diff --git a/java/src/com/android/inputmethod/latin/LatinIME.java b/java/src/com/android/inputmethod/latin/LatinIME.java
index 1211ea5..44185e8 100644
--- a/java/src/com/android/inputmethod/latin/LatinIME.java
+++ b/java/src/com/android/inputmethod/latin/LatinIME.java
@@ -76,7 +76,7 @@
 import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
 import com.android.inputmethod.latin.define.ProductionFlag;
 import com.android.inputmethod.latin.personalization.PersonalizationDictionaryHelper;
-import com.android.inputmethod.latin.personalization.UserHistoryDictionary;
+import com.android.inputmethod.latin.personalization.UserHistoryPredictionDictionary;
 import com.android.inputmethod.latin.settings.Settings;
 import com.android.inputmethod.latin.settings.SettingsActivity;
 import com.android.inputmethod.latin.settings.SettingsValues;
@@ -169,7 +169,7 @@
 
     private boolean mIsMainDictionaryAvailable;
     private UserBinaryDictionary mUserDictionary;
-    private UserHistoryDictionary mUserHistoryDictionary;
+    private UserHistoryPredictionDictionary mUserHistoryPredictionDictionary;
     private boolean mIsUserDictionaryAvailable;
 
     private LastComposedWord mLastComposedWord = LastComposedWord.NOT_A_COMPOSED_WORD;
@@ -565,9 +565,9 @@
         resetContactsDictionary(oldContactsDictionary);
 
         final SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
-        mUserHistoryDictionary =
-                PersonalizationDictionaryHelper.getUserHistoryDictionary(this, localeStr, prefs);
-        mSuggest.setUserHistoryDictionary(mUserHistoryDictionary);
+        mUserHistoryPredictionDictionary = PersonalizationDictionaryHelper
+                .getUserHistoryPredictionDictionary(this, localeStr, prefs);
+        mSuggest.setUserHistoryPredictionDictionary(mUserHistoryPredictionDictionary);
     }
 
     /**
@@ -2507,7 +2507,8 @@
         if (!currentSettings.mCorrectionEnabled) return null;
 
         final Suggest suggest = mSuggest;
-        final UserHistoryDictionary userHistoryDictionary = mUserHistoryDictionary;
+        final UserHistoryPredictionDictionary userHistoryDictionary =
+                mUserHistoryPredictionDictionary;
         if (suggest == null || userHistoryDictionary == null) {
             // Avoid concurrent issue
             return null;
@@ -2657,7 +2658,7 @@
         }
         mConnection.deleteSurroundingText(deleteLength, 0);
         if (!TextUtils.isEmpty(previousWord) && !TextUtils.isEmpty(committedWord)) {
-            mUserHistoryDictionary.cancelAddingUserHistory(previousWord, committedWord);
+            mUserHistoryPredictionDictionary.cancelAddingUserHistory(previousWord, committedWord);
         }
         mConnection.commitText(originallyTypedWord + mLastComposedWord.mSeparatorString, 1);
         if (mSettings.isInternal()) {
diff --git a/java/src/com/android/inputmethod/latin/Suggest.java b/java/src/com/android/inputmethod/latin/Suggest.java
index 647c6f6..6b01667 100644
--- a/java/src/com/android/inputmethod/latin/Suggest.java
+++ b/java/src/com/android/inputmethod/latin/Suggest.java
@@ -22,7 +22,7 @@
 import com.android.inputmethod.annotations.UsedForTesting;
 import com.android.inputmethod.keyboard.ProximityInfo;
 import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
-import com.android.inputmethod.latin.personalization.UserHistoryDictionary;
+import com.android.inputmethod.latin.personalization.UserHistoryPredictionDictionary;
 import com.android.inputmethod.latin.utils.AutoCorrectionUtils;
 import com.android.inputmethod.latin.utils.BoundedTreeSet;
 import com.android.inputmethod.latin.utils.CollectionUtils;
@@ -168,8 +168,10 @@
         addOrReplaceDictionary(mDictionaries, Dictionary.TYPE_CONTACTS, contactsDictionary);
     }
 
-    public void setUserHistoryDictionary(final UserHistoryDictionary userHistoryDictionary) {
-        addOrReplaceDictionary(mDictionaries, Dictionary.TYPE_USER_HISTORY, userHistoryDictionary);
+    public void setUserHistoryPredictionDictionary(
+            final UserHistoryPredictionDictionary userHistoryPredictionDictionary) {
+        addOrReplaceDictionary(mDictionaries, Dictionary.TYPE_USER_HISTORY,
+                userHistoryPredictionDictionary);
     }
 
     public void setAutoCorrectionThreshold(float threshold) {
diff --git a/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryHelper.java b/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryHelper.java
index e09e834..f5dae99 100644
--- a/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryHelper.java
+++ b/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryHelper.java
@@ -29,15 +29,16 @@
     private static final String TAG = PersonalizationDictionaryHelper.class.getSimpleName();
     private static final boolean DEBUG = false;
 
