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/*
This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this file,
You can obtain one at http://mozilla.org/MPL/2.0/.
Copyright (c) 2013, Armon Dadgar armon.dadgar@gmail.com
Copyright (c) 2013, Mitchell Hashimoto mitchell.hashimoto@gmail.com
Alternatively, the contents of this file may be used under the terms
of the GNU General Public License Version 3 or later, as described below:
This file is free software: you may copy, redistribute and/or modify
it under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.
This file is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see http://www.gnu.org/licenses/.
*/
// Note: this code originally originate from Hashicorp's Serf but has been changed since to fit git-bug's need.
// Note: this Lamport clock implementation is different than the algorithms you can find, notably Wikipedia or the
// original Serf implementation. The reason is lie to what constitute an event in this distributed system.
// Commonly, events happen when messages are sent or received, whereas in git-bug events happen when some data is
// written, but *not* when read. This is why Witness set the time to the max seen value instead of max seen value +1.
// See https://cs.stackexchange.com/a/133730/129795
package lamport
import (
"sync/atomic"
)
var _ Clock = &MemClock{}
// MemClock is a thread safe implementation of a lamport clock. It
// uses efficient atomic operations for all of its functions, falling back
// to a heavy lock only if there are enough CAS failures.
type MemClock struct {
counter uint64
}
// NewMemClock create a new clock with the value 1.
// Value 0 is considered as invalid.
func NewMemClock() *MemClock {
return &MemClock{
counter: 1,
}
}
// NewMemClockWithTime create a new clock with a value.
func NewMemClockWithTime(time uint64) *MemClock {
return &MemClock{
counter: time,
}
}
// Time is used to return the current value of the lamport clock
func (mc *MemClock) Time() Time {
return Time(atomic.LoadUint64(&mc.counter))
}
// Increment is used to return the value of the lamport clock and increment it afterwards
func (mc *MemClock) Increment() (Time, error) {
return Time(atomic.AddUint64(&mc.counter, 1)), nil
}
// Witness is called to update our local clock if necessary after
// witnessing a clock value received from another process
func (mc *MemClock) Witness(v Time) error {
WITNESS:
// If the other value is old, we do not need to do anything
cur := atomic.LoadUint64(&mc.counter)
other := uint64(v)
if other <= cur {
return nil
}
// Ensure that our local clock is at least one ahead.
if !atomic.CompareAndSwapUint64(&mc.counter, cur, other) {
// CAS: CompareAndSwap
// The CAS failed, so we just retry. Eventually our CAS should
// succeed or a future witness will pass us by and our witness
// will end.
goto WITNESS
}
return nil
}
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