feat: translation middleware

internal/components/translate/context.go

@@ -0,0 +1,81 @@
+package translate
+
+import (
+	"context"
+	"log/slog"
+	"path"
+)
+
+type contextKey string
+
+const (
+	serviceKey contextKey = "translationService"
+	langKey    contextKey = "language"
+)
+
+func Language(ctx context.Context) string {
+	lang, ok := ctx.Value(langKey).(string)
+	if !ok {
+		return ""
+	}
+	return lang
+}
+
+func SupportedLanguages(ctx context.Context) []string {
+	s, ok := ctx.Value(serviceKey).(*Service)
+	if !ok {
+		slog.Error("could not extract translation service from context for getting supported languages")
+	}
+	return s.SupportedLanguages()
+}
+
+func T(ctx context.Context, key string) string {
+	s, ok := ctx.Value(serviceKey).(*Service)
+	if !ok {
+		slog.Error("could not extract translation service from context", "ctx", ctx, "key", key)
+		return key
+	}
+
+	lang := Language(ctx)
+
+	translation, ok := s.translations[lang][key]
+	if !ok {
+		slog.Warn("could not translate", "key", key, "language", lang)
+		return key
+	}
+
+	return translation
+}
+
+func Path(ctx context.Context, pp ...string) string {
+	return PathForLang(ctx, Language(ctx), pp...)
+}
+
+func PathForLang(ctx context.Context, lang string, pp ...string) string {
+	slog.Info("path for lang", "path", pp)
+	s, ok := ctx.Value(serviceKey).(*Service)
+	if !ok {
+		slog.Error("could not extract translation service from context for translating path", "ctx", ctx, "path", pp)
+		return path.Join(pp...)
+	}
+
+	if len(pp) > 0 {
+		if _, ok := s.translations[pp[0]]; ok {
+			pp = pp[1:]
+		}
+	}
+
+	return path.Join(append([]string{lang}, pp...)...)
+}
+
+func (s *Service) SupportedLanguages() (languages []string) {
+	for lang := range s.translations {
+		languages = append(languages, lang)
+	}
+
+	return
+}
+
+func (s *Service) DefaultLanguage() string {
+	return s.defaultLanguage
+}

internal/components/translate/middleware.go

@@ -0,0 +1,85 @@
+package translate
+
+import (
+	"context"
+	"net/http"
+	"strings"
+)
+
+func (s *Service) Middleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		path := r.URL.Path
+		segments := []string{}
+		for segment := range strings.SplitSeq(path, "/") {
+			if segment != "" {
+				segments = append(segments, segment)
+			}
+		}
+
+		// 1. Check if the first segment is a supported language.
+		var currentLang string
+		if len(segments) >= 1 {
+			potentialLang := segments[0]
+			if _, ok := s.translations[potentialLang]; ok {
+				currentLang = potentialLang
+			}
+		}
+
+		if currentLang != "" {
+			// CASE A: Language is present in the URL.
+			// Remove the language from the path so the router sees the clean path.
+			newPath := "/" + strings.Join(segments[1:], "/")
+			if newPath == "" {
+				newPath = "/"
+			}
+
+			// Update the request URL path.
+			r.URL.Path = newPath
+
+			// Inject translator into context.
+			ctx := context.WithValue(r.Context(), serviceKey, s)
+			ctx = context.WithValue(ctx, langKey, currentLang)
+
+			// Pass control to the next handler with the modified request.
+			next.ServeHTTP(w, r.WithContext(ctx))
+			return
+		}
+
+		// CASE B: Language is NOT in the URL.
+		// Detect best language from Accept-Language header.
+		detectedLang := s.detectLanguage(r.Header.Get("Accept-Language"))
+
+		// Redirect to the URL with the language prefix.
+		targetURL := "/" + detectedLang + path
+		if r.URL.RawQuery != "" {
+			targetURL += "?" + r.URL.RawQuery
+		}
+
+		http.Redirect(w, r, targetURL, http.StatusFound)
+	})
+}
+
+// detectLanguage parses the Accept-Language header and returns the best matching supported language.
+func (s *Service) detectLanguage(header string) string {
+	if header == "" {
+		return s.defaultLanguage
+	}
+
+	// Simple parser for Accept-Language: en-US,en;q=0.9,de;q=0.8
+	// We only care about the first match for simplicity here, or we could rank them.
+	parts := strings.Split(header, ",")
+	for _, part := range parts {
+		tag := strings.Split(strings.TrimSpace(part), ";")[0]
+		// Check for exact match (e.g., "en-US")
+		if _, ok := s.translations[tag]; ok {
+			return tag
+		}
+		// Check for base match (e.g., "en" from "en-US")
+		base := strings.Split(tag, "-")[0]
+		if _, ok := s.translations[base]; ok {
+			return base
+		}
+	}
+
+	return s.defaultLanguage
+}

internal/components/translate/resource.go

@@ -0,0 +1,22 @@
+package translate
+
+type Translations map[string]string // key -> translation
+
+type Service struct {
+	translations    map[string]Translations // language -> translations
+	defaultLanguage string
+}
+
+func New() *Service {
+	return &Service{
+		defaultLanguage: "de",
+		translations: map[string]Translations{
+			"de": {
+				"home": "Startseite",
+			},
+			"en": {
+				"home": "Home",
+			},
+		},
+	}
+}