-    private static final ConcurrentHashMap<String, SoftReference<UserHistoryDictionary>>
+    private static final ConcurrentHashMap<String, SoftReference<UserHistoryPredictionDictionary>>
             sLangDictCache = CollectionUtils.newConcurrentHashMap();
 
-    public static UserHistoryDictionary getUserHistoryDictionary(
+    public static UserHistoryPredictionDictionary getUserHistoryPredictionDictionary(
             final Context context, final String locale, final SharedPreferences sp) {
         synchronized (sLangDictCache) {
             if (sLangDictCache.containsKey(locale)) {
-                final SoftReference<UserHistoryDictionary> ref = sLangDictCache.get(locale);
-                final UserHistoryDictionary dict = ref == null ? null : ref.get();
+                final SoftReference<UserHistoryPredictionDictionary> ref =
+                        sLangDictCache.get(locale);
+                final UserHistoryPredictionDictionary dict = ref == null ? null : ref.get();
                 if (dict != null) {
                     if (DEBUG) {
                         Log.w(TAG, "Use cached UserHistoryDictionary for " + locale);
@@ -45,8 +46,9 @@
                     return dict;
                 }
             }
-            final UserHistoryDictionary dict = new UserHistoryDictionary(context, locale, sp);
-            sLangDictCache.put(locale, new SoftReference<UserHistoryDictionary>(dict));
+            final UserHistoryPredictionDictionary dict =
+                    new UserHistoryPredictionDictionary(context, locale, sp);
+            sLangDictCache.put(locale, new SoftReference<UserHistoryPredictionDictionary>(dict));
             return dict;
         }
     }
diff --git a/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryUpdateListener.java b/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryUpdateListener.java
index 2ec0dc0..c78e5a9 100644
--- a/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryUpdateListener.java
+++ b/java/src/com/android/inputmethod/latin/personalization/PersonalizationDictionaryUpdateListener.java
@@ -16,6 +16,6 @@
 
 package com.android.inputmethod.latin.personalization;
 
-public class PersonalizationDictionaryUpdateListener {
+public interface PersonalizationDictionaryUpdateListener {
     // TODO: Implement
 }
diff --git a/java/src/com/android/inputmethod/latin/personalization/PersonalizationPredictionDictionary.java b/java/src/com/android/inputmethod/latin/personalization/PersonalizationPredictionDictionary.java
index 7bce979..083ce58 100644
--- a/java/src/com/android/inputmethod/latin/personalization/PersonalizationPredictionDictionary.java
+++ b/java/src/com/android/inputmethod/latin/personalization/PersonalizationPredictionDictionary.java
@@ -16,11 +16,34 @@
 
 package com.android.inputmethod.latin.personalization;
 
-import com.android.inputmethod.latin.Dictionary;
-import com.android.inputmethod.latin.ExpandableDictionary;
-
 import android.content.Context;
 import android.content.SharedPreferences;
+import android.os.AsyncTask;
+import android.util.Log;
+
+import com.android.inputmethod.annotations.UsedForTesting;
+import com.android.inputmethod.keyboard.ProximityInfo;
+import com.android.inputmethod.latin.Constants;
+import com.android.inputmethod.latin.ExpandableDictionary;
+import com.android.inputmethod.latin.LatinImeLogger;
+import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
+import com.android.inputmethod.latin.WordComposer;
+import com.android.inputmethod.latin.makedict.FormatSpec.FormatOptions;
+import com.android.inputmethod.latin.settings.Settings;
+import com.android.inputmethod.latin.utils.ByteArrayWrapper;
+import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils;
+import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils.BigramDictionaryInterface;
+import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils.OnAddWordListener;
+import com.android.inputmethod.latin.utils.UserHistoryForgettingCurveUtils;
+import com.android.inputmethod.latin.utils.UserHistoryForgettingCurveUtils.ForgettingCurveParams;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.concurrent.locks.ReentrantLock;
 
 /**
  * This class is a dictionary for the personalized prediction language model implemented in Java.
@@ -30,17 +53,348 @@
         // TODO: Implement
     }
 
+    private static final String TAG = PersonalizationPredictionDictionary.class.getSimpleName();
+    private static final String NAME = PersonalizationPredictionDictionary.class.getSimpleName();
+    public static final boolean DBG_SAVE_RESTORE = false;
+    public static final boolean DBG_STRESS_TEST = false;
+    public static final boolean DBG_ALWAYS_WRITE = false;
+    public static final boolean PROFILE_SAVE_RESTORE = LatinImeLogger.sDBG;
+
+    private static final FormatOptions VERSION3 = new FormatOptions(3,
+            true /* supportsDynamicUpdate */);
+
+    /** Any pair being typed or picked */
+    private static final int FREQUENCY_FOR_TYPED = 2;
+
+    /** Maximum number of pairs. Pruning will start when databases goes above this number. */
+    public static final int MAX_HISTORY_BIGRAMS = 10000;
+
+    /**
+     * When it hits maximum bigram pair, it will delete until you are left with
+     * only (sMaxHistoryBigrams - sDeleteHistoryBigrams) pairs.
+     * Do not keep this number small to avoid deleting too often.
+     */
+    public static final int DELETE_HISTORY_BIGRAMS = 1000;
+
     /** Locale for which this user history dictionary is storing words */
     private final String mLocale;
+
+    private final UserHistoryDictionaryBigramList mBigramList =
+            new UserHistoryDictionaryBigramList();
+    private final ReentrantLock mBigramListLock = new ReentrantLock();
     private final SharedPreferences mPrefs;
 
