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      1 /*
      2  * Copyright (C) 2012 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 #ifndef SINGLE_STATE_QUEUE_H
     18 #define SINGLE_STATE_QUEUE_H
     19 
     20 // Non-blocking single element state queue, or
     21 // Non-blocking single-reader / single-writer multi-word atomic load / store
     22 
     23 #include <stdint.h>
     24 
     25 namespace android {
     26 
     27 template<typename T> class SingleStateQueue {
     28 
     29 public:
     30 
     31     class Mutator;
     32     class Observer;
     33 
     34     struct Shared {
     35         // needs to be part of a union so don't define constructor or destructor
     36 
     37         friend class Mutator;
     38         friend class Observer;
     39 
     40 private:
     41         void                init() { mAck = 0; mSequence = 0; }
     42 
     43         volatile int32_t    mAck;
     44 #if 0
     45         int                 mPad[7];
     46         // cache line boundary
     47 #endif
     48         volatile int32_t    mSequence;
     49         T                   mValue;
     50     };
     51 
     52     class Mutator {
     53     public:
     54         Mutator(Shared *shared);
     55         /*virtual*/ ~Mutator() { }
     56 
     57         // push new value onto state queue, overwriting previous value;
     58         // returns a sequence number which can be used with ack()
     59         int32_t push(const T& value);
     60 
     61         // return true if most recent push has been observed
     62         bool ack();
     63 
     64         // return true if a push with specified sequence number or later has been observed
     65         bool ack(int32_t sequence);
     66 
     67     private:
     68         int32_t     mSequence;
     69         Shared * const mShared;
     70     };
     71 
     72     class Observer {
     73     public:
     74         Observer(Shared *shared);
     75         /*virtual*/ ~Observer() { }
     76 
     77         // return true if value has changed
     78         bool poll(T& value);
     79 
     80     private:
     81         int32_t     mSequence;
     82         int         mSeed;  // for PRNG
     83         Shared * const mShared;
     84     };
     85 
     86 #if 0
     87     SingleStateQueue(void /*Shared*/ *shared);
     88     /*virtual*/ ~SingleStateQueue() { }
     89 
     90     static size_t size() { return sizeof(Shared); }
     91 #endif
     92 
     93 };
     94 
     95 }   // namespace android
     96 
     97 #endif  // SINGLE_STATE_QUEUE_H
     98