diff --git a/library/include/mobilisis/calculate_price.h b/library/include/mobilisis/calculate_price.h
index 5f05367..bbcda42 100644
--- a/library/include/mobilisis/calculate_price.h
+++ b/library/include/mobilisis/calculate_price.h
@@ -4,6 +4,8 @@
 #include <time.h>
 #include <inttypes.h>
 
+#include <QString>
+
 #ifdef WIN32
 #ifdef CALCULATE_LIBRARY_EXPORTS
 #define CALCULATE_LIBRARY_API __declspec(dllexport)
@@ -30,14 +32,79 @@ struct CALCULATE_LIBRARY_API price_t {
     double vat;
 };
 
-bool CALCULATE_LIBRARY_API init_tariff(parking_tariff_t **tariff, char const *config_file);
+struct CALCULATE_LIBRARY_API CalcState {
+    enum class State : uint8_t {
+        SUCCESS,
+        ERROR_PARSING_ZONE_NR,
+        ERROR_LOADING_TARIFF,
+        ERROR_PARSING_TARIFF,
+        NEGATIVE_PARING_TIME,
+        WRONG_PARAM_VALUES,
+        ABOVE_MAX_PARKING_TIME,
+        BELOW_MIN_PARKING_TIME,
+        BELOW_MIN_PARKING_PRICE,
+        OVERPAID
+    };
+
+    State m_status;
+    QString m_desc;
+
+    explicit CalcState() : m_status(State::SUCCESS), m_desc("") {}
+
+    explicit operator bool() const noexcept {
+        return (m_status == State::SUCCESS);
+    }
+
+    explicit operator QString () const noexcept {
+        QString s;
+        switch (m_status) {
+        case State::SUCCESS:
+            s = "SUCCESS";
+            break;
+        case State::ERROR_PARSING_ZONE_NR:
+            s = "ERROR_PARSING_ZONE_NR";
+            break;
+        case State::ERROR_LOADING_TARIFF:
+            s = "ERROR_LOADING_TARIFF";
+            break;
+        case State::ERROR_PARSING_TARIFF:
+            s = "ERROR_PARSING_TARIFF";
+            break;
+        case State::NEGATIVE_PARING_TIME:
+            s = "NEGATIVE_PARKING_TIME";
+            break;
+        case State::ABOVE_MAX_PARKING_TIME:
+            s = "ABOVE_MAX_PARKING_TIME";
+            break;
+        case State::WRONG_PARAM_VALUES:
+            s = "WRONG_PARAM_VALUES";
+            break;
+        case State::BELOW_MIN_PARKING_TIME:
+            s = "BELOW_MIN_PARKING_TIME";
+            break;
+        case State::BELOW_MIN_PARKING_PRICE:
+            s = "BELOW_MIN_PARKING_PRICE";
+            break;
+        case State::OVERPAID:
+            s = "OVERPAID";
+            break;
+        }
+        return s + ":" + m_desc;
+    }
+
+    CalcState &set(State s) { m_status = s; return *this; }
+    CalcState &setDesc(QString s) { m_desc = s; return *this; }
+};
+
+CalcState CALCULATE_LIBRARY_API init_tariff(parking_tariff_t **tariff,
+                                            char const *config_file);
 void CALCULATE_LIBRARY_API free_tariff(parking_tariff_t *tariff);
 int CALCULATE_LIBRARY_API get_zone_nr();
 
-bool CALCULATE_LIBRARY_API compute_price_for_parking_ticket(parking_tariff_t *tariff,
-                                      time_t start_parking_time,
-                                      time_t end_parking_time,
-                                      struct price_t *price);
+CalcState CALCULATE_LIBRARY_API compute_price_for_parking_ticket(parking_tariff_t *tariff,
+                                            time_t start_parking_time,
+                                            time_t end_parking_time,
+                                            struct price_t *price);
 #ifdef __cplusplus
 } // extern "C"
 #endif