package resolver

import "strings"

func BuildResolverFallbackPool(raw string, fallbackDefaults []string, normalize func(string) string) []string {
	switch strings.ToLower(strings.TrimSpace(raw)) {
	case "off", "none", "0":
		return nil
	}

	candidates := fallbackDefaults
	if strings.TrimSpace(raw) != "" {
		candidates = nil
		fields := strings.FieldsFunc(raw, func(r rune) bool {
			return r == ',' || r == ';' || r == ' ' || r == '\n' || r == '\t'
		})
		for _, f := range fields {
			if normalize == nil {
				continue
			}
			if n := normalize(f); n != "" {
				candidates = append(candidates, n)
			}
		}
	}
	return UniqueStrings(candidates)
}

func MergeDNSUpstreamPools(primary, fallback []string, maxUpstreams int, normalize func(string) string) []string {
	if maxUpstreams < 1 {
		maxUpstreams = 1
	}
	out := make([]string, 0, len(primary)+len(fallback))
	seen := map[string]struct{}{}
	add := func(items []string) {
		for _, item := range items {
			if len(out) >= maxUpstreams {
				return
			}
			if normalize == nil {
				continue
			}
			n := normalize(item)
			if n == "" {
				continue
			}
			if _, ok := seen[n]; ok {
				continue
			}
			seen[n] = struct{}{}
			out = append(out, n)
		}
	}
	add(primary)
	add(fallback)
	return out
}