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feature: 쿠폰 도메인 설계 및 단위 테스트 작성
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‎apps/commerce-api/src/main/java/com/loopers/domain/coupon/Coupon.java‎

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package com.loopers.domain.coupon;
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import com.loopers.domain.BaseEntity;
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import com.loopers.support.error.CoreException;
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import com.loopers.support.error.ErrorType;
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import jakarta.persistence.Column;
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import jakarta.persistence.Entity;
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import jakarta.persistence.EnumType;
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import jakarta.persistence.Enumerated;
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import jakarta.persistence.Table;
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import lombok.AccessLevel;
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import lombok.NoArgsConstructor;
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@Entity
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@Table(name = "coupon")
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@NoArgsConstructor(access = AccessLevel.PROTECTED)
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public class Coupon extends BaseEntity {
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@Column(name = "ref_user_id", nullable = false)
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private long userId;
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@Enumerated(EnumType.STRING)
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@Column(nullable = false)
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private CouponType type;
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private long discountValue;
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private boolean used;
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public Coupon(long userId, CouponType type, long discountValue) {
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if (type == CouponType.PERCENTAGE && (discountValue < 0 || discountValue > 100)) {
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throw new CoreException(ErrorType.BAD_REQUEST, "할인율은 0~100% 사이여야 합니다.");
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}
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this.userId = userId;
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this.type = type;
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this.discountValue = discountValue;
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this.used = false;
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}
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public boolean isUsed() {
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return used;
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}
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public long calculateDiscountAmount(long totalOrderAmount) {
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return this.type.calculate(totalOrderAmount, this.discountValue);
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}
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public void use() {
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if (this.used) {
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throw new CoreException(ErrorType.BAD_REQUEST, "이미 사용된 쿠폰입니다.");
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}
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this.used = true;
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}
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}

‎apps/commerce-api/src/main/java/com/loopers/domain/coupon/CouponType.java‎

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package com.loopers.domain.coupon;
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public enum CouponType {
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FIXED_AMOUNT {
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@Override
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public long calculate(long totalAmount, long discountValue) {
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return Math.min(totalAmount, discountValue);
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}
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},
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PERCENTAGE {
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@Override
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public long calculate(long totalAmount, long discountValue) {
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return (long) (totalAmount * (discountValue / 100.0));
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}
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};
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public abstract long calculate(long totalAmount, long discountValue);
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}

‎apps/commerce-api/src/test/java/com/loopers/domain/coupon/CouponTest.java‎

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package com.loopers.domain.coupon;
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import static org.assertj.core.api.Assertions.assertThat;
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import static org.assertj.core.api.Assertions.assertThatThrownBy;
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import static org.junit.jupiter.api.Assertions.*;
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import com.loopers.support.error.CoreException;
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import com.loopers.support.error.ErrorType;
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import org.junit.jupiter.api.DisplayName;
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import org.junit.jupiter.api.Nested;
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import org.junit.jupiter.api.Test;
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class CouponTest {
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@Nested
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@DisplayName("쿠폰 할인 금액 계산 테스트")
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class CalculateDiscount {
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@Test
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@DisplayName("정액 쿠폰: 주문 금액에서 할인 금액만큼 차감된 값을 계산한다.")
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void fixed_amount_discount() {
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// given
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long orderAmount = 10000L;
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long discountAmount = 1000L;
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Coupon coupon = new Coupon(1l, CouponType.FIXED_AMOUNT, discountAmount);
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// when
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long result = coupon.calculateDiscountAmount(orderAmount);
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// then
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assertThat(result).isEqualTo(1000L);
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}
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@Test
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@DisplayName("정액 쿠폰: 주문 금액보다 할인 금액이 크면, 주문 금액만큼만 할인된다(결제금액 0원 보장).")
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void fixed_amount_discount_capped() {
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// given
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long orderAmount = 500L;
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long discountAmount = 1000L;
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Coupon coupon = new Coupon(1l, CouponType.FIXED_AMOUNT, discountAmount);
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// when
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long result = coupon.calculateDiscountAmount(orderAmount);
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// then
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assertThat(result).isEqualTo(500L);
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}
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@Test
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@DisplayName("정률 쿠폰: 주문 금액의 비율만큼 할인 금액이 계산된다.")
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void percentage_discount() {
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// given
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long orderAmount = 20000L;
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long discountRate = 10L; // 10%
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Coupon coupon = new Coupon(1l, CouponType.PERCENTAGE, discountRate);
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// when
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long result = coupon.calculateDiscountAmount(orderAmount);
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// then
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assertThat(result).isEqualTo(2000L);
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}
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}
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@Nested
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@DisplayName("쿠폰 생성 및 유효성 검증 테스트")
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class Validation {
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@Test
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@DisplayName("정률 쿠폰 생성 시 할인율이 100을 초과하면 예외가 발생한다.")
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void create_percentage_coupon_fail_over_100() {
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assertThatThrownBy(() ->
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new Coupon(1l, CouponType.PERCENTAGE, 101L)
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)
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.isInstanceOf(CoreException.class)
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.hasMessage("할인율은 0~100% 사이여야 합니다.");
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}
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}
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@Nested
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@DisplayName("쿠폰 사용 처리 테스트")
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class UseCoupon {
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@Test
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@DisplayName("쿠폰을 사용하면 used 상태가 true로 변경된다.")
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void use_coupon_success() {
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// given
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Coupon coupon = new Coupon(1l, CouponType.FIXED_AMOUNT, 1000L);
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// when
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coupon.use();
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// then
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assertThat(coupon.isUsed()).isTrue();
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}
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@Test
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@DisplayName("이미 사용된 쿠폰을 다시 사용하려고 하면 예외가 발생한다.")
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void use_coupon_fail_already_used() {
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// given
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Coupon coupon = new Coupon(1l, CouponType.FIXED_AMOUNT, 1000L);
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coupon.use();
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// when & then
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assertThatThrownBy(coupon::use)
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.isInstanceOf(CoreException.class)
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.hasFieldOrPropertyWithValue("errorType", ErrorType.BAD_REQUEST)
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.hasMessage("이미 사용된 쿠폰입니다.");
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}
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}
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}

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