-    // Singleton
-    private PersonalizationPredictionDictionary(final Context context, final String locale,
-            final SharedPreferences sp) {
-        super(context, Dictionary.TYPE_PERSONALIZATION_PREDICTION_IN_JAVA);
+    // Should always be false except when we use this class for test
+    @UsedForTesting boolean isTest = false;
+
+    /* package */ PersonalizationPredictionDictionary(final Context context, final String locale,
+            final SharedPreferences sp, final String dictionaryType) {
+        super(context, dictionaryType);
         mLocale = locale;
         mPrefs = sp;
+        if (mLocale != null && mLocale.length() > 1) {
+            loadDictionary();
+        }
     }
 
-    // TODO: Implement
+    @Override
+    public void close() {
+        flushPendingWrites();
+        // 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.
+        // mOpenHelper.close();
+        // Ignore close because we cache PersonalizationPredictionDictionary for each language.
+        // See getInstance() above.
+        // super.close();
+    }
+
+    @Override
+    protected ArrayList<SuggestedWordInfo> getWordsInner(final WordComposer composer,
+            final String prevWord, final ProximityInfo proximityInfo) {
+        // Inhibit suggestions (not predictions) for user history for now. Removing this method
+        // is enough to use it through the standard ExpandableDictionary way.
+        return null;
+    }
+
+    /**
+     * Return whether the passed charsequence is in the dictionary.
+     */
+    @Override
+    public synchronized boolean isValidWord(final String word) {
+        // TODO: figure out what is the correct thing to do here.
+        return false;
+    }
+
+    /**
+     * Pair will be added to the user history dictionary.
+     *
+     * The first word may be null. That means we don't know the context, in other words,
+     * it's only a unigram. The first word may also be an empty string : this means start
+     * context, as in beginning of a sentence for example.
+     * The second word may not be null (a NullPointerException would be thrown).
+     */
+    public int addToUserHistory(final String word1, final String word2, final boolean isValid) {
+        if (word2.length() >= Constants.DICTIONARY_MAX_WORD_LENGTH ||
+                (word1 != null && word1.length() >= Constants.DICTIONARY_MAX_WORD_LENGTH)) {
+            return -1;
+        }
+        if (mBigramListLock.tryLock()) {
+            try {
+                super.addWord(
+                        word2, null /* the "shortcut" parameter is null */, FREQUENCY_FOR_TYPED);
+                mBigramList.addBigram(null, word2, (byte)FREQUENCY_FOR_TYPED);
+                // Do not insert a word as a bigram of itself
+                if (word2.equals(word1)) {
+                    return 0;
+                }
+                final int freq;
+                if (null == word1) {
+                    freq = FREQUENCY_FOR_TYPED;
+                } else {
+                    freq = super.setBigramAndGetFrequency(
+                            word1, word2, new ForgettingCurveParams(isValid));
+                }
+                mBigramList.addBigram(word1, word2);
+                return freq;
+            } finally {
+                mBigramListLock.unlock();
+            }
+        }
+        return -1;
+    }
+
+    public boolean cancelAddingUserHistory(final String word1, final String word2) {
+        if (mBigramListLock.tryLock()) {
+            try {
+                if (mBigramList.removeBigram(word1, word2)) {
+                    return super.removeBigram(word1, word2);
+                }
+            } finally {
+                mBigramListLock.unlock();
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Schedules a background thread to write any pending words to the database.
+     */
+    private void flushPendingWrites() {
+        // Create a background thread to write the pending entries
+        new UpdateBinaryTask(mBigramList, mLocale, this, mPrefs, getContext()).execute();
+    }
+
+    @Override
+    public void loadDictionaryAsync() {
+        // This must be run on non-main thread
+        mBigramListLock.lock();
+        try {
+            loadDictionaryAsyncLocked();
+        } finally {
+            mBigramListLock.unlock();
+        }
+    }
+
+    private int profTotal;
+
+    private void loadDictionaryAsyncLocked() {
+        if (DBG_STRESS_TEST) {
+            try {
+                Log.w(TAG, "Start stress in loading: " + mLocale);
+                Thread.sleep(15000);
+                Log.w(TAG, "End stress in loading");
+            } catch (InterruptedException e) {
+            }
+        }
+        final long last = Settings.readLastUserHistoryWriteTime(mPrefs, mLocale);
+        final boolean initializing = last == 0;
+        final long now = System.currentTimeMillis();
+        profTotal = 0;
+        final String fileName = NAME + "." + mLocale + ".dict";
+        final ExpandableDictionary dictionary = this;
+        final OnAddWordListener listener = new OnAddWordListener() {
+            @Override
+            public void setUnigram(final String word, final String shortcutTarget,
+                    final int frequency) {
+                profTotal++;
+                if (DBG_SAVE_RESTORE) {
+                    Log.d(TAG, "load unigram: " + word + "," + frequency);
+                }
+                dictionary.addWord(word, shortcutTarget, frequency);
+                mBigramList.addBigram(null, word, (byte)frequency);
+            }
+
+            @Override
+            public void setBigram(final String word1, final String word2, final int frequency) {
+                if (word1.length() < Constants.DICTIONARY_MAX_WORD_LENGTH
+                        && word2.length() < Constants.DICTIONARY_MAX_WORD_LENGTH) {
+                    profTotal++;
+                    if (DBG_SAVE_RESTORE) {
+                        Log.d(TAG, "load bigram: " + word1 + "," + word2 + "," + frequency);
+                    }
+                    dictionary.setBigramAndGetFrequency(
+                            word1, word2, initializing ? new ForgettingCurveParams(true)
+                            : new ForgettingCurveParams(frequency, now, last));
+                }
+                mBigramList.addBigram(word1, word2, (byte)frequency);
