package i18n

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

type ctxKey string

const langKey ctxKey = "lang"

func WithLang(ctx context.Context, lang string) context.Context {
	return context.WithValue(ctx, langKey, lang)
}

func LangFromContext(ctx context.Context) string {
	v, _ := ctx.Value(langKey).(string)
	if v == "" {
		return fallback
	}
	return v
}

type langWeight struct {
	lang   string
	weight float64
}

func ParseAcceptLanguage(header string) string {
	if header == "" {
		return fallback
	}

	var items []langWeight
	for _, part := range strings.Split(header, ",") {
		part = strings.TrimSpace(part)
		if part == "" {
			continue
		}
		segments := strings.SplitN(part, ";", 2)
		lang := strings.TrimSpace(segments[0])
		w := 1.0
		if len(segments) == 2 {
			qPart := strings.TrimSpace(segments[1])
			if strings.HasPrefix(qPart, "q=") {
				if v, err := strconv.ParseFloat(qPart[2:], 64); err == nil {
					w = v
				}
			}
		}
		items = append(items, langWeight{lang: normalizeLang(lang), weight: w})
	}

	sort.Slice(items, func(i, j int) bool {
		return items[i].weight > items[j].weight
	})

	mu.RLock()
	defer mu.RUnlock()

	for _, item := range items {
		if _, ok := messages[item.lang]; ok {
			return item.lang
		}
		base := strings.SplitN(item.lang, "-", 2)[0]
		if _, ok := messages[base]; ok {
			return base
		}
	}

	return fallback
}

func normalizeLang(lang string) string {
	lang = strings.ToLower(strings.TrimSpace(lang))
	lang = strings.ReplaceAll(lang, "_", "-")
	return lang
}