID },
+                                MAIN_COLUMN_WORD1 + " IS NULL AND " + MAIN_COLUMN_WORD2 + "=? AND "
+                                        + MAIN_COLUMN_LOCALE + "=?",
+                                        new String[] { bi.mWord2, mLocale }, null, null, null);
+                    }
 
-                int pairId;
-                if (c.moveToFirst()) {
-                    // existing pair
-                    pairId = c.getInt(c.getColumnIndex(MAIN_COLUMN_ID));
-                    db.delete(FREQ_TABLE_NAME, FREQ_COLUMN_PAIR_ID + "=?",
-                            new String[] { Integer.toString(pairId) });
-                } else {
-                    // new pair
-                    Long pairIdLong = db.insert(MAIN_TABLE_NAME, null,
-                            getContentValues(bi.mWord1, bi.mWord2, mLocale));
-                    pairId = pairIdLong.intValue();
+                    final int pairId;
+                    if (c.moveToFirst()) {
+                        // existing pair
+                        pairId = c.getInt(c.getColumnIndex(MAIN_COLUMN_ID));
+                        db.delete(FREQ_TABLE_NAME, FREQ_COLUMN_PAIR_ID + "=?",
+                                new String[] { Integer.toString(pairId) });
+                    } else {
+                        // new pair
+                        Long pairIdLong = db.insert(MAIN_TABLE_NAME, null,
+                                getContentValues(bi.mWord1, bi.mWord2, mLocale));
+                        pairId = pairIdLong.intValue();
+                    }
+                    // insert new frequency
+                    final int freq;
+                    if (bi.mWord1 == null) {
+                        freq = FREQUENCY_FOR_TYPED;
+                    } else {
+                        final NextWord nw = mUserHistoryDictionary.getBigramWord(
+                                bi.mWord1, bi.mWord2);
+                        if (nw != null) {
+                            final int tempFreq = nw.getFcValue();
+                            // TODO: Check whether the word is valid or not
+                            if (UserHistoryForgettingCurveUtils.needsToSave(
+                                    (byte)tempFreq, false)) {
+                                freq = tempFreq;
+                            } else {
+                                freq = -1;
+                            }
+                        } else {
+                            freq = -1;
+                        }
+                    }
+                    if (freq > 0) {
+                        db.insert(FREQ_TABLE_NAME, null, getFrequencyContentValues(pairId, freq));
+                    }
+                } finally {
+                    if (c != null) {
+                        c.close();
+                    }
                 }
-                c.close();
-
-                // insert new frequency
-                db.insert(FREQ_TABLE_NAME, null, getFrequencyContentValues(pairId, bi.mFrequency));
             }
+
             checkPruneData(db);
             sUpdatingDB = false;
 
diff --git a/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java b/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java
index eb3881726..f30fee23e 100644
--- a/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java
+++ b/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java
@@ -16,14 +16,78 @@
 
 package com.android.inputmethod.latin;
 
+import android.text.format.DateUtils;
+import android.util.Log;
+
 public class UserHistoryForgettingCurveUtils {
+    private static final String TAG = UserHistoryForgettingCurveUtils.class.getSimpleName();
+    private static final boolean DEBUG = false;
     private static final int FC_FREQ_MAX = 127;
     /* package */ static final int COUNT_MAX = 3;
     private static final int FC_LEVEL_MAX = 3;
     /* package */ static final int ELAPSED_TIME_MAX = 15;
     private static final int ELAPSED_TIME_INTERVAL_HOURS = 6;
+    private static final long ELAPSED_TIME_INTERVAL_MILLIS = ELAPSED_TIME_INTERVAL_HOURS
+            * DateUtils.HOUR_IN_MILLIS;
     private static final int HALF_LIFE_HOURS = 48;
 
