[NTP suggestions UI] Track precise count of suggestions seen.

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java
index d0406d625ed5517fbf681827825046b9b4601ed0..e393cafc04fa1e8155fcffbc27b8ea47a4557c23 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/SuggestionsSection.java
@@ -45,12 +45,9 @@ public class SuggestionsSection extends InnerNode {
 
     private boolean mIsNtpDestroyed;
 
-    // Keep track of impressions of the suggestions so that we replace only suggestions that
-    // have not been impressed, yet. We keep track of the first suggestion separately as the
-    // first is often impressed in the form of a peeking card and we still want to be able to
-    // replace something in this case.
-    private boolean mFirstSuggestionSeen;
-    private boolean mSubsequentSuggestionsSeen;
+    // Keep track of how many suggestions have been seen by the user so that we replace only
+    // suggestions that have not been seen, yet.
+    private int mNumberOfSuggestionsSeen;
 
     /**
      * Delegate interface that allows dismissing this section without introducing
@@ -136,13 +133,13 @@ public class SuggestionsSection extends InnerNode {
         }
 
         /**
-         * Clears all suggestions except the first one.
+         * Clears all suggestions except for the first {@code n} suggestions.
          */
-        private void clearAllButFirst() {
+        private void clearAllButFirstN(int n) {
             int itemCount = mSuggestions.size();
-            if (itemCount > 1) {
-                mSuggestions.subList(1, itemCount).clear();
-                notifyItemRangeRemoved(1, itemCount - 1);
+            if (itemCount > n) {
+                mSuggestions.subList(n, itemCount).clear();
+                notifyItemRangeRemoved(n, itemCount - 1);
             }
         }
 
@@ -268,18 +265,15 @@ public class SuggestionsSection extends InnerNode {
      * @param position Position in the list being shown (the first suggestion being at index 1,
      * as at index 0, there is a non-suggestion).
      */
-    @VisibleForTesting
-    public void childSeen(int position) {
+    private void childSeen(int position) {
         Log.d(TAG, "childSeen: position %d in category %d", position, mCategoryInfo.getCategory());
         assert getStartingOffsetForChild(mSuggestionsList) == 1;
+        // We assume all non-snippet cards come after all cards of type SNIPPET.
         if (getItemViewType(position) == ItemViewType.SNIPPET) {
-            // We assume all non-snippet cards come after all cards of type SNIPPET.
-            int positionAmongSuggestions = position - 1;
-            if (positionAmongSuggestions == 0) {
-                mFirstSuggestionSeen = true;
-            } else if (positionAmongSuggestions > 0) {
-                mSubsequentSuggestionsSeen = true;
-            }
+            // As asserted above, first suggestion has position 1, etc., so the position of this
+            // child coincides with the number of suggestions above this child (including this one).
+            mNumberOfSuggestionsSeen =
+                    Math.max(mNumberOfSuggestionsSeen, position);
         }
     }
 
@@ -332,26 +326,34 @@ public class SuggestionsSection extends InnerNode {
 
         // Remove suggestions to be replaced.
         if (replaceExisting && hasSuggestions()) {
-            if (CardsVariationParameters.ignoreUpdatesForExistingSuggestions()
-                    || mSubsequentSuggestionsSeen) {
-                Log.d(TAG, "setSuggestions: replacing existing suggestion not possible");
+            if (CardsVariationParameters.ignoreUpdatesForExistingSuggestions()) {
+                Log.d(TAG, "setSuggestions: replacing existing suggestion disabled");
                 return;
             }
 
-            if (mFirstSuggestionSeen) {
-                Log.d(TAG, "setSuggestions: keeping the first suggestion");
-                mSuggestionsList.clearAllButFirst();
+            if (mNumberOfSuggestionsSeen >= getSuggestionsCount()) {
+                Log.d(TAG, "setSuggestions: replacing existing suggestion not possible, all seen");
+                return;
+            }
 
-                // Make sure that {@code mSuggestionsList} will contain as many elements as newly
-                // provided in {@code suggestions}. Remove the kept first element from the new
-                // collection, if it repeats there. Otherwise, remove the last element of the new
-                // collection.
-                if (!suggestions.remove(mSuggestionsList.getSuggestionAt(0))) {
-                    suggestions.remove(suggestions.size() - 1);
+            Log.d(TAG, "setSuggestions: keeping the first %d suggestion",
+                        mNumberOfSuggestionsSeen);
+            mSuggestionsList.clearAllButFirstN(mNumberOfSuggestionsSeen);
+
+            if (mNumberOfSuggestionsSeen > 0) {
+                // Make sure that mSuggestionsList will contain as many elements as newly provided
+                // in suggestions. Remove the kept first element from the new collection, if it
+                // repeats there. Otherwise, remove the last element of the new collection.
+                int targetCountToAppend =
+                        Math.max(0, suggestions.size() - mNumberOfSuggestionsSeen);
+                for (SnippetArticle suggestion : mSuggestionsList) {
+                    suggestions.remove(suggestion);
+                }
+                if (suggestions.size() > targetCountToAppend) {
+                    Log.d(TAG, "setSuggestions: removing %d excess elements from the end",
+                            suggestions.size() - targetCountToAppend);
+                    suggestions.subList(targetCountToAppend, suggestions.size()).clear();
                 }
-            } else {
-                Log.d(TAG, "setSuggestions: removing all suggestions");
-                mSuggestionsList.clear();
             }
         }