1/*
2
3 This Source Code Form is subject to the terms of the Mozilla Public
4 License, v. 2.0. If a copy of the MPL was not distributed with this file,
5 You can obtain one at http://mozilla.org/MPL/2.0/.
6
7 Copyright (c) 2013, Armon Dadgar armon.dadgar@gmail.com
8 Copyright (c) 2013, Mitchell Hashimoto mitchell.hashimoto@gmail.com
9
10 Alternatively, the contents of this file may be used under the terms
11 of the GNU General Public License Version 3 or later, as described below:
12
13 This file is free software: you may copy, redistribute and/or modify
14 it under the terms of the GNU General Public License as published by the
15 Free Software Foundation, either version 3 of the License, or (at your
16 option) any later version.
17
18 This file is distributed in the hope that it will be useful, but
19 WITHOUT ANY WARRANTY; without even the implied warranty of
20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
21 Public License for more details.
22
23 You should have received a copy of the GNU General Public License
24 along with this program. If not, see http://www.gnu.org/licenses/.
25
26*/
27
28package lamport
29
30import (
31 "sync/atomic"
32)
33
34var _ Clock = &MemClock{}
35
36// MemClock is a thread safe implementation of a lamport clock. It
37// uses efficient atomic operations for all of its functions, falling back
38// to a heavy lock only if there are enough CAS failures.
39type MemClock struct {
40 counter uint64
41}
42
43// NewMemClock create a new clock with the value 1.
44// Value 0 is considered as invalid.
45func NewMemClock() *MemClock {
46 return &MemClock{
47 counter: 1,
48 }
49}
50
51// NewMemClockWithTime create a new clock with a value.
52func NewMemClockWithTime(time uint64) *MemClock {
53 return &MemClock{
54 counter: time,
55 }
56}
57
58// Time is used to return the current value of the lamport clock
59func (mc *MemClock) Time() Time {
60 return Time(atomic.LoadUint64(&mc.counter))
61}
62
63// Increment is used to return the value of the lamport clock and increment it afterwards
64func (mc *MemClock) Increment() (Time, error) {
65 return Time(atomic.AddUint64(&mc.counter, 1) - 1), nil
66}
67
68// Witness is called to update our local clock if necessary after
69// witnessing a clock value received from another process
70func (mc *MemClock) Witness(v Time) error {
71WITNESS:
72 // If the other value is old, we do not need to do anything
73 cur := atomic.LoadUint64(&mc.counter)
74 other := uint64(v)
75 if other < cur {
76 return nil
77 }
78
79 // Ensure that our local clock is at least one ahead.
80 if !atomic.CompareAndSwapUint64(&mc.counter, cur, other+1) {
81 // CAS: CompareAndSwap
82 // The CAS failed, so we just retry. Eventually our CAS should
83 // succeed or a future witness will pass us by and our witness
84 // will end.
85 goto WITNESS
86 }
87
88 return nil
89}