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      1 /*
      2  * Copyright 2016 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 UTILITY_MONOTONIC_COUNTER_H
     18 #define UTILITY_MONOTONIC_COUNTER_H
     19 
     20 #include <stdint.h>
     21 
     22 /**
     23  * Maintain a 64-bit monotonic counter.
     24  * Can be used to track a 32-bit counter that wraps or gets reset.
     25  *
     26  * Note that this is not atomic and has no interior locks.
     27  * A caller will need to provide their own exterior locking
     28  * if they need to use it from multiple threads.
     29  */
     30 class MonotonicCounter {
     31 
     32 public:
     33     MonotonicCounter() {};
     34     virtual ~MonotonicCounter() {};
     35 
     36     /**
     37      * @return current value of the counter
     38      */
     39     int64_t get() const {
     40         return mCounter64;
     41     }
     42 
     43     /**
     44      * set the current value of the counter
     45      */
     46     void set(int64_t counter) {
     47         mCounter64 = counter;
     48     }
     49 
     50     /**
     51      * Advance the counter if delta is positive.
     52      * @return current value of the counter
     53      */
     54     int64_t increment(int64_t delta) {
     55         if (delta > 0) {
     56             mCounter64 += delta;
     57         }
     58         return mCounter64;
     59     }
     60 
     61     /**
     62      * Advance the 64-bit counter if (current32 - previousCurrent32) > 0.
     63      * This can be used to convert a 32-bit counter that may be wrapping into
     64      * a monotonic 64-bit counter.
     65      *
     66      * This counter32 should NOT be allowed to advance by more than 0x7FFFFFFF between calls.
     67      * Think of the wrapping counter like a sine wave. If the frequency of the signal
     68      * is more than half the sampling rate (Nyquist rate) then you cannot measure it properly.
     69      * If the counter wraps around every 24 hours then we should measure it with a period
     70      * of less than 12 hours.
     71      *
     72      * @return current value of the 64-bit counter
     73      */
     74     int64_t update32(int32_t counter32) {
     75         int32_t delta = counter32 - mCounter32;
     76         // protect against the mCounter64 going backwards
     77         if (delta > 0) {
     78             mCounter64 += delta;
     79             mCounter32 = counter32;
     80         }
     81         return mCounter64;
     82     }
     83 
     84     /**
     85      * Reset the stored value of the 32-bit counter.
     86      * This is used if your counter32 has been reset to zero.
     87      */
     88     void reset32() {
     89         mCounter32 = 0;
     90     }
     91 
     92     /**
     93      * Round 64-bit counter up to a multiple of the period.
     94      *
     95      * @param period might be, for example, a buffer capacity
     96      */
     97     void roundUp64(int32_t period) {
     98         if (period > 0) {
     99             int64_t numPeriods = (mCounter64 + period - 1) / period;
    100             mCounter64 = numPeriods * period;
    101         }
    102     }
    103 
    104 private:
    105     int64_t mCounter64 = 0;
    106     int32_t mCounter32 = 0;
    107 };
    108 
    109 
    110 #endif //UTILITY_MONOTONIC_COUNTER_H
    111