+            }
+        };
+
+        // Load the dictionary from binary file
+        FileInputStream inStream = null;
+        try {
+            final File file = new File(getContext().getFilesDir(), fileName);
+            final byte[] buffer = new byte[(int)file.length()];
+            inStream = new FileInputStream(file);
+            inStream.read(buffer);
+            UserHistoryDictIOUtils.readDictionaryBinary(
+                    new ByteArrayWrapper(buffer), listener);
+        } catch (FileNotFoundException e) {
+            // This is an expected condition: we don't have a user history dictionary for this
+            // language yet. It will be created sometime later.
+        } catch (IOException e) {
+            Log.e(TAG, "IOException on opening a bytebuffer", e);
+        } finally {
+            if (inStream != null) {
+                try {
+                    inStream.close();
+                } catch (IOException e) {
+                    // do nothing
+                }
+            }
+            if (PROFILE_SAVE_RESTORE) {
+                final long diff = System.currentTimeMillis() - now;
+                Log.d(TAG, "PROF: Load UserHistoryDictionary: "
+                        + mLocale + ", " + diff + "ms. load " + profTotal + "entries.");
+            }
+        }
+    }
+
+    /**
+     * Async task to write pending words to the binarydicts.
+     */
+    private static final class UpdateBinaryTask extends AsyncTask<Void, Void, Void>
+            implements BigramDictionaryInterface {
+        private final UserHistoryDictionaryBigramList mBigramList;
+        private final boolean mAddLevel0Bigrams;
+        private final String mLocale;
+        private final PersonalizationPredictionDictionary mPersonalizationPredictionDictionary;
+        private final SharedPreferences mPrefs;
+        private final Context mContext;
+
+        public UpdateBinaryTask(final UserHistoryDictionaryBigramList pendingWrites,
+                final String locale, final PersonalizationPredictionDictionary dict,
+                final SharedPreferences prefs, final Context context) {
+            mBigramList = pendingWrites;
+            mLocale = locale;
+            mPersonalizationPredictionDictionary = dict;
+            mPrefs = prefs;
+            mContext = context;
+            mAddLevel0Bigrams = mBigramList.size() <= MAX_HISTORY_BIGRAMS;
+        }
+
+        @Override
+        protected Void doInBackground(final Void... v) {
+            if (mPersonalizationPredictionDictionary.isTest) {
+                // If isTest == true, wait until the lock is released.
+                mPersonalizationPredictionDictionary.mBigramListLock.lock();
+                try {
+                    doWriteTaskLocked();
+                } finally {
+                    mPersonalizationPredictionDictionary.mBigramListLock.unlock();
+                }
+            } else if (mPersonalizationPredictionDictionary.mBigramListLock.tryLock()) {
+                try {
+                    doWriteTaskLocked();
+                } finally {
+                    mPersonalizationPredictionDictionary.mBigramListLock.unlock();
+                }
+            }
+            return null;
+        }
+
+        private void doWriteTaskLocked() {
+            if (DBG_STRESS_TEST) {
+                try {
+                    Log.w(TAG, "Start stress in closing: " + mLocale);
+                    Thread.sleep(15000);
+                    Log.w(TAG, "End stress in closing");
+                } catch (InterruptedException e) {
+                    Log.e(TAG, "In stress test", e);
+                }
+            }
+
+            final long now = PROFILE_SAVE_RESTORE ? System.currentTimeMillis() : 0;
+            final String fileName = NAME + "." + mLocale + ".dict";
+            final File file = new File(mContext.getFilesDir(), fileName);
+            FileOutputStream out = null;
+
+            try {
+                out = new FileOutputStream(file);
+                UserHistoryDictIOUtils.writeDictionaryBinary(out, this, mBigramList, VERSION3);
+                out.flush();
+                out.close();
+            } catch (IOException e) {
+                Log.e(TAG, "IO Exception while writing file", e);
+            } finally {
+                if (out != null) {
+                    try {
+                        out.close();
+                    } catch (IOException e) {
+                        // ignore
+                    }
+                }
+            }
+
+            // Save the timestamp after we finish writing the binary dictionary.
+            Settings.writeLastUserHistoryWriteTime(mPrefs, mLocale);
+            if (PROFILE_SAVE_RESTORE) {
+                final long diff = System.currentTimeMillis() - now;
+                Log.w(TAG, "PROF: Write User HistoryDictionary: " + mLocale + ", " + diff + "ms.");
+            }
+        }
+
+        @Override
+        public int getFrequency(final String word1, final String word2) {
+            final int freq;
+            if (word1 == null) { // unigram
+                freq = FREQUENCY_FOR_TYPED;
+                final byte prevFc = mBigramList.getBigrams(word1).get(word2);
+            } else { // bigram
+                final NextWord nw =
+                        mPersonalizationPredictionDictionary.getBigramWord(word1, word2);
+                if (nw != null) {
+                    final ForgettingCurveParams fcp = nw.getFcParams();
+                    final byte prevFc = mBigramList.getBigrams(word1).get(word2);
+                    final byte fc = fcp.getFc();
+                    final boolean isValid = fcp.isValid();
+                    if (prevFc > 0 && prevFc == fc) {
+                        freq = fc & 0xFF;
+                    } else if (UserHistoryForgettingCurveUtils.
+                            needsToSave(fc, isValid, mAddLevel0Bigrams)) {
+                        freq = fc & 0xFF;
+                    } else {
+                        // Delete this entry
+                        freq = -1;
+                    }
+                } else {
+                    // Delete this entry
+                    freq = -1;
+                }
+            }
+            return freq;
+        }
+    }
+
+    @UsedForTesting
+    /* package for test */ void forceAddWordForTest(
