Store message count, unread count and timestamp of latest message

in group metadata to be able to speed up group listings.

Closes #584, #586, #585

briar-api/src/org/briarproject/api/clients/MessageTracker.java

@@ -0,0 +1,44 @@
+package org.briarproject.api.clients;
+
+import org.briarproject.api.db.DbException;
+import org.briarproject.api.db.Transaction;
+import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.Message;
+import org.briarproject.api.sync.MessageId;
+
+public interface MessageTracker {
+
+	/**
+	 * Gets the number of visible and unread messages in the group
+	 * as well as the timestamp of the latest message
+	 **/
+	GroupCount getGroupCount(GroupId g) throws DbException;
+
+	/**
+	 * Marks a message as read or unread and updates the group counts in g.
+	 **/
+	void setReadFlag(GroupId g, MessageId m, boolean read) throws DbException;
+
+	class GroupCount {
+		private final long msgCount, unreadCount, latestMsgTime;
+
+		public GroupCount(long msgCount, long unreadCount, long latestMsgTime) {
+			this.msgCount = msgCount;
+			this.unreadCount = unreadCount;
+			this.latestMsgTime = latestMsgTime;
+		}
+
+		public long getMsgCount() {
+			return msgCount;
+		}
+
+		public long getUnreadCount() {
+			return unreadCount;
+		}
+
+		public long getLatestMsgTime() {
+			return latestMsgTime;
+		}
+	}
+
+}