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app/src/main/java/eu/faircode/email/ActivityView.java

@@ -75,6 +75,7 @@ import androidx.fragment.app.FragmentTransaction;
 import androidx.lifecycle.Lifecycle;
 import androidx.lifecycle.Observer;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
+import androidx.paging.PagedList;
 
 public class ActivityView extends ActivityBilling implements FragmentManager.OnBackStackChangedListener {
     private View view;
@@ -83,6 +84,7 @@ public class ActivityView extends ActivityBilling implements FragmentManager.OnB
     private ActionBarDrawerToggle drawerToggle;
 
     private long attachment = -1;
+    private PagedList<TupleMessageEx> messages = null;
 
     private static final int ATTACHMENT_BUFFER_SIZE = 8192; // bytes
 
@@ -883,4 +885,29 @@ public class ActivityView extends ActivityBilling implements FragmentManager.OnB
                 }.load(this, args);
             }
     }
+
+    void setMessages(PagedList<TupleMessageEx> messages) {
+        this.messages = messages;
+    }
+
+    public String[] getPrevNext(String thread) {
+        boolean found = false;
+        TupleMessageEx prev = null;
+        TupleMessageEx next = null;
+        for (int i = 0; i < messages.size(); i++) {
+            TupleMessageEx item = messages.get(i);
+            if (item == null)
+                continue;
+            if (found) {
+                next = item;
+                messages.loadAround(i);
+                break;
+            }
+            if (thread.equals(item.thread))
+                found = true;
+            else
+                prev = item;
+        }
+        return new String[]{prev == null ? null : prev.thread, next == null ? null : next.thread};
+    }
 }