+            final String word1, final String word2, final boolean isValid) {
+        mBigramListLock.lock();
+        try {
+            addToUserHistory(word1, word2, isValid);
+        } finally {
+            mBigramListLock.unlock();
+        }
+    }
 }
diff --git a/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionary.java b/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionary.java
deleted file mode 100644
index c76dea0..0000000
--- a/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionary.java
+++ /dev/null
@@ -1,397 +0,0 @@
-/*
- * Copyright (C) 2010 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.
- */
-
-package com.android.inputmethod.latin.personalization;
-
-import android.content.Context;
-import android.content.SharedPreferences;
-import android.os.AsyncTask;
-import android.util.Log;
-
-import com.android.inputmethod.annotations.UsedForTesting;
-import com.android.inputmethod.keyboard.ProximityInfo;
-import com.android.inputmethod.latin.Constants;
-import com.android.inputmethod.latin.Dictionary;
-import com.android.inputmethod.latin.ExpandableDictionary;
-import com.android.inputmethod.latin.LatinImeLogger;
-import com.android.inputmethod.latin.SuggestedWords.SuggestedWordInfo;
-import com.android.inputmethod.latin.WordComposer;
-import com.android.inputmethod.latin.makedict.FormatSpec.FormatOptions;
-import com.android.inputmethod.latin.settings.Settings;
-import com.android.inputmethod.latin.utils.ByteArrayWrapper;
-import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils;
-import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils.BigramDictionaryInterface;
-import com.android.inputmethod.latin.utils.UserHistoryDictIOUtils.OnAddWordListener;
-import com.android.inputmethod.latin.utils.UserHistoryForgettingCurveUtils;
-import com.android.inputmethod.latin.utils.UserHistoryForgettingCurveUtils.ForgettingCurveParams;
-
-import java.io.File;
-import java.io.FileInputStream;
-import java.io.FileNotFoundException;
-import java.io.FileOutputStream;
-import java.io.IOException;
-import java.util.ArrayList;
-import java.util.concurrent.locks.ReentrantLock;
-
-/**
- * Locally gathers stats about the words user types and various other signals like auto-correction
- * cancellation or manual picks. This allows the keyboard to adapt to the typist over time.
- */
-public class UserHistoryDictionary extends ExpandableDictionary {
-    private static final String TAG = UserHistoryDictionary.class.getSimpleName();
-    private static final String NAME = UserHistoryDictionary.class.getSimpleName();
-    public static final boolean DBG_SAVE_RESTORE = false;
-    public static final boolean DBG_STRESS_TEST = false;
-    public static final boolean DBG_ALWAYS_WRITE = false;
-    public static final boolean PROFILE_SAVE_RESTORE = LatinImeLogger.sDBG;
-
-    private static final FormatOptions VERSION3 = new FormatOptions(3,
-            true /* supportsDynamicUpdate */);
-
-    /** Any pair being typed or picked */
-    private static final int FREQUENCY_FOR_TYPED = 2;
-
-    /** Maximum number of pairs. Pruning will start when databases goes above this number. */
-    public static final int MAX_HISTORY_BIGRAMS = 10000;
-
-    /**
-     * When it hits maximum bigram pair, it will delete until you are left with
-     * only (sMaxHistoryBigrams - sDeleteHistoryBigrams) pairs.
-     * Do not keep this number small to avoid deleting too often.
-     */
-    public static final int DELETE_HISTORY_BIGRAMS = 1000;
-
-    /** Locale for which this user history dictionary is storing words */
-    private final String mLocale;
-
-    private final UserHistoryDictionaryBigramList mBigramList =
-            new UserHistoryDictionaryBigramList();
-    private final ReentrantLock mBigramListLock = new ReentrantLock();
-    private final SharedPreferences mPrefs;
-
-    // Should always be false except when we use this class for test
-    @UsedForTesting boolean isTest = false;
-
-    /* package */ UserHistoryDictionary(final Context context, final String locale,
-            final SharedPreferences sp) {
-        super(context, Dictionary.TYPE_USER_HISTORY);
-        mLocale = locale;
-        mPrefs = sp;
-        if (mLocale != null && mLocale.length() > 1) {
-            loadDictionary();
-        }
-    }
-
-    @Override
-    public void close() {
-        flushPendingWrites();
-        // 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.
-        // mOpenHelper.close();
-        // Ignore close because we cache UserHistoryDictionary for each language. See getInstance()
-        // above.
-        // super.close();
-    }
-
-    @Override
-    protected ArrayList<SuggestedWordInfo> getWordsInner(final WordComposer composer,
-            final String prevWord, final ProximityInfo proximityInfo) {
-        // Inhibit suggestions (not predictions) for user history for now. Removing this method
-        // is enough to use it through the standard ExpandableDictionary way.
-        return null;
-    }
-
-    /**
-     * Return whether the passed charsequence is in the dictionary.
-     */
-    @Override
-    public synchronized boolean isValidWord(final String word) {
-        // TODO: figure out what is the correct thing to do here.
-        return false;
-    }
-
-    /**
-     * Pair will be added to the user history dictionary.
-     *
-     * The first word may be null. That means we don't know the context, in other words,
-     * it's only a unigram. The first word may also be an empty string : this means start
-     * context, as in beginning of a sentence for example.
-     * The second word may not be null (a NullPointerException would be thrown).
-     */
-    public int addToUserHistory(final String word1, final String word2, final boolean isValid) {
-        if (word2.length() >= Constants.DICTIONARY_MAX_WORD_LENGTH ||
-                (word1 != null && word1.length() >= Constants.DICTIONARY_MAX_WORD_LENGTH)) {
-            return -1;
-        }
-        if (mBigramListLock.tryLock()) {
-            try {
-                super.addWord(
-                        word2, null /* the "shortcut" parameter is null */, FREQUENCY_FOR_TYPED);
-                mBigramList.addBigram(null, word2, (byte)FREQUENCY_FOR_TYPED);
-                // Do not insert a word as a bigram of itself
