package serf
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import (
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"sync/atomic"
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)
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// LamportClock is a thread safe implementation of a lamport clock. It
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// uses efficient atomic operations for all of its functions, falling back
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// to a heavy lock only if there are enough CAS failures.
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type LamportClock struct {
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counter uint64
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}
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// LamportTime is the value of a LamportClock.
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type LamportTime uint64
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// Time is used to return the current value of the lamport clock
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func (l *LamportClock) Time() LamportTime {
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return LamportTime(atomic.LoadUint64(&l.counter))
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}
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// Increment is used to increment and return the value of the lamport clock
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func (l *LamportClock) Increment() LamportTime {
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return LamportTime(atomic.AddUint64(&l.counter, 1))
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}
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// Witness is called to update our local clock if necessary after
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// witnessing a clock value received from another process
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func (l *LamportClock) Witness(v LamportTime) {
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WITNESS:
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// If the other value is old, we do not need to do anything
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cur := atomic.LoadUint64(&l.counter)
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other := uint64(v)
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if other < cur {
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return
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}
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// Ensure that our local clock is at least one ahead.
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if !atomic.CompareAndSwapUint64(&l.counter, cur, other+1) {
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// The CAS failed, so we just retry. Eventually our CAS should
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// succeed or a future witness will pass us by and our witness
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// will end.
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goto WITNESS
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}
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}
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