Basic implementation of sync queue

astrid/plugin-src/com/todoroo/astrid/actfm/sync/ActFmSyncQueue.java

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+package com.todoroo.astrid.actfm.sync;
+
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+
+import com.todoroo.astrid.data.RemoteModel;
+
+public class ActFmSyncQueue {
+
+    private static final ActFmSyncQueue INSTANCE = new ActFmSyncQueue();
+
+    public static class SyncQueueEntry {
+        public Class<? extends RemoteModel> modelType;
+        public Long id;
+
+        public SyncQueueEntry(Class<? extends RemoteModel> modelType, Long id) {
+            this.modelType = modelType;
+            this.id = id;
+        }
+
+        @Override
+        public int hashCode() {
+            final int prime = 31;
+            int result = 1;
+            result = prime * result
+                    + ((modelType == null) ? 0 : modelType.hashCode());
+            result = prime * result + ((id == null) ? 0 : id.hashCode());
+            return result;
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj)
+                return true;
+            if (obj == null)
+                return false;
+            if (getClass() != obj.getClass())
+                return false;
+            SyncQueueEntry other = (SyncQueueEntry) obj;
+            if (modelType == null) {
+                if (other.modelType != null)
+                    return false;
+            } else if (!modelType.equals(other.modelType))
+                return false;
+            if (id == null) {
+                if (other.id != null)
+                    return false;
+            } else if (!id.equals(other.id))
+                return false;
+            return true;
+        }
+
+    }
+
+    private final List<SyncQueueEntry> queue;
+    private final HashSet<SyncQueueEntry> elements;
+
+    private ActFmSyncQueue() {
+        queue = new LinkedList<SyncQueueEntry>();
+        elements = new HashSet<SyncQueueEntry>();
+    }
+
+    public synchronized void enqueue(SyncQueueEntry entry) {
+        if (!elements.contains(entry)) {
+            queue.add(entry);
+            elements.add(entry);
+        }
+    }
+
+    public synchronized SyncQueueEntry dequeue() {
+        if (queue.isEmpty())
+            return null;
+        SyncQueueEntry entry = queue.remove(0);
+        elements.remove(entry);
+        return entry;
+    }
+
+    public static ActFmSyncQueue getInstance() {
+        return INSTANCE;
+    }
+
+}