Kargo ücreti servisi: if/else'ten Strategy'ye

// ===== Carrier.java =====
public enum Carrier {
    ARAS, YURTICI, MNG;

    public static Carrier from(String value) {
        try {
            return valueOf(value.trim().toUpperCase(Locale.ROOT));    // Türk locale'inde "i" tuzağına dikkat
        } catch (IllegalArgumentException | NullPointerException e) {
            throw new UnsupportedCarrierException(value);
        }
    }
}

// ===== ShippingFeePolicy.java =====
public interface ShippingFeePolicy {
    Carrier carrier();
    BigDecimal feeFor(ShipmentQuote quote);
}

// ===== ShipmentQuote.java =====
public record ShipmentQuote(BigDecimal weightKg, BigDecimal cartTotal) {}

// ===== ArasFeePolicy.java =====
@Component
class ArasFeePolicy implements ShippingFeePolicy {
    private static final BigDecimal HEAVY_THRESHOLD_KG = new BigDecimal("5");
    private static final BigDecimal HEAVY_FEE = new BigDecimal("89.90");
    private static final BigDecimal STANDARD_FEE = new BigDecimal("49.90");

    @Override public Carrier carrier() { return Carrier.ARAS; }

    @Override
    public BigDecimal feeFor(ShipmentQuote quote) {
        return quote.weightKg().compareTo(HEAVY_THRESHOLD_KG) > 0 ? HEAVY_FEE : STANDARD_FEE;
    }
}

// ===== YurticiFeePolicy.java =====
@Component
class YurticiFeePolicy implements ShippingFeePolicy {
    private static final BigDecimal FREE_SHIPPING_THRESHOLD = new BigDecimal("500");
    private static final BigDecimal STANDARD_FEE = new BigDecimal("54.90");

    @Override public Carrier carrier() { return Carrier.YURTICI; }

    @Override
    public BigDecimal feeFor(ShipmentQuote quote) {
        return quote.cartTotal().compareTo(FREE_SHIPPING_THRESHOLD) >= 0 ? BigDecimal.ZERO : STANDARD_FEE;
    }
}

// ===== MngFeePolicy.java =====
@Component
class MngFeePolicy implements ShippingFeePolicy {
    private static final BigDecimal BASE_FEE = new BigDecimal("39.90");
    private static final BigDecimal PER_KG = new BigDecimal("4.50");

    @Override public Carrier carrier() { return Carrier.MNG; }

    @Override
    public BigDecimal feeFor(ShipmentQuote quote) {
        return BASE_FEE.add(PER_KG.multiply(quote.weightKg())).setScale(2, RoundingMode.HALF_UP);
    }
}

// ===== ShippingFeeCalculator.java =====
@Service
public class ShippingFeeCalculator {

    private final Map<Carrier, ShippingFeePolicy> policies;

    public ShippingFeeCalculator(List<ShippingFeePolicy> policies) {
        // Aynı carrier iki kez tanımlanırsa uygulama açılırken hata verir (fail fast)
        this.policies = policies.stream()
                .collect(Collectors.toUnmodifiableMap(ShippingFeePolicy::carrier, Function.identity()));
    }

    public BigDecimal calculate(Carrier carrier, ShipmentQuote quote) {
        ShippingFeePolicy policy = policies.get(carrier);
        if (policy == null) {
            throw new UnsupportedCarrierException(carrier.name());
        }
        return policy.feeFor(quote);
    }
}

// ===== ProductController.java =====
@RestController
@RequestMapping("/api/v1/products")
public class ProductController {

    private final ProductRepository productRepository;
    private final ShippingFeeCalculator shippingFeeCalculator;

    public ProductController(ProductRepository productRepository, ShippingFeeCalculator shippingFeeCalculator) {
        this.productRepository = productRepository;
        this.shippingFeeCalculator = shippingFeeCalculator;
    }

    @GetMapping("/{productId}/shipping-fee")
    public ShippingFeeResponse shippingFee(@PathVariable long productId, @RequestParam String carrier) {
        Product product = productRepository.findById(productId)
                .orElseThrow(() -> new ProductNotFoundException(productId));
        Carrier parsed = Carrier.from(carrier);                         // bilinmeyense UnsupportedCarrierException
        BigDecimal fee = shippingFeeCalculator.calculate(parsed, new ShipmentQuote(product.getWeightKg(), product.getPrice()));
        return new ShippingFeeResponse(productId, parsed, fee);
    }
}
// Not: İş mantığı büyürse controller yerine bir ProductShippingService'e taşınır.

// ===== GlobalExceptionHandler.java =====
@RestControllerAdvice
class GlobalExceptionHandler {
    @ExceptionHandler(ProductNotFoundException.class)
    ProblemDetail notFound(ProductNotFoundException e) {
        return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, e.getMessage());
    }
    @ExceptionHandler(UnsupportedCarrierException.class)
    ProblemDetail unsupported(UnsupportedCarrierException e) {
        return ProblemDetail.forStatusAndDetail(HttpStatus.BAD_REQUEST, e.getMessage());
    }
}

// ===== ShippingFeeCalculatorTest.java =====
class ShippingFeeCalculatorTest {

    private final ShippingFeeCalculator calculator =
            new ShippingFeeCalculator(List.of(new ArasFeePolicy(), new YurticiFeePolicy()));

    @Test
    void yurticiIsFreeAboveThreshold() {
        BigDecimal fee = calculator.calculate(Carrier.YURTICI, new ShipmentQuote(BigDecimal.ONE, new BigDecimal("500")));
        assertThat(fee).isEqualByComparingTo("0");
    }

    @Test
    void arasChargesHeavyFeeAboveFiveKg() {
        BigDecimal fee = calculator.calculate(Carrier.ARAS, new ShipmentQuote(new BigDecimal("5.01"), BigDecimal.TEN));
        assertThat(fee).isEqualByComparingTo("89.90");
    }

    @Test
    void rejectsCarrierWithoutPolicy() {
        assertThatThrownBy(() -> calculator.calculate(Carrier.MNG, new ShipmentQuote(BigDecimal.ONE, BigDecimal.TEN)))
                .isInstanceOf(UnsupportedCarrierException.class);
    }
}