-                if (word2.equals(word1)) {
-                    return 0;
-                }
-                final int freq;
-                if (null == word1) {
-                    freq = FREQUENCY_FOR_TYPED;
-                } else {
-                    freq = super.setBigramAndGetFrequency(
-                            word1, word2, new ForgettingCurveParams(isValid));
-                }
-                mBigramList.addBigram(word1, word2);
-                return freq;
-            } finally {
-                mBigramListLock.unlock();
-            }
-        }
-        return -1;
-    }
-
-    public boolean cancelAddingUserHistory(final String word1, final String word2) {
-        if (mBigramListLock.tryLock()) {
-            try {
-                if (mBigramList.removeBigram(word1, word2)) {
-                    return super.removeBigram(word1, word2);
-                }
-            } finally {
-                mBigramListLock.unlock();
-            }
-        }
-        return false;
-    }
-
-    /**
-     * Schedules a background thread to write any pending words to the database.
-     */
-    private void flushPendingWrites() {
-        // Create a background thread to write the pending entries
-        new UpdateBinaryTask(mBigramList, mLocale, this, mPrefs, getContext()).execute();
-    }
-
-    @Override
-    public void loadDictionaryAsync() {
-        // This must be run on non-main thread
-        mBigramListLock.lock();
-        try {
-            loadDictionaryAsyncLocked();
-        } finally {
-            mBigramListLock.unlock();
-        }
-    }
-
-    private int profTotal;
-
-    private void loadDictionaryAsyncLocked() {
-        if (DBG_STRESS_TEST) {
-            try {
-                Log.w(TAG, "Start stress in loading: " + mLocale);
-                Thread.sleep(15000);
-                Log.w(TAG, "End stress in loading");
-            } catch (InterruptedException e) {
-            }
-        }
-        final long last = Settings.readLastUserHistoryWriteTime(mPrefs, mLocale);
-        final boolean initializing = last == 0;
-        final long now = System.currentTimeMillis();
-        profTotal = 0;
-        final String fileName = NAME + "." + mLocale + ".dict";
-        final ExpandableDictionary dictionary = this;
-        final OnAddWordListener listener = new OnAddWordListener() {
-            @Override
-            public void setUnigram(final String word, final String shortcutTarget,
-                    final int frequency) {
-                profTotal++;
-                if (DBG_SAVE_RESTORE) {
-                    Log.d(TAG, "load unigram: " + word + "," + frequency);
-                }
-                dictionary.addWord(word, shortcutTarget, frequency);
-                mBigramList.addBigram(null, word, (byte)frequency);
-            }
-
-            @Override
-            public void setBigram(final String word1, final String word2, final int frequency) {
-                if (word1.length() < Constants.DICTIONARY_MAX_WORD_LENGTH
-                        && word2.length() < Constants.DICTIONARY_MAX_WORD_LENGTH) {
-                    profTotal++;
-                    if (DBG_SAVE_RESTORE) {
-                        Log.d(TAG, "load bigram: " + word1 + "," + word2 + "," + frequency);
-                    }
-                    dictionary.setBigramAndGetFrequency(
-                            word1, word2, initializing ? new ForgettingCurveParams(true)
-                            : new ForgettingCurveParams(frequency, now, last));
-                }
-                mBigramList.addBigram(word1, word2, (byte)frequency);
-            }
-        };
-
-        // Load the dictionary from binary file
-        FileInputStream inStream = null;
-        try {
-            final File file = new File(getContext().getFilesDir(), fileName);
-            final byte[] buffer = new byte[(int)file.length()];
-            inStream = new FileInputStream(file);
-            inStream.read(buffer);
-            UserHistoryDictIOUtils.readDictionaryBinary(
-                    new ByteArrayWrapper(buffer), listener);
-        } catch (FileNotFoundException e) {
-            // This is an expected condition: we don't have a user history dictionary for this
-            // language yet. It will be created sometime later.
-        } catch (IOException e) {
-            Log.e(TAG, "IOException on opening a bytebuffer", e);
-        } finally {
-            if (inStream != null) {
-                try {
-                    inStream.close();
-                } catch (IOException e) {
-                    // do nothing
-                }
-            }
-            if (PROFILE_SAVE_RESTORE) {
-                final long diff = System.currentTimeMillis() - now;
-                Log.d(TAG, "PROF: Load UserHistoryDictionary: "
-                        + mLocale + ", " + diff + "ms. load " + profTotal + "entries.");
-            }
-        }
-    }
-
-    /**
-     * Async task to write pending words to the binarydicts.
-     */
-    private static final class UpdateBinaryTask extends AsyncTask<Void, Void, Void>
-            implements BigramDictionaryInterface {
-        private final UserHistoryDictionaryBigramList mBigramList;
-        private final boolean mAddLevel0Bigrams;
-        private final String mLocale;
-        private final UserHistoryDictionary mUserHistoryDictionary;
-        private final SharedPreferences mPrefs;
-        private final Context mContext;
-
-        public UpdateBinaryTask(final UserHistoryDictionaryBigramList pendingWrites,
-                final String locale, final UserHistoryDictionary dict,
-                final SharedPreferences prefs, final Context context) {
-            mBigramList = pendingWrites;
-            mLocale = locale;
-            mUserHistoryDictionary = dict;
-            mPrefs = prefs;
-            mContext = context;
-            mAddLevel0Bigrams = mBigramList.size() <= MAX_HISTORY_BIGRAMS;
-        }
-
-        @Override
-        protected Void doInBackground(final Void... v) {
-            if (mUserHistoryDictionary.isTest) {
-                // If isTest == true, wait until the lock is released.
-                mUserHistoryDictionary.mBigramListLock.lock();
-                try {
-                    doWriteTaskLocked();
-                } finally {
