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// Package backoff provides an exponential-backoff implementation.
package backoff
import (
"math"
"math/rand"
"sync/atomic"
"time"
)
// Backoff is a time.Duration counter, starting at Min. After every call to
// the Duration method the current timing is multiplied by Factor, but it
// never exceeds Max.
//
// Backoff is not generally concurrent-safe, but the ForAttempt method can
// be used concurrently.
type Backoff struct {
attempt uint64
// Factor is the multiplying factor for each increment step
Factor float64
// Jitter eases contention by randomizing backoff steps
Jitter bool
// Min and Max are the minimum and maximum values of the counter
Min, Max time.Duration
}
// Duration returns the duration for the current attempt before incrementing
// the attempt counter. See ForAttempt.
func (b *Backoff) Duration() time.Duration {
d := b.ForAttempt(float64(atomic.AddUint64(&b.attempt, 1) - 1))
return d
}
const maxInt64 = float64(math.MaxInt64 - 512)
// ForAttempt returns the duration for a specific attempt. This is useful if
// you have a large number of independent Backoffs, but don't want use
// unnecessary memory storing the Backoff parameters per Backoff. The first
// attempt should be 0.
//
// ForAttempt is concurrent-safe.
func (b *Backoff) ForAttempt(attempt float64) time.Duration {
// Zero-values are nonsensical, so we use
// them to apply defaults
min := b.Min
if min <= 0 {
min = 100 * time.Millisecond
}
max := b.Max
if max <= 0 {
max = 10 * time.Second
}
if min >= max {
// short-circuit
return max
}
factor := b.Factor
if factor <= 0 {
factor = 2
}
//calculate this duration
minf := float64(min)
durf := minf * math.Pow(factor, attempt)
if b.Jitter {
durf = rand.Float64()*(durf-minf) + minf
}
//ensure float64 wont overflow int64
if durf > maxInt64 {
return max
}
dur := time.Duration(durf)
//keep within bounds
if dur < min {
return min
}
if dur > max {
return max
}
return dur
}
// Reset restarts the current attempt counter at zero.
func (b *Backoff) Reset() {
atomic.StoreUint64(&b.attempt, 0)
}
// Attempt returns the current attempt counter value.
func (b *Backoff) Attempt() float64 {
return float64(atomic.LoadUint64(&b.attempt))
}
// Copy returns a backoff with equals constraints as the original
func (b *Backoff) Copy() *Backoff {
return &Backoff{
Factor: b.Factor,
Jitter: b.Jitter,
Min: b.Min,
Max: b.Max,
}
}