package retry

import (
	"math"
	"time"

	"github.com/aws/aws-sdk-go-v2/internal/rand"
	"github.com/aws/aws-sdk-go-v2/internal/timeconv"
)

// ExponentialJitterBackoff provides backoff delays with jitter based on the
// number of attempts.
type ExponentialJitterBackoff struct {
	maxBackoff time.Duration
	// precomputed number of attempts needed to reach max backoff.
	maxBackoffAttempts float64

	randFloat64 func() (float64, error)
}

// NewExponentialJitterBackoff returns an ExponentialJitterBackoff configured
// for the max backoff.
func NewExponentialJitterBackoff(maxBackoff time.Duration) *ExponentialJitterBackoff {
	return &ExponentialJitterBackoff{
		maxBackoff: maxBackoff,
		maxBackoffAttempts: math.Log2(
			float64(maxBackoff) / float64(time.Second)),
		randFloat64: rand.CryptoRandFloat64,
	}
}

// BackoffDelay returns the duration to wait before the next attempt should be
// made. Returns an error if unable get a duration.
func (j *ExponentialJitterBackoff) BackoffDelay(attempt int, err error) (time.Duration, error) {
	if attempt > int(j.maxBackoffAttempts) {
		return j.maxBackoff, nil
	}

	b, err := j.randFloat64()
	if err != nil {
		return 0, err
	}

	// [0.0, 1.0) * 2 ^ attempts
	ri := int64(1 << uint64(attempt))
	delaySeconds := b * float64(ri)

	return timeconv.FloatSecondsDur(delaySeconds), nil
}