-                    mUserHistoryDictionary.mBigramListLock.unlock();
-                }
-            } else if (mUserHistoryDictionary.mBigramListLock.tryLock()) {
-                try {
-                    doWriteTaskLocked();
-                } finally {
-                    mUserHistoryDictionary.mBigramListLock.unlock();
-                }
-            }
-            return null;
-        }
-
-        private void doWriteTaskLocked() {
-            if (DBG_STRESS_TEST) {
-                try {
-                    Log.w(TAG, "Start stress in closing: " + mLocale);
-                    Thread.sleep(15000);
-                    Log.w(TAG, "End stress in closing");
-                } catch (InterruptedException e) {
-                    Log.e(TAG, "In stress test", e);
-                }
-            }
-
-            final long now = PROFILE_SAVE_RESTORE ? System.currentTimeMillis() : 0;
-            final String fileName = NAME + "." + mLocale + ".dict";
-            final File file = new File(mContext.getFilesDir(), fileName);
-            FileOutputStream out = null;
-
-            try {
-                out = new FileOutputStream(file);
-                UserHistoryDictIOUtils.writeDictionaryBinary(out, this, mBigramList, VERSION3);
-                out.flush();
-                out.close();
-            } catch (IOException e) {
-                Log.e(TAG, "IO Exception while writing file", e);
-            } finally {
-                if (out != null) {
-                    try {
-                        out.close();
-                    } catch (IOException e) {
-                        // ignore
-                    }
-                }
-            }
-
-            // Save the timestamp after we finish writing the binary dictionary.
-            Settings.writeLastUserHistoryWriteTime(mPrefs, mLocale);
-            if (PROFILE_SAVE_RESTORE) {
-                final long diff = System.currentTimeMillis() - now;
-                Log.w(TAG, "PROF: Write User HistoryDictionary: " + mLocale + ", " + diff + "ms.");
-            }
-        }
-
-        @Override
-        public int getFrequency(final String word1, final String word2) {
-            final int freq;
-            if (word1 == null) { // unigram
-                freq = FREQUENCY_FOR_TYPED;
-                final byte prevFc = mBigramList.getBigrams(word1).get(word2);
-            } else { // bigram
-                final NextWord nw = mUserHistoryDictionary.getBigramWord(word1, word2);
-                if (nw != null) {
-                    final ForgettingCurveParams fcp = nw.getFcParams();
-                    final byte prevFc = mBigramList.getBigrams(word1).get(word2);
-                    final byte fc = fcp.getFc();
-                    final boolean isValid = fcp.isValid();
-                    if (prevFc > 0 && prevFc == fc) {
-                        freq = fc & 0xFF;
-                    } else if (UserHistoryForgettingCurveUtils.
-                            needsToSave(fc, isValid, mAddLevel0Bigrams)) {
-                        freq = fc & 0xFF;
-                    } else {
-                        // Delete this entry
-                        freq = -1;
-                    }
-                } else {
-                    // Delete this entry
-                    freq = -1;
-                }
-            }
-            return freq;
-        }
-    }
-
-    @UsedForTesting
-    /* package for test */ void forceAddWordForTest(
-            final String word1, final String word2, final boolean isValid) {
-        mBigramListLock.lock();
-        try {
-            addToUserHistory(word1, word2, isValid);
-        } finally {
-            mBigramListLock.unlock();
-        }
-    }
-}
diff --git a/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryBigramList.java b/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryBigramList.java
index b93630a..f21db25 100644
--- a/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryBigramList.java
+++ b/java/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryBigramList.java
@@ -53,7 +53,7 @@
      * Called when loaded from the SQL DB.
      */
     public void addBigram(String word1, String word2, byte fcValue) {
-        if (UserHistoryDictionary.DBG_SAVE_RESTORE) {
+        if (UserHistoryPredictionDictionary.DBG_SAVE_RESTORE) {
             Log.d(TAG, "--- add bigram: " + word1 + ", " + word2 + ", " + fcValue);
         }
         final HashMap<String, Byte> map;
@@ -73,7 +73,7 @@
      * Called when inserted to the SQL DB.
      */
     public void updateBigram(String word1, String word2, byte fcValue) {
-        if (UserHistoryDictionary.DBG_SAVE_RESTORE) {
+        if (UserHistoryPredictionDictionary.DBG_SAVE_RESTORE) {
             Log.d(TAG, "--- update bigram: " + word1 + ", " + word2 + ", " + fcValue);
         }
         final HashMap<String, Byte> map;
diff --git a/java/src/com/android/inputmethod/latin/personalization/UserHistoryPredictionDictionary.java b/java/src/com/android/inputmethod/latin/personalization/UserHistoryPredictionDictionary.java
new file mode 100644
index 0000000..38987b6
--- /dev/null
+++ b/java/src/com/android/inputmethod/latin/personalization/UserHistoryPredictionDictionary.java
@@ -0,0 +1,33 @@
+/*
+ * 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.
+ */
+
+package com.android.inputmethod.latin.personalization;
+
+import com.android.inputmethod.latin.Dictionary;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+
+/**
+ * Locally gathers stats about the words user types and various other signals like auto-correction
+ * cancellation or manual picks. This allows the keyboard to adapt to the typist over time.
+ */
+public class UserHistoryPredictionDictionary extends PersonalizationPredictionDictionary {
+    /* package */ UserHistoryPredictionDictionary(final Context context, final String locale,
+            final SharedPreferences sp) {
+        super(context, locale, sp, Dictionary.TYPE_USER_HISTORY);
+    }
+}
diff --git a/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java b/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
index 7af83d0..8f9ef1d 100644
--- a/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
+++ b/tests/src/com/android/inputmethod/latin/personalization/UserHistoryDictionaryTests.java
@@ -70,7 +70,7 @@
         return new ArrayList<String>(wordSet);
     }
 
