diff --git a/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java b/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java
new file mode 100644
index 0000000000000000000000000000000000000000..eb388172698579860f826b060220079523663213
--- /dev/null
+++ b/java/src/com/android/inputmethod/latin/UserHistoryForgettingCurveUtils.java
@@ -0,0 +1,120 @@
+/*
+ * Copyright (C) 2012 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;
+
+public class UserHistoryForgettingCurveUtils {
+ 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 int HALF_LIFE_HOURS = 48;
+
+ /* package */ static int fcToElapsedTime(byte fc) {
+ return fc & 0x0F;
+ }
+
+ /* package */ static int fcToCount(byte fc) {
+ return (fc >> 4) & 0x03;
+ }
+
+ /* package */ static int fcToLevel(byte fc) {
+ return (fc >> 6) & 0x03;
+ }
+
+ private static int calcFreq(int elapsedTime, int count, int level) {
+ if (level <= 0) {
+ // Reserved words, just return 0
+ return 0;
+ }
+ if (count == COUNT_MAX) {
+ // Temporary promote because it's frequently typed recently
+ ++level;
+ }
+ final int et = Math.min(FC_FREQ_MAX, Math.max(0, elapsedTime));
+ final int l = Math.min(FC_LEVEL_MAX, Math.max(0, level));
+ return MathUtils.SCORE_TABLE[l - 1][et];
+ }
+
+ /* pakcage */ static byte calcFc(int elapsedTime, int count, int level) {
+ final int et = Math.min(FC_FREQ_MAX, Math.max(0, elapsedTime));
+ final int c = Math.min(COUNT_MAX, Math.max(0, count));
+ final int l = Math.min(FC_LEVEL_MAX, Math.max(0, level));
+ return (byte)(et | (c << 4) | (l << 6));
+ }
+
+ public static int fcToFreq(byte fc) {
+ final int elapsedTime = fcToElapsedTime(fc);
+ final int count = fcToCount(fc);
+ final int level = fcToLevel(fc);
+ return calcFreq(elapsedTime, count, level);
+ }
+
+ public static byte pushElapsedTime(byte fc) {
+ int elapsedTime = fcToElapsedTime(fc);
+ int count = fcToCount(fc);
+ int level = fcToLevel(fc);
+ if (elapsedTime >= ELAPSED_TIME_MAX) {
+ // Downgrade level
+ elapsedTime = 0;
+ count = COUNT_MAX;
+ --level;
+ } else {
+ ++elapsedTime;
+ }
+ return calcFc(elapsedTime, count, level);
+ }
+
+ public static byte pushCount(byte fc, boolean isValid) {
+ final int elapsedTime = fcToElapsedTime(fc);
+ int count = fcToCount(fc);
+ int level = fcToLevel(fc);
+ if ((elapsedTime == 0 && count >= COUNT_MAX) || (isValid && level == 0)) {
+ // Upgrade level
+ ++level;
+ count = 0;
+ } else {
+ ++count;
+ }
+ return calcFc(0, count, level);
+ }
+
+ private static class MathUtils {
+ public static final int[][] SCORE_TABLE = new int[FC_LEVEL_MAX][ELAPSED_TIME_MAX + 1];
+ static {
+ for (int i = 0; i < FC_LEVEL_MAX; ++i) {
+ final double initialFreq;
+ if (i >= 2) {
+ initialFreq = (double)FC_FREQ_MAX;
+ } else if (i == 1) {
+ initialFreq = (double)FC_FREQ_MAX / 2;
+ } else if (i == 0) {
+ initialFreq = (double)FC_FREQ_MAX / 4;
+ } else {
+ continue;
+ }
+ for (int j = 0; j < ELAPSED_TIME_MAX; ++j) {
+ final double elapsedHour = j * ELAPSED_TIME_INTERVAL_HOURS;
+ final double freq =
+ initialFreq * Math.pow(initialFreq, elapsedHour / HALF_LIFE_HOURS);
+ final int intFreq = Math.min(FC_FREQ_MAX, Math.max(0, (int)freq));
+ SCORE_TABLE[i][j] = intFreq;
+ }
+ }
+ }
+ }
+}
diff --git a/tests/src/com/android/inputmethod/latin/ForgettingCurveTests.java b/tests/src/com/android/inputmethod/latin/ForgettingCurveTests.java
new file mode 100644
index 0000000000000000000000000000000000000000..a2e71c11a7510f081204f42f0c9996a06dd212d5
--- /dev/null
+++ b/tests/src/com/android/inputmethod/latin/ForgettingCurveTests.java
@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2012 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;
+
+import android.test.AndroidTestCase;
+
+public class ForgettingCurveTests extends AndroidTestCase {
+ public void testFcToFreq() {
+ for (int i = 0; i < Byte.MAX_VALUE; ++i) {
+ final byte fc = (byte)i;
+ final int e = UserHistoryForgettingCurveUtils.fcToElapsedTime(fc);
+ final int c = UserHistoryForgettingCurveUtils.fcToCount(fc);
+ final int l = UserHistoryForgettingCurveUtils.fcToLevel(fc);
+ final byte fc2 = UserHistoryForgettingCurveUtils.calcFc(e, c, l);
+ assertEquals(fc, fc2);
+ }
+ byte fc = 0;
+ int l;
+ for (int i = 0; i < 4; ++i) {
+ for (int j = 0; j < (UserHistoryForgettingCurveUtils.COUNT_MAX + 1); ++j) {
+ fc = UserHistoryForgettingCurveUtils.pushCount(fc, true);
+ }
+ l = UserHistoryForgettingCurveUtils.fcToLevel(fc);
+ assertEquals(l, Math.max(1, Math.min(i + 1, 3)));
+ }
+ fc = 0;
+ for (int i = 0; i < 4; ++i) {
+ for (int j = 0; j < (UserHistoryForgettingCurveUtils.COUNT_MAX + 1); ++j) {
+ fc = UserHistoryForgettingCurveUtils.pushCount(fc, false);
+ }
+ l = UserHistoryForgettingCurveUtils.fcToLevel(fc);
+ assertEquals(l, Math.min(i + 1, 3));
+ }
+ for (int i = 0; i < 4; ++i) {
+ for (int j = 0; j < (UserHistoryForgettingCurveUtils.ELAPSED_TIME_MAX + 1); ++j) {
+ fc = UserHistoryForgettingCurveUtils.pushElapsedTime(fc);
+ }
+ l = UserHistoryForgettingCurveUtils.fcToLevel(fc);
+ assertEquals(l, Math.max(0, 2 - i));
+ }
+ }
+}