class CouponServiceTest {

    @Test
    void testApplyCoupon_fixedAmount_shouldDeductCorrectly() {
        // Arrange
        CouponService couponService = new CouponService();
        Coupon coupon = Coupon.builder()
            .code("SAVE20")
            .type(CouponType.FIXED_AMOUNT)
            .discountValue(new BigDecimal("20"))
            .minOrderAmount(new BigDecimal("100"))
            .build();
        BigDecimal orderAmount = new BigDecimal("150");

        // Act
        BigDecimal finalAmount = couponService.apply(coupon, orderAmount);

        // Assert
        assertEquals(new BigDecimal("130.00"), finalAmount);
    }
}

运行：红色。CouponService 和 Coupon 类都不存在，编译失败。

第二步：让测试通过（Green）——写最少的代码

先创建 Coupon 类和 CouponService，写最简单的实现让测试通过：

@Data
@Builder
public class Coupon {
    private String code;
    private CouponType type;
    private BigDecimal discountValue;
    private BigDecimal minOrderAmount;
}

public enum CouponType {
    FIXED_AMOUNT, PERCENTAGE
}

public class CouponService {
    public BigDecimal apply(Coupon coupon, BigDecimal orderAmount) {
        // 最简单的实现，只处理 FIXED_AMOUNT 类型
        if (CouponType.FIXED_AMOUNT == coupon.getType()) {
            if (orderAmount.compareTo(coupon.getMinOrderAmount()) >= 0) {
                return orderAmount.subtract(coupon.getDiscountValue());
            }
        }
        return orderAmount;
    }
}

运行：绿色。

第三步：加更多测试，驱动更多实现

@Test
void testApplyCoupon_whenOrderAmountBelowMinimum_shouldNotApply() {
    CouponService couponService = new CouponService();
    Coupon coupon = Coupon.builder()
        .code("SAVE20").type(CouponType.FIXED_AMOUNT)
        .discountValue(new BigDecimal("20"))
        .minOrderAmount(new BigDecimal("100"))
        .build();

    BigDecimal finalAmount = couponService.apply(coupon, new BigDecimal("80")); // 低于门槛

    assertEquals(new BigDecimal("80"), finalAmount, "未达门槛，不应该打折");
}

@Test
void testApplyCoupon_percentage_shouldApplyCorrectDiscount() {
    CouponService couponService = new CouponService();
    Coupon coupon = Coupon.builder()
        .code("DISCOUNT80").type(CouponType.PERCENTAGE)
        .discountValue(new BigDecimal("0.8")) // 8折
        .minOrderAmount(BigDecimal.ZERO)
        .build();

    BigDecimal finalAmount = couponService.apply(coupon, new BigDecimal("200"));

    assertEquals(new BigDecimal("160.00"), finalAmount, "8折应该是 200 * 0.8 = 160");
}

第二个测试会失败——当前实现没有处理 PERCENTAGE 类型。追加实现：

public BigDecimal apply(Coupon coupon, BigDecimal orderAmount) {
    if (orderAmount.compareTo(coupon.getMinOrderAmount()) < 0) {
        return orderAmount; // 未达门槛
    }
    return switch (coupon.getType()) {
        case FIXED_AMOUNT -> orderAmount.subtract(coupon.getDiscountValue())
            .max(BigDecimal.ZERO); // 不能为负
        case PERCENTAGE -> orderAmount.multiply(coupon.getDiscountValue())
            .setScale(2, RoundingMode.HALF_UP);
    };
}

第四步：加有效期逻辑

@Test
void testApplyCoupon_expired_shouldThrowException() {
    CouponService couponService = new CouponService();
    Coupon coupon = Coupon.builder()
        .code("OLD10")
        .type(CouponType.FIXED_AMOUNT)
        .discountValue(new BigDecimal("10"))
        .minOrderAmount(BigDecimal.ZERO)
        .expireTime(LocalDateTime.now().minusDays(1)) // 已过期
        .build();

    assertThrows(CouponExpiredException.class,
        () -> couponService.apply(coupon, new BigDecimal("100")),
        "已过期的优惠券应该抛出 CouponExpiredException");
}

红色→追加实现：

public BigDecimal apply(Coupon coupon, BigDecimal orderAmount) {
    // 检查有效期
    if (coupon.getExpireTime() != null && coupon.getExpireTime().isBefore(LocalDateTime.now())) {
        throw new CouponExpiredException("优惠券[" + coupon.getCode() + "]已过期");
    }
    // ... 其余逻辑
}

第五步：加"不能重复使用"逻辑

@Test
void testApplyCoupon_alreadyUsed_shouldThrowException() {
    UsageRepository mockRepo = mock(UsageRepository.class);
    CouponService couponService = new CouponService(mockRepo); // 注入依赖

    Coupon coupon = validCoupon();
    Long userId = 1L;

    // 模拟：这个用户已经用过这张券
    when(mockRepo.existsByUserIdAndCouponCode(userId, coupon.getCode())).thenReturn(true);

    assertThrows(CouponAlreadyUsedException.class,
        () -> couponService.apply(userId, coupon, new BigDecimal("200")));
}