-    private void addToDict(final UserHistoryDictionary dict, final List<String> words) {
+    private void addToDict(final UserHistoryPredictionDictionary dict, final List<String> words) {
         String prevWord = null;
         for (String word : words) {
             dict.forceAddWordForTest(prevWord, word, true);
@@ -90,8 +90,8 @@
             final String locale = "testRandomWords";
             final String fileName = "UserHistoryDictionary." + locale + ".dict";
             dictFile = new File(getContext().getFilesDir(), fileName);
-            final UserHistoryDictionary dict =
-                    PersonalizationDictionaryHelper.getUserHistoryDictionary(
+            final UserHistoryPredictionDictionary dict =
+                    PersonalizationDictionaryHelper.getUserHistoryPredictionDictionary(
                             getContext(), locale, mPrefs);
             dict.isTest = true;
 
@@ -142,8 +142,8 @@
             for (int i = 0; i < numberOfLanguageSwitching; i++) {
                 final int index = i % numberOfLanguages;
                 // Switch languages to locales[index].
-                final UserHistoryDictionary dict =
-                        PersonalizationDictionaryHelper.getUserHistoryDictionary(
+                final UserHistoryPredictionDictionary dict =
+                        PersonalizationDictionaryHelper.getUserHistoryPredictionDictionary(
                                 getContext(), locales[index], mPrefs);
                 final List<String> words = generateWords(
                         numberOfWordsIntertedForEachLanguageSwitch, random);