+    private UserHistoryForgettingCurveUtils() {
+        // This utility class is not publicly instantiable.
+    }
+
+    public static class ForgettingCurveParams {
+        private byte mFc;
+        long mLastTouchedTime = 0;
+
+        private void updateLastTouchedTime() {
+            mLastTouchedTime = System.currentTimeMillis();
+        }
+
+        public ForgettingCurveParams() {
+            // TODO: Check whether this word is valid or not
+            this(System.currentTimeMillis());
+        }
+
+        private ForgettingCurveParams(long now) {
+            this((int)pushCount((byte)0, false), now, now);
+        }
+
+        public ForgettingCurveParams(int fc, long now, long last) {
+            mFc = (byte)fc;
+            mLastTouchedTime = last;
+            updateElapsedTime(now);
+        }
+
+        public byte getFc() {
+            updateElapsedTime(System.currentTimeMillis());
+            return mFc;
+        }
+
+        public int getFrequency() {
+            updateElapsedTime(System.currentTimeMillis());
+            return UserHistoryForgettingCurveUtils.fcToFreq(mFc);
+        }
+
+        public int notifyTypedAgainAndGetFrequency() {
+            updateLastTouchedTime();
+            // TODO: Check whether this word is valid or not
+            mFc = pushCount(mFc, false);
+            return UserHistoryForgettingCurveUtils.fcToFreq(mFc);
+        }
+
+        private void updateElapsedTime(long now) {
+            final int elapsedTimeCount =
+                    (int)((now - mLastTouchedTime) / ELAPSED_TIME_INTERVAL_MILLIS);
+            if (elapsedTimeCount <= 0) {
+                return;
+            }
+            for (int i = 0; i < elapsedTimeCount; ++i) {
+                mLastTouchedTime += ELAPSED_TIME_INTERVAL_MILLIS;
+                mFc = pushElapsedTime(mFc);
+            }
+        }
+    }
+
     /* package */ static  int fcToElapsedTime(byte fc) {
         return fc & 0x0F;
     }
@@ -38,8 +102,8 @@ public class UserHistoryForgettingCurveUtils {
 
     private static int calcFreq(int elapsedTime, int count, int level) {
         if (level <= 0) {
-            // Reserved words, just return 0
-            return 0;
+            // Reserved words, just return -1
+            return -1;
         }
         if (count == COUNT_MAX) {
             // Temporary promote because it's frequently typed recently
@@ -87,12 +151,26 @@ public class UserHistoryForgettingCurveUtils {
             // Upgrade level
             ++level;
             count = 0;
+            if (DEBUG) {
+                Log.d(TAG, "Upgrade level.");
+            }
         } else {
             ++count;
         }
         return calcFc(0, count, level);
     }
 
+    // TODO: isValid should be false for a word whose frequency is 0,
+    // or that is not in the dictionary.
+    public static boolean needsToSave(byte fc, boolean isValid) {
+        int level = fcToLevel(fc);
+        if (isValid && level == 0) {
+            return false;
+        }
+        final int elapsedTime = fcToElapsedTime(fc);
+        return (elapsedTime < ELAPSED_TIME_MAX - 1 || level > 0);
+    }
+
     private static class MathUtils {
         public static final int[][] SCORE_TABLE = new int[FC_LEVEL_MAX][ELAPSED_TIME_MAX + 1];
         static {
diff --git a/java/src/com/android/inputmethod/latin/Utils.java b/java/src/com/android/inputmethod/latin/Utils.java
index 036ff74b8..91996674b 100644
--- a/java/src/com/android/inputmethod/latin/Utils.java
+++ b/java/src/com/android/inputmethod/latin/Utils.java
@@ -44,8 +44,10 @@ import java.io.IOException;
 import java.io.PrintWriter;
 import java.nio.channels.FileChannel;
 import java.text.SimpleDateFormat;
+import java.util.Collections;
 import java.util.Date;
 import java.util.HashMap;
+import java.util.Map;
 
 public class Utils {
     private Utils() {
@@ -484,4 +486,40 @@ public class Utils {
         }
         return sDeviceOverrideValueMap.get(key);
     }
+
+    private static final HashMap<String, Long> EMPTY_LT_HASH_MAP = new HashMap<String, Long>();
+    private static final String LOCALE_AND_TIME_STR_SEPARATER = ",";
+    public static HashMap<String, Long> localeAndTimeStrToHashMap(String str) {
+        if (TextUtils.isEmpty(str)) {
+            return EMPTY_LT_HASH_MAP;
+        }
+        final String[] ss = str.split(LOCALE_AND_TIME_STR_SEPARATER);
+        final int N = ss.length;
+        if (N < 2 || N % 2 != 0) {
+            return EMPTY_LT_HASH_MAP;
+        }
+        final HashMap<String, Long> retval = new HashMap<String, Long>();
+        for (int i = 0; i < N / 2; ++i) {
+            final String localeStr = ss[i];
+            final long time = Long.valueOf(ss[i + 1]);
+            retval.put(localeStr, time);
+        }
+        return retval;
+    }
+
+    public static String localeAndTimeHashMapToStr(HashMap<String, Long> map) {
+        if (map == null || map.isEmpty()) {
+            return "";
+        }
+        final StringBuilder builder = new StringBuilder();
+        for (String localeStr : map.keySet()) {
+            if (builder.length() > 0) {
+                builder.append(LOCALE_AND_TIME_STR_SEPARATER);
+            }
+            final Long time = map.get(localeStr);
+            builder.append(localeStr).append(LOCALE_AND_TIME_STR_SEPARATER);
+            builder.append(String.valueOf(time));
+        }
+        return builder.toString();
+    }
 }