aboutsummaryrefslogtreecommitdiffstats
path: root/util/lamport.go
diff options
context:
space:
mode:
authorMichael Muré <batolettre@gmail.com>2018-08-05 15:27:46 +0200
committerMichael Muré <batolettre@gmail.com>2018-08-05 15:27:46 +0200
commitb2f20c9a61715b0a60f9e55688dbed221d775c7e (patch)
tree827d9a62a86cdd10061cb5f0289935c158f54191 /util/lamport.go
parent90fb85e067326b0f0a121e781f87df762f385592 (diff)
downloadgit-bug-b2f20c9a61715b0a60f9e55688dbed221d775c7e.tar.gz
util: add a Lamport clock implementation as well as a persistable one
Diffstat (limited to 'util/lamport.go')
-rw-r--r--util/lamport.go85
1 files changed, 85 insertions, 0 deletions
diff --git a/util/lamport.go b/util/lamport.go
new file mode 100644
index 00000000..c75e1d15
--- /dev/null
+++ b/util/lamport.go
@@ -0,0 +1,85 @@
+/*
+
+ 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/.
+
+*/
+
+package util
+
+import (
+ "sync/atomic"
+)
+
+// LamportClock 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 LamportClock struct {
+ counter uint64
+}
+
+// LamportTime is the value of a LamportClock.
+type LamportTime uint64
+
+func NewLamportClock() LamportClock {
+ return LamportClock{
+ counter: 1,
+ }
+}
+
+func NewLamportClockWithTime(time uint64) LamportClock {
+ return LamportClock{
+ counter: time,
+ }
+}
+
+// Time is used to return the current value of the lamport clock
+func (l *LamportClock) Time() LamportTime {
+ return LamportTime(atomic.LoadUint64(&l.counter))
+}
+
+// Increment is used to increment and return the value of the lamport clock
+func (l *LamportClock) Increment() LamportTime {
+ return LamportTime(atomic.AddUint64(&l.counter, 1))
+}
+
+// Witness is called to update our local clock if necessary after
+// witnessing a clock value received from another process
+func (l *LamportClock) Witness(v LamportTime) {
+WITNESS:
+ // If the other value is old, we do not need to do anything
+ cur := atomic.LoadUint64(&l.counter)
+ other := uint64(v)
+ if other < cur {
+ return
+ }
+
+ // Ensure that our local clock is at least one ahead.
+ if !atomic.CompareAndSwapUint64(&l.counter, cur, other+1) {
+ // 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
+ }
+}