// An SRV represents a single DNS SRV record.
type SRV struct {
	Target   string
	Port     uint16
	Priority uint16
	Weight   uint16
}

// byPriorityWeight sorts SRV records by ascending priority and weight.
type byPriorityWeight []*SRV

// shuffleByWeight shuffles SRV records by weight using the algorithm
// described in RFC 2782.
func (addrs byPriorityWeight) shuffleByWeight() {
	sum := 0
	for _, addr := range addrs {
		sum += int(addr.Weight)
	}
	for sum > 0 && len(addrs) > 1 {
		s := 0
		n := randIntn(sum)
		for i := range addrs {
			s += int(addrs[i].Weight)
			if s > n {
				if i > 0 {
					addrs[0], addrs[i] = addrs[i], addrs[0]
				}
				break
			}
		}
		sum -= int(addrs[0].Weight)
		addrs = addrs[1:]
	}
}

// sort reorders SRV records as specified in RFC 2782.
func (addrs byPriorityWeight) sort() {
	slices.SortFunc(addrs, func(a, b *SRV) int {
		if r := cmp.Compare(a.Priority, b.Priority); r != 0 {
			return r
		}
		return cmp.Compare(a.Weight, b.Weight)
	})
	i := 0
	for j := 1; j < len(addrs); j++ {
		if addrs[i].Priority != addrs[j].Priority {
			addrs[i:j].shuffleByWeight()
			i = j
		}
	}
	addrs[i:].shuffleByWeight()
}

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