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diff --git a/services/audioflinger/StateQueue.h b/services/audioflinger/StateQueue.h
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+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ANDROID_AUDIO_STATE_QUEUE_H
+#define ANDROID_AUDIO_STATE_QUEUE_H
+
+namespace android {
+
+// manages a FIFO queue of states
+template<typename T> class StateQueue {
+
+public:
+            StateQueue();
+    virtual ~StateQueue();
+
+    // Observer APIs
+
+    // Poll for a state change.  Returns a pointer to a read-only state,
+    // or NULL if the state has not been initialized yet.
+    // If a new state has not pushed by mutator since the previous poll,
+    // then the returned pointer will be unchanged.
+    // The previous state pointer is guaranteed to still be valid;
+    // this allows the observer to diff the previous and new states.
+    const T* poll();
+
+    // Mutator APIs
+
+    // Begin a mutation.  Returns a pointer to a read/write state, except the
+    // first time it is called the state is write-only and _must_ be initialized.
+    // Mutations cannot be nested.
+    // If the state is dirty and has not been pushed onto the state queue yet, then
+    // this new mutation will be squashed together with the previous one.
+    T*      begin();
+
+    // End the current mutation and indicate whether caller modified the state.
+    // If didModify is true, then the state is marked dirty (in need of pushing).
+    // There is no rollback option because modifications are done in place.
+    // Does not automatically push the new state onto the state queue.
+    void    end(bool didModify = true);
+
+    // Push a new state, if any, out to the observer via the state queue.
+    // For BLOCK_NEVER, returns:
+    //      true if not dirty, or dirty and pushed successfully
+    //      false if dirty and not pushed because that would block; remains dirty
+    // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true.
+    // No-op if there are no pending modifications (not dirty), except
+    //      for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged.
+    // Must not be called in the middle of a mutation.
+    enum block_t {
+        BLOCK_NEVER,        // do not block
+        BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push
+        BLOCK_UNTIL_ACKED,  // also block until the push is acknowledged by the observer
+    };
+    bool    push(block_t block = BLOCK_NEVER);
+
+    // Return whether the current state is dirty (modified and not pushed).
+    bool    isDirty() const { return mIsDirty; }
+
+private:
+    static const unsigned kN = 4;       // values != 4 are not supported by this code
+    T                 mStates[kN];      // written by mutator, read by observer
+
+    // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
+    volatile const T* mNext; // written by mutator to advance next, read by observer
+    volatile const T* mAck;  // written by observer to acknowledge advance of next, read by mutator
+
+    // only used by observer
+    const T*          mCurrent;         // most recent value returned by poll()
+
+    // only used by mutator
+    T*                mMutating;        // where updates by mutator are done in place
+    const T*          mExpecting;       // what the mutator expects mAck to be set to
+    bool              mInMutation;      // whether we're currently in the middle of a mutation
+    bool              mIsDirty;         // whether mutating state has been modified since last push
+    bool              mIsInitialized;   // whether mutating state has been initialized yet
+
+};  // class StateQueue
+
+}   // namespace android
+
+#endif  // ANDROID_AUDIO_STATE_QUEUE_H