Finished first part of balance calculation

handlers/balance.go

@@ -1,12 +1,32 @@
 package handlers
 
-import "net/http"
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+	"shap-planner-backend/storage"
+)
 
 func GetBalance(w http.ResponseWriter, r *http.Request) {
 	query := r.URL.Query()
 	userParam := query.Get("user")
 
-	if userParam == "" {
-
+	if userParam == "all" {
+		// TODO: add later
+	} else {
+		balance, err := storage.ComputeBalance(userParam)
+		if err != nil {
+			log.Println("GET [api/balance] " + r.RemoteAddr + ": " + err.Error())
+			http.Error(w, "Invalid request query", http.StatusBadRequest)
+			return
+		}
+		err = json.NewEncoder(w).Encode(map[string]interface{}{
+			"balance": balance,
+		})
+		if err != nil {
+			log.Println("GET [api/balance] " + r.RemoteAddr + ": " + err.Error())
+			return
+		}
+		log.Println("GET [api/balance] " + r.RemoteAddr + ": Successfully retrieved balance")
 	}
 }