with file db

.gitignore

@@ -1,2 +1,4 @@
 dist/
 node_modules/
+
+*.local*

main.go

@@ -20,8 +20,65 @@ func main() {
 	log.Fatal(server.ListenAndServe())
 }
 
+type Store interface {
+	Count() (int, error)
+	Increment() (int, error)
+	Decrement() (int, error)
+}
+
+const StoreFilename = "store.local.json"
+
+type fileStore struct {
+	Counter int `json:"counter"`
+}
+
+func (db *fileStore) Count() (int, error) {
+	if err := readOrCreateJson(StoreFilename, db); err != nil {
+		return 0, err
+	}
+
+	return db.Counter, nil
+}
+
+func (db *fileStore) Increment() (int, error) {
+	if err := readOrCreateJson(StoreFilename, db); err != nil {
+		return 0, err
+	}
+
+	db.Counter++
+
+	if err := writeJson(StoreFilename, db); err != nil {
+		return 0, err
+	}
+
+	return db.Counter, nil
+}
+
+func (db *fileStore) Decrement() (int, error) {
+	if err := readOrCreateJson(StoreFilename, db); err != nil {
+		return 0, err
+	}
+
+	db.Counter--
+
+	if err := writeJson(StoreFilename, db); err != nil {
+		return 0, err
+	}
+
+	return db.Counter, nil
+}
+
 func setupRoutes(mux *http.ServeMux) {
-	counter := 0
+	var db Store = &fileStore{Counter: 0}
+	{
+		initialCount, err := db.Count()
+		if err != nil {
+			log.Fatalf(`Unable to create store: %v`, err)
+			return
+		}
+
+		log.Printf(`Store correctly initialized, count: %v`, initialCount)
+	}
 
 	// API Routes
 
@@ -46,12 +103,17 @@ func setupRoutes(mux *http.ServeMux) {
 			return
 		}
 
-		w.Header().Set("Content-Type", "application/json")
-		err := json.NewEncoder(w).Encode(counter)
+		count, err := db.Count()
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
+
+		w.Header().Set("Content-Type", "application/json")
+		if err := json.NewEncoder(w).Encode(count); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
 	})
 
 	mux.HandleFunc("/api/increment", func(w http.ResponseWriter, r *http.Request) {
@@ -60,11 +122,14 @@ func setupRoutes(mux *http.ServeMux) {
 			return
 		}
 
-		counter++
+		newCount, err := db.Increment()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
 
 		w.Header().Set("Content-Type", "application/json")
-		err := json.NewEncoder(w).Encode(counter)
-		if err != nil {
+		if err := json.NewEncoder(w).Encode(newCount); err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
@@ -76,11 +141,14 @@ func setupRoutes(mux *http.ServeMux) {
 			return
 		}
 
-		counter--
+		newCount, err := db.Decrement()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
 
 		w.Header().Set("Content-Type", "application/json")
-		err := json.NewEncoder(w).Encode(counter)
-		if err != nil {
+		if err := json.NewEncoder(w).Encode(newCount); err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}

utils.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+)
+
+func readOrCreateJson(filename string, value any) error {
+	f, err := os.Open(filename)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return writeJson(filename, value)
+		}
+
+		return err
+	}
+	defer f.Close()
+
+	if json.NewDecoder(f).Decode(value); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func writeJson(filename string, value any) error {
+	f, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	enc := json.NewEncoder(f)
+	enc.SetIndent("", "    ")
+	if err := enc.Encode(value); err != nil {
+		return err
+	}
+
+	return nil
+}