configuration.h is a header, so we need make sure it follows ODR rule

Here we can use c++17 inline variable to obey ODR

Author: haohaolee

rest/configuration.h

@@ -8,37 +8,37 @@
 
 #include <unordered_map>
 
-constexpr auto DR_EVILS_DARK_ENERGY_ENERGY_SUPPLIER = "Dr Evil's Dark Energy";
-constexpr auto THE_GREEN_ECO_ENERGY_SUPPLIER = "The Green Eco";
-constexpr auto POWER_FOR_EVERYONE_ENERGY_SUPPLIER = "Power for Everyone";
+inline constexpr auto DR_EVILS_DARK_ENERGY_ENERGY_SUPPLIER = "Dr Evil's Dark Energy";
+inline constexpr auto THE_GREEN_ECO_ENERGY_SUPPLIER = "The Green Eco";
+inline constexpr auto POWER_FOR_EVERYONE_ENERGY_SUPPLIER = "Power for Everyone";
 
-constexpr auto MOST_EVIL_PRICE_PLAN_ID = "price-plan-0";
-constexpr auto RENEWABLES_PRICE_PLAN_ID = "price-plan-1";
-constexpr auto STANDARD_PRICE_PLAN_ID = "price-plan-2";
+inline constexpr auto MOST_EVIL_PRICE_PLAN_ID = "price-plan-0";
+inline constexpr auto RENEWABLES_PRICE_PLAN_ID = "price-plan-1";
+inline constexpr auto STANDARD_PRICE_PLAN_ID = "price-plan-2";
 
-constexpr auto SARAHS_SMART_METER_ID = "smart-meter-0";
-constexpr auto PETERS_SMART_METER_ID = "smart-meter-1";
-constexpr auto CHARLIES_SMART_METER_ID = "smart-meter-2";
-constexpr auto ANDREAS_SMART_METER_ID = "smart-meter-3";
-constexpr auto ALEXS_SMART_METER_ID = "smart-meter-4";
+inline constexpr auto SARAHS_SMART_METER_ID = "smart-meter-0";
+inline constexpr auto PETERS_SMART_METER_ID = "smart-meter-1";
+inline constexpr auto CHARLIES_SMART_METER_ID = "smart-meter-2";
+inline constexpr auto ANDREAS_SMART_METER_ID = "smart-meter-3";
+inline constexpr auto ALEXS_SMART_METER_ID = "smart-meter-4";
 
-std::vector<PricePlan> pricePlans() {
+inline std::vector<PricePlan> pricePlans() {
   std::vector<PricePlan> price_plans;
   price_plans.push_back(PricePlan(MOST_EVIL_PRICE_PLAN_ID, DR_EVILS_DARK_ENERGY_ENERGY_SUPPLIER, 10, {}));
   price_plans.push_back(PricePlan(RENEWABLES_PRICE_PLAN_ID, THE_GREEN_ECO_ENERGY_SUPPLIER, 2, {}));
   price_plans.push_back(PricePlan(STANDARD_PRICE_PLAN_ID, POWER_FOR_EVERYONE_ENERGY_SUPPLIER, 1, {}));
   return price_plans;
 }
 
-std::unordered_map<std::string, std::string> smart_meter_to_price_plan_accounts() {
+inline std::unordered_map<std::string, std::string> smart_meter_to_price_plan_accounts() {
   return {{SARAHS_SMART_METER_ID, MOST_EVIL_PRICE_PLAN_ID},
           {PETERS_SMART_METER_ID, RENEWABLES_PRICE_PLAN_ID},
           {CHARLIES_SMART_METER_ID, MOST_EVIL_PRICE_PLAN_ID},
           {ANDREAS_SMART_METER_ID, STANDARD_PRICE_PLAN_ID},
           {ALEXS_SMART_METER_ID, RENEWABLES_PRICE_PLAN_ID}};
 };
 
-auto readings() {
+inline auto readings() {
   std::unordered_map<std::string, std::vector<ElectricityReading>> result;
   for (auto &[meter, plan] : smart_meter_to_price_plan_accounts()) {
     result[meter] = generator{}.generate(21);