package locale

import (
	"context"
	"net/http"
	"strings"
)

// Default is the fallback language when no supported language is detected.
const Default = "en"

// Supported is the set of language codes the application currently handles.
// Keys are ISO 639-1 two-letter codes (lower-case).
var Supported = map[string]bool{
	"en": true,
	"ru": true,
	"es": true,
	"de": true,
	"fr": true,
	"it": true,
	"pt": true,
	"zh": true,
	"ja": true,
	"ko": true,
	"ar": true,
	"hi": true,
}

type contextKey struct{}

// Parse returns the best-matching supported language from an Accept-Language
// header value.  It iterates through the comma-separated list in preference
// order and returns the first entry whose primary subtag is in Supported.
// Returns Default when the header is empty or no match is found.
func Parse(acceptLang string) string {
	if acceptLang == "" {
		return Default
	}
	for part := range strings.SplitSeq(acceptLang, ",") {
		// Strip quality value (e.g. ";q=0.9").
		tag := strings.TrimSpace(strings.SplitN(part, ";", 2)[0])
		// Use only the primary subtag (e.g. "ru" from "ru-RU").
		lang := strings.ToLower(strings.SplitN(tag, "-", 2)[0])
		if Supported[lang] {
			return lang
		}
	}
	return Default
}

// WithLang returns a copy of ctx carrying the given language code.
func WithLang(ctx context.Context, lang string) context.Context {
	return context.WithValue(ctx, contextKey{}, lang)
}

// FromContext returns the language stored in ctx.
// Returns Default when no language has been set.
func FromContext(ctx context.Context) string {
	if lang, ok := ctx.Value(contextKey{}).(string); ok && lang != "" {
		return lang
	}
	return Default
}

// FromRequest extracts the preferred language from the request's
// Accept-Language header.
func FromRequest(r *http.Request) string {
	return Parse(r.Header.Get("Accept-Language"))
}