// Package choose picks the host for a new lease: among the healthy, non-draining, known hosts it
// prefers those that have the model loaded over those that would only be able to serve it, then the
// one with the most free slots times weight, breaking ties by shortest queue and finally list
// order.
package choose

// Info is one candidate host's view of itself for a single model.
type Info struct {
	Healthy  bool // answered its last poll
	Draining bool // operator is draining it; no new leases
	Loaded   bool // the model is resident here
	CanServe bool // config lists the model, so we may load it
	Free     int
	Queued   int
	Weight   float64
}

// Best returns the best host for a new lease, or ok=false when nothing is eligible. It runs two
// passes over the candidates in order: the first over those that have the model loaded, the second,
// only if the first found nothing, over those that can serve it. A host must be healthy, not
// draining, and known (info reported ok) to be eligible in either pass; a host with zero free slots
// is still eligible, since it will queue. Among the eligible ones the highest Free*Weight wins, ties
// go to the lowest queue, and a remaining tie keeps the earlier candidate.
func Best(candidates []string, info func(host string) (Info, bool)) (string, bool) {
	const (
		passLoaded   = 0
		passCanServe = 1
	)
	best := ""
	var (
		bestScore  float64
		bestQueued int
		found      bool
	)
	for pass := passLoaded; pass <= passCanServe; pass++ {
		for _, host := range candidates {
			v, ok := info(host)
			if !ok || !v.Healthy || v.Draining {
				continue
			}
			switch pass {
			case passLoaded:
				if !v.Loaded {
					continue
				}
			case passCanServe:
				if !v.CanServe {
					continue
				}
			}
			score := float64(v.Free) * v.Weight
			// Replace only when strictly better on score, or equal score with a shorter queue; a
			// further tie keeps the earlier candidate because we scan in order and use "<".
			if !found || score > bestScore || (score == bestScore && v.Queued < bestQueued) {
				best, bestScore, bestQueued, found = host, score, v.Queued, true
			}
		}
		if found {
			break
		}
	}
	return best, found
}