Store last cryptid render in pref

This CL replaces the default hardcoded value for last render timestamp
with one tied to an Android preference.

(see go/probabilistic-cryptid-renderer for design; sorry, Googlers only)

Bug: 1061949
Change-Id: I5a975fbb96ea5bf24c4201f576928a3b0af33f4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118810Reviewed-by: sebsg <sebsg@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753213}

chrome/android/java/src/org/chromium/chrome/browser/cryptids/ProbabilisticCryptidRenderer.java

@@ -7,6 +7,8 @@ package org.chromium.chrome.browser.cryptids;
 import androidx.annotation.VisibleForTesting;
 
 import org.chromium.base.Log;
+import org.chromium.chrome.browser.preferences.ChromePreferenceKeys;
+import org.chromium.chrome.browser.preferences.SharedPreferencesManager;
 
 /**
  * Allows for cryptids to be displayed on the New Tab Page under certain probabilistic conditions.
@@ -25,15 +27,28 @@ public class ProbabilisticCryptidRenderer {
      */
     public boolean shouldUseCryptidRendering() {
         // TODO: This should be disabled unless enabled by variations.
-        return getRandom() < calculateProbability(getLastRenderTimestampMillis(),
-                       System.currentTimeMillis(), getRenderingMoratoriumLengthMillis(),
-                       getRampUpLengthMillis(), getMaxProbability());
+        return getRandom() < calculateProbability();
+    }
+
+    /**
+     * Records the current timestamp as the last render time in the appropriate pref.
+     */
+    public void recordRenderEvent() {
+        recordRenderEvent(System.currentTimeMillis());
     }
 
     // Protected for testing
     protected long getLastRenderTimestampMillis() {
-        // TODO: Store last instance timestamp in a pref and return it.
-        return 0;
+        long lastRenderTimestamp = SharedPreferencesManager.getInstance().readLong(
+                ChromePreferenceKeys.CRYPTID_LAST_RENDER_TIMESTAMP,
+                /* defaultValue = */ 0);
+        if (lastRenderTimestamp == 0) {
+            // If no render has ever been recorded, set the timestamp such that the current time is
+            // the end of the moratorium period.
+            lastRenderTimestamp = System.currentTimeMillis() - getRenderingMoratoriumLengthMillis();
+            recordRenderEvent(lastRenderTimestamp);
+        }
+        return lastRenderTimestamp;
     }
 
     protected long getRenderingMoratoriumLengthMillis() {
@@ -55,6 +70,12 @@ public class ProbabilisticCryptidRenderer {
         return (int) (Math.random() * DENOMINATOR);
     }
 
+    @VisibleForTesting
+    void recordRenderEvent(long timestamp) {
+        SharedPreferencesManager.getInstance().writeLong(
+                ChromePreferenceKeys.CRYPTID_LAST_RENDER_TIMESTAMP, timestamp);
+    }
+
     /**
      * Calculates the probability of display at a particular moment, based on various timestamp/
      * length factors. Roughly speaking, the probability starts at 0 after a rendering event,
@@ -90,4 +111,13 @@ public class ProbabilisticCryptidRenderer {
         float fractionOfRampUp = (float) (currentTimestamp - windowStartTimestamp) / rampUpLength;
         return (int) Math.round(fractionOfRampUp * maxProbability);
     }
+
+    /**
+     * Convenience method calling the static method above using the appropriate values.
+     */
+    @VisibleForTesting
+    int calculateProbability() {
+        return calculateProbability(getLastRenderTimestampMillis(), System.currentTimeMillis(),
+                getRenderingMoratoriumLengthMillis(), getRampUpLengthMillis(), getMaxProbability());
+    }
 }

chrome/android/junit/src/org/chromium/chrome/browser/cryptids/ProbabilisticCryptidRendererUnitTest.java

@@ -68,10 +68,40 @@ public class ProbabilisticCryptidRendererUnitTest {
         Assert.assertEquals(MAX_PROBABILITY, probPostRampup);
     }
 
+    @Test
+    @SmallTest
+    public void testPrefStorageDefaultValue() {
+        ProbabilisticCryptidRenderer render = new ProbabilisticCryptidRenderer();
+        long defaultRenderTimestamp = render.getLastRenderTimestampMillis();
+
+        long howLongSinceDefaultTime = System.currentTimeMillis() - defaultRenderTimestamp;
+        // We expect |howLongSinceDefaultTime| to be one moratorium length, because the default
+        // case should be rigged up to start users at the end of the moratorium.
+        long delta = howLongSinceDefaultTime - render.getRenderingMoratoriumLengthMillis();
+        Assert.assertTrue(String.format("Delta %d was larger than 10 seconds (10000)", delta),
+                delta < 10000); // Allow a 10 second grace period in case the test is slow.
+    }
+
+    @Test
+    @SmallTest
+    public void testPrefStorage() {
+        ProbabilisticCryptidRenderer render = new ProbabilisticCryptidRenderer();
+        // Set the last render event to be a crazy long time ago, ensuring max probability.
+        render.recordRenderEvent(1);
+        Assert.assertEquals(render.getMaxProbability(), render.calculateProbability());
+
+        // Immediately after an event, the probability should have dropped to 0.
+        render.recordRenderEvent();
+        Assert.assertEquals(0, render.calculateProbability());
+    }
+
     // Aggregator tests: these tests run |NUM_RUNS| invocations of shouldUseCryptidRendering to
     // verify that the number of trues returned is consistent with our assumptions about how often
     // this should happen, within |TOLERANCE|.
 
+    /**
+     * This fake bypasses any calls to pref logic, and also uses a seeded RNG.
+     */
     private static class FakeProbabilisticCrpytidRenderer extends ProbabilisticCryptidRenderer {
         public FakeProbabilisticCrpytidRenderer(long lastRenderDeltaFromNow) {
             mLastRenderTimestamp = System.currentTimeMillis() + lastRenderDeltaFromNow;

chrome/browser/preferences/android/java/src/org/chromium/chrome/browser/preferences/ChromePreferenceKeys.java

@@ -190,6 +190,8 @@ public final class ChromePreferenceKeys {
     public static final String CRASH_UPLOAD_SUCCESS_OTHER = "other_crash_success_upload";
     public static final String CRASH_UPLOAD_SUCCESS_RENDERER = "renderer_crash_success_upload";
 
+    public static final String CRYPTID_LAST_RENDER_TIMESTAMP = "Chrome.Cryptid.LastRenderTimestamp";
+
     public static final KeyPrefix CUSTOM_TABS_DEX_LAST_UPDATE_TIME_PREF_PREFIX =
             new KeyPrefix("pref_local_custom_tabs_module_dex_last_update_time_*");
     public static final String CUSTOM_TABS_LAST_URL = "pref_last_custom_tab_url";
@@ -674,6 +676,7 @@ public final class ChromePreferenceKeys {
                 CONTEXT_MENU_OPEN_IMAGE_IN_EPHEMERAL_TAB_CLICKED,
                 CONTEXT_MENU_OPEN_IN_EPHEMERAL_TAB_CLICKED,
                 CONTEXT_MENU_SEARCH_WITH_GOOGLE_LENS_CLICKED,
+                CRYPTID_LAST_RENDER_TIMESTAMP,
                 EXPLORE_OFFLINE_CONTENT_AVAILABILITY_STATUS,
                 FLAGS_CACHED.pattern(),
                 FLAGS_CACHED_DUET_TABSTRIP_INTEGRATION_ANDROID_ENABLED,