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 */
30class MonotonicCounter {
31
32public:
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     * Advance the counter if delta is positive.
45     * @return current value of the counter
46     */
47    int64_t increment(int64_t delta) {
48        if (delta > 0) {
49            mCounter64 += delta;
50        }
51        return mCounter64;
52    }
53
54    /**
55     * Advance the 64-bit counter if (current32 - previousCurrent32) > 0.
56     * This can be used to convert a 32-bit counter that may be wrapping into
57     * a monotonic 64-bit counter.
58     *
59     * This counter32 should NOT be allowed to advance by more than 0x7FFFFFFF between calls.
60     * Think of the wrapping counter like a sine wave. If the frequency of the signal
61     * is more than half the sampling rate (Nyquist rate) then you cannot measure it properly.
62     * If the counter wraps around every 24 hours then we should measure it with a period
63     * of less than 12 hours.
64     *
65     * @return current value of the 64-bit counter
66     */
67    int64_t update32(int32_t counter32) {
68        int32_t delta = counter32 - mCounter32;
69        // protect against the mCounter64 going backwards
70        if (delta > 0) {
71            mCounter64 += delta;
72            mCounter32 = counter32;
73        }
74        return mCounter64;
75    }
76
77    /**
78     * Reset the stored value of the 32-bit counter.
79     * This is used if your counter32 has been reset to zero.
80     */
81    void reset32() {
82        mCounter32 = 0;
83    }
84
85private:
86    int64_t mCounter64 = 0;
87    int32_t mCounter32 = 0;
88};
89
90
91#endif //UTILITY_MONOTONIC_COUNTER_H
92