package i18n

import (
	"strings"
)

// Lang is a canonical UI language. Only these three are served.
type Lang string

const (
	ZhCN    Lang = "zh-CN"
	En      Lang = "en"
	ZhTW    Lang = "zh-TW"
	Default      = En
)

func (l Lang) String() string {
	if l == "" {
		return string(Default)
	}
	return string(l)
}

// Parse maps Accept-Language, X-Language, or TCP AUTH language to ZhCN / En / ZhTW.
// Unknown or empty input falls back to Default (en).
func Parse(raw string) Lang {
	for _, part := range strings.Split(raw, ",") {
		tag, _, _ := strings.Cut(strings.TrimSpace(part), ";")
		if lang, ok := match(tag); ok {
			return lang
		}
	}
	return Default
}

// ParseExplicit requires a recognized language tag; ok is false when raw is empty or unknown.
func ParseExplicit(raw string) (Lang, bool) {
	raw = strings.TrimSpace(raw)
	if raw == "" {
		return Default, false
	}
	for _, part := range strings.Split(raw, ",") {
		tag, _, _ := strings.Cut(strings.TrimSpace(part), ";")
		if lang, ok := match(tag); ok {
			return lang, true
		}
	}
	return Default, false
}

func match(tag string) (Lang, bool) {
	t := strings.ToLower(strings.ReplaceAll(strings.TrimSpace(tag), "_", "-"))
	if t == "" {
		return "", false
	}
	switch {
	case t == "en" || strings.HasPrefix(t, "en-"):
		return En, true
	case t == "zh-tw" || t == "zh-hk" || t == "zh-mo" || t == "zh-hant" || strings.HasPrefix(t, "zh-hant"):
		return ZhTW, true
	case t == "zh" || t == "zh-cn" || t == "zh-sg" || t == "zh-hans" || strings.HasPrefix(t, "zh-hans"):
		return ZhCN, true
	default:
		return "", false
	}
}