package i18n

import (
	"sort"
	"strconv"
	"strings"
)

// weightedTag is one entry of a parsed Accept-Language header.
type weightedTag struct {
	primary string // "fr" out of "fr-FR"
	q       float64
}

// parseAcceptLanguage turns a raw "Accept-Language" header value (RFC
// 9110 §12.5.4 — e.g. "fr-FR,fr;q=0.9,en-US;q=0.8,en;q=0.7") into its
// tags' primary subtags, ordered by descending quality weight. A
// malformed segment is skipped rather than aborting the whole parse —
// browsers have been known to send odd values, and the worst case of
// ignoring one bad segment is falling back to Default, never a panic
// or a rejected request.
func parseAcceptLanguage(header string) []weightedTag {
	var tags []weightedTag
	for _, part := range strings.Split(header, ",") {
		part = strings.TrimSpace(part)
		if part == "" {
			continue
		}
		tag, qStr, hasQ := strings.Cut(part, ";")
		tag = strings.TrimSpace(tag)
		if tag == "" || tag == "*" {
			continue
		}
		q := 1.0
		if hasQ {
			qStr = strings.TrimSpace(qStr)
			if v, ok := strings.CutPrefix(qStr, "q="); ok {
				if parsed, err := strconv.ParseFloat(strings.TrimSpace(v), 64); err == nil {
					q = parsed
				}
			}
		}
		primary, _, _ := strings.Cut(tag, "-")
		tags = append(tags, weightedTag{primary: strings.ToLower(primary), q: q})
	}
	sort.SliceStable(tags, func(i, j int) bool { return tags[i].q > tags[j].q })
	return tags
}

// NegotiateAcceptLanguage picks the best Supported locale for a raw
// "Accept-Language" header value, falling back to Default if nothing
// in the header is one enkly ships a catalog for (or the header is
// empty / unparseable). Callers only reach for this when there's no
// stronger signal available — see the gateway's localeFrom, which
// tries the enkly_lang cookie first and only falls through to this
// for a visitor who has never touched the language switcher.
func NegotiateAcceptLanguage(header string) Locale {
	for _, tag := range parseAcceptLanguage(header) {
		if IsSupported(tag.primary) {
			return Locale(tag.primary)
		}
	}
	return Default
}