Unify NewTabPageItem and ItemGroup into a single tree-structured interface.

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/InnerNode.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/InnerNode.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/InnerNode.java
new file mode 100644
index 0000000000000000000000000000000000000000..8c0078ae9007e8e6ce91dd69738dc4b2dede56a6
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/InnerNode.java
@@ -0,0 +1,90 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.ntp.cards;
+
+import org.chromium.chrome.browser.ntp.snippets.SnippetArticle;
+
+import java.util.List;
+
+/**
+ * An inner node in the tree: the root of a subtree, with a list of child nodes.
+ */
+public abstract class InnerNode extends ChildNode implements NodeParent {
+    public InnerNode(NodeParent parent) {
+        super(parent);
+    }
+
+    protected abstract List<TreeNode> getChildren();
+
+    int getChildIndexForPosition(int position) {
+        List<TreeNode> children = getChildren();
+        int numItems = 0;
+        int numChildren = children.size();
+        for (int i = 0; i < numChildren; i++) {
+            numItems += children.get(i).getItemCount();
+            if (position < numItems) return i;
+        }
+        return -1;
+    }
+
+    private int getStartingOffsetForChildIndex(int childIndex) {
+        List<TreeNode> children = getChildren();
+        int offset = 0;
+        for (int i = 0; i < childIndex; i++) {
+            offset += children.get(i).getItemCount();
+        }
+        return offset;
+    }
+
+    int getStartingOffsetForChild(TreeNode child) {
+        return getStartingOffsetForChildIndex(getChildren().indexOf(child));
+    }
+
+    @Override
+    public int getItemCount() {
+        int numItems = 0;
+        for (TreeNode child : getChildren()) {
+            numItems += child.getItemCount();
+        }
+        return numItems;
+    }
+
+    @Override
+    @ItemViewType
+    public int getItemViewType(int position) {
+        int index = getChildIndexForPosition(position);
+        return getChildren().get(index).getItemViewType(
+                position - getStartingOffsetForChildIndex(index));
+    }
+
+    @Override
+    public void onBindViewHolder(NewTabPageViewHolder holder, int position) {
+        int index = getChildIndexForPosition(position);
+        getChildren().get(index).onBindViewHolder(
+                holder, position - getStartingOffsetForChildIndex(index));
+    }
+
+    @Override
+    public SnippetArticle getSuggestionAt(int position) {
+        int index = getChildIndexForPosition(position);
+        return getChildren().get(index).getSuggestionAt(
+                position - getStartingOffsetForChildIndex(index));
+    }
+
+    @Override
+    public void onItemRangeChanged(TreeNode child, int index, int count) {
+        notifyItemRangeChanged(getStartingOffsetForChild(child) + index, count);
+    }
+
+    @Override
+    public void onItemRangeInserted(TreeNode child, int index, int count) {
+        notifyItemRangeInserted(getStartingOffsetForChild(child) + index, count);
+    }
+
+    @Override
+    public void onItemRangeRemoved(TreeNode child, int index, int count) {
+        notifyItemRangeRemoved(getStartingOffsetForChild(child) + index, count);
+    }
+}