package geo

import "math"

// Candidate is one region a guess may pick. Code is in the record
// vocabulary ("" is the deployment's home); the caller has already
// filtered to eligible, policy-allowed candidates.
type Candidate struct {
	Code      string
	Home      bool
	AWSRegion string
}

// NearestWith returns the candidate nearest p, measured to each
// candidate's AWS region through pointOf.
//
//   - A candidate whose region has no point is left out.
//   - If the HOME is among cands and has no point, ok is false: picking
//     among the rest could choose a far region over an unmeasured near
//     home, and the home is where today's behaviour lives (spec §6.3).
//   - Distances are compared in whole kilometres. A tie goes to the home,
//     then to the lexically smallest code, so every console answers alike.
func NearestWith(p Point, cands []Candidate, pointOf func(string) (Point, bool)) (code string, ok bool) {
	best := -1
	bestKm := math.MaxInt
	for i, c := range cands {
		cp, has := pointOf(c.AWSRegion)
		if !has {
			if c.Home {
				return "", false
			}
			continue
		}
		km := int(math.Round(DistanceKm(p, cp)))
		switch {
		case best < 0, km < bestKm:
		case km == bestKm && better(c, cands[best]):
		default:
			continue
		}
		best, bestKm = i, km
	}
	if best < 0 {
		return "", false
	}
	return cands[best].Code, true
}

// better breaks an exact tie: the home first, then the smaller code.
func better(a, b Candidate) bool {
	if a.Home != b.Home {
		return a.Home
	}
	return a.Code < b.Code
}