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package waiter
import (
"fmt"
"math"
"time"
"github.com/aws/smithy-go/rand"
)
// ComputeDelay computes delay between waiter attempts. The function takes in a current attempt count,
// minimum delay, maximum delay, and remaining wait time for waiter as input. The inputs minDelay and maxDelay
// must always be greater than 0, along with minDelay lesser than or equal to maxDelay.
//
// Returns the computed delay and if next attempt count is possible within the given input time constraints.
// Note that the zeroth attempt results in no delay.
func ComputeDelay(attempt int64, minDelay, maxDelay, remainingTime time.Duration) (delay time.Duration, err error) {
// zeroth attempt, no delay
if attempt <= 0 {
return 0, nil
}
// remainingTime is zero or less, no delay
if remainingTime <= 0 {
return 0, nil
}
// validate min delay is greater than 0
if minDelay == 0 {
return 0, fmt.Errorf("minDelay must be greater than zero when computing Delay")
}
// validate max delay is greater than 0
if maxDelay == 0 {
return 0, fmt.Errorf("maxDelay must be greater than zero when computing Delay")
}
// Get attempt ceiling to prevent integer overflow.
attemptCeiling := (math.Log(float64(maxDelay/minDelay)) / math.Log(2)) + 1
if attempt > int64(attemptCeiling) {
delay = maxDelay
} else {
// Compute exponential delay based on attempt.
ri := 1 << uint64(attempt-1)
// compute delay
delay = minDelay * time.Duration(ri)
}
if delay != minDelay {
// randomize to get jitter between min delay and delay value
d, err := rand.CryptoRandInt63n(int64(delay - minDelay))
if err != nil {
return 0, fmt.Errorf("error computing retry jitter, %w", err)
}
delay = time.Duration(d) + minDelay
}
// check if this is the last attempt possible and compute delay accordingly
if remainingTime-delay <= minDelay {
delay = remainingTime - minDelay
}
return delay, nil
}
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