i18n/detect/http.go

package detect

import (
	"net/http"
	"strings"
)

/* NewHttpDetector initializes new language detector */
func NewHttpDetector(languages []string, path string, maxAge int) func(http.ResponseWriter, *http.Request) string {
	const cookieName = "lang"
	if len(languages) == 0 {
		panic("empty languages list")
	}
	/* initialize state */
	defaultLang := strings.ToLower(languages[0])
	langSet := make(map[string]struct{}, len(languages))
	for _, lang := range languages {
		langSet[strings.ToLower(lang)] = struct{}{}
	}

	/* detectLanguage returns a language string */
	return func(w http.ResponseWriter, r *http.Request) string {
		/* check for query language override */
		if lang := strings.ToLower(r.URL.Query().Get(cookieName)); lang != "" {
			if _, ok := langSet[lang]; ok {
				/* language provided by query string, enforce it and send cookie */
				http.SetCookie(w, &http.Cookie{
					Name:     cookieName,
					Value:    lang,
					Path:     path,
					HttpOnly: true,
					SameSite: http.SameSiteLaxMode,
					Secure:   r.TLS != nil,
					MaxAge:   maxAge,
				})
				return lang
			}
		}
		/* check for cookie preferences */
		cookie, err := r.Cookie(cookieName)
		if err == nil {
			cookieLang := strings.ToLower(cookie.Value)
			if _, ok := langSet[cookieLang]; ok {
				return cookieLang
			}
		}
		/* fall back to browser preferences */
		acceptLanguage := strings.ToLower(r.Header.Get("Accept-Language"))
		parts := strings.Split(acceptLanguage, ",")
		for _, part := range parts {
			part = strings.TrimSpace(part)
			if part == "" {
				continue
			}
			/* cut off ";q=..." */
			if idx := strings.IndexByte(part, ';'); idx >= 0 {
				part = part[:idx]
			}
			/* try exact match first */
			if _, ok := langSet[part]; ok {
				return part
			}
			/* try base language */
			if i := strings.IndexByte(part, '-'); i >= 0 {
				base := part[:i]
				if _, ok := langSet[base]; ok {
					return base
				}
			}
		}
		return defaultLang
	}
}