package pool

import "time"

const (
	// arbitrary values to be most likely out of sync with anything else:
	minNightshiftTime = 729 * time.Millisecond // ~1sec
	maxNightshiftTime = 59049 * time.Second    // ~2/3day
)

type adaptive struct {
	lastTurnedActive time.Time
	nightshiftTime   time.Duration
	idle             bool
	average          int
	deviation        int
}

type maxTimeout struct {
	max   int
	to    time.Duration
	items []time.Time
}

func makeAdaptiveAlgo() *adaptive {
	return &adaptive{idle: true}
}

func abs(v int) int {
	if v >= 0 {
		return v
	}

	return 0 - v
}

func divE(v int) int {
	return (3 * v) >> 3 // 1 / 2.72 => 3 / 8
}

func mulE(v int) int {
	return (11 * v) >> 2 // 2.72 => 11 / 4
}

func movingAverage(prev, currv int) int {
	return prev + divE(currv-prev)
}

func movingAbsoluteDeviation(prev, currv, currav int) int {
	return prev + divE(abs(currv-currav)-prev)
}

func targetCapacity(av, dev int) int {
	return av + mulE(dev)
}

func (a *adaptive) target(s Stats) int {
	av := movingAverage(a.average, s.Active)
	dev := movingAbsoluteDeviation(a.deviation, s.Active, av)
	a.average = av
	a.deviation = dev
	return targetCapacity(av, dev)
}

func (a *adaptive) nightshift(s Stats) time.Duration {
	if a.idle && s.Active == 0 {
		return a.nightshiftTime
	}

	if !a.idle && s.Active > 0 {
		return 0
	}

	now := time.Now()
	a.idle = !a.idle
	if !a.idle {
		a.lastTurnedActive = now
		return 0
	}

	a.nightshiftTime = now.Sub(a.lastTurnedActive)
	a.nightshiftTime = (3 * a.nightshiftTime) >> 3
	if a.nightshiftTime < minNightshiftTime {
		a.nightshiftTime = minNightshiftTime
	}

	if a.nightshiftTime > maxNightshiftTime {
		a.nightshiftTime = maxNightshiftTime
	}

	return a.nightshiftTime
}

func (a *adaptive) Target(s Stats) (int, time.Duration) {
	t := a.target(s)
	ns := a.nightshift(s)
	return t, ns
}

func makeMaxTimeout(max int, to time.Duration) *maxTimeout {
	return &maxTimeout{
		max: max,
		to:  to,
	}
}

func (a *maxTimeout) Target(s Stats) (int, time.Duration) {
	if a.max <= 0 && a.to <= 0 {
		return s.Idle, 0
	}

	if a.max > 0 && a.to <= 0 {
		t := s.Idle
		if t > a.max {
			t = a.max
		}

		return t, 0
	}

	var zero time.Time
	if len(a.items) > s.Idle {
		for i := s.Idle; i < len(a.items); i++ {
			a.items[i] = zero
		}

		a.items = a.items[:s.Idle]
		if len(a.items) == 0 {
			a.items = nil
		}
	}

	now := time.Now()
	for len(a.items) < s.Idle {
		a.items = append(a.items, now)
	}

	var drop int
	for drop < len(a.items) && a.items[drop].Add(a.to).Before(now) {
		a.items[drop] = zero
		drop++
	}

	if drop > 0 {
		for i := 0; i < drop; i++ {
			a.items[i] = zero
		}

		a.items = a.items[drop:]
	}

	if len(a.items) == 0 {
		a.items = nil
		return 0, 0
	}

	t := len(a.items)
	if a.max > 0 && t > a.max {
		t = a.max
	}

	return t, a.items[0].Add(a.to).Sub(now)
}