var (
	Fast = New(
		WithSlotDuration(fastSlot),
		WithCeiling(6),
	)
	Slow = New(
		WithSlotDuration(slowSlot),
		WithCeiling(6),
	)
)

type Backoff interface {
	// Wait maps index of the retry to a channel which fulfillment means that
	// Delay is over.
	//
	// Note that retry index begins from 0 and 0-th index means that it is the
	// first retry attempt after an initial error.
	Wait(n int) <-chan time.Time

	// Delay returns mapping of i to Delay.
	Delay(i int) time.Duration
}

type Type uint8

const (
	TypeNoBackoff Type = 1 << iota >> 1

	TypeFast
	TypeSlow

	TypeAny = TypeFast | TypeSlow
)