[Feature/Fix] 클라이언트 정산 계산 로직 구현 및 지출 수정 버그 해결

Data/Sources/DTO/Expense/Request/UpdateExpenseRequestDTO.swift

@@ -14,6 +14,7 @@ public struct UpdateExpenseRequestDTO: Encodable {
     let currency: String
     let expenseDate: String
     let category: String
+    let payerId: String
     let participantIds: [String]
 }
 
@@ -33,6 +34,7 @@ extension Expense {
             currency: currency,
             expenseDate: dateString,
             category: category.rawValue,
+            payerId: payerId,
             participantIds: participants.map { $0.id }
         )
     }

Domain/Sources/UseCase/Settlement/CalculateSettlementUseCase.swift

@@ -0,0 +1,192 @@
+//
+//  CalculateSettlementUseCase.swift
+//  Domain
+//
+//  Created by 홍석현 on 12/10/25.
+//
+
+import Foundation
+import ComposableArchitecture
+
+public struct CalculateSettlementUseCase: CalculateSettlementUseCaseProtocol {
+
+    public init() {}
+
+    public func execute(
+        expenses: [Expense],
+        members: [TravelMember],
+        currentUserId: String?
+    ) -> SettlementCalculation {
+
+        // 1. 총 지출 금액
+        let totalExpenseAmount = expenses.reduce(0) { $0 + $1.convertedAmount }
+
+        // 2. 내가 부담해야 할 금액 (내가 참여한 지출들의 분담금 합계)
+        let myShareAmount: Double
+        if let userId = currentUserId {
+            myShareAmount = expenses
+                .filter { expense in
+                    expense.participants.contains(where: { $0.id == userId })
+                }
+                .reduce(0) { sum, expense in
+                    let participantCount = Double(expense.participants.count)
+                    guard participantCount > 0 else { return sum }
+                    return sum + (expense.convertedAmount / participantCount)
+                }
+        } else {
+            myShareAmount = 0
+        }
+
+        // 3. 인원수
+        let totalPersonCount = members.count
+
+        // 4. 1인 평균 지출
+        let averagePerPerson = totalPersonCount > 0 ? totalExpenseAmount / Double(totalPersonCount) : 0
+
+        // 5. 각 멤버의 순 차액 계산 (Net Balance = Pay - Owe)
+        var memberBalances: [String: Double] = [:]
+
+        // 모든 멤버 초기화
+        for member in members {
+            memberBalances[member.id] = 0.0
+        }
+
+        // 각 지출에 대해 계산
+        for expense in expenses {
+            let participantCount = Double(expense.participants.count)
+            guard participantCount > 0 else { continue }
+
+            let amountPerPerson = expense.convertedAmount / participantCount
+
+            // 결제자는 전체 금액을 지불한 것으로 (+)
+            memberBalances[expense.payerId, default: 0] += expense.convertedAmount
+
+            // 참여자들은 각자 분담금을 빚진 것으로 (-)
+            for participant in expense.participants {
+                memberBalances[participant.id, default: 0] -= amountPerPerson
+            }
+        }
+
+        // 6. 내 순 차액
+        let myNetBalance = currentUserId.flatMap { memberBalances[$0] } ?? 0
+
+        // 7. 지급 예정 금액 계산
+        let paymentsToMake = calculatePaymentsToMake(
+            myBalance: myNetBalance,
+            memberBalances: memberBalances,
+            members: members,
+            currentUserId: currentUserId
+        )
+
+        // 8. 수령 예정 금액 계산
+        let paymentsToReceive = calculatePaymentsToReceive(
+            myBalance: myNetBalance,
+            memberBalances: memberBalances,
+            members: members,
+            currentUserId: currentUserId
+        )
+
+        return SettlementCalculation(
+            totalExpenseAmount: totalExpenseAmount,
+            myShareAmount: myShareAmount,
+            totalPersonCount: totalPersonCount,
+            averagePerPerson: averagePerPerson,
+            myNetBalance: myNetBalance,
+            memberBalances: memberBalances,
+            paymentsToMake: paymentsToMake,
+            paymentsToReceive: paymentsToReceive
+        )
+    }
+
+    // MARK: - Private Helper Methods
+
+    // 지급 예정 금액 (내가 빚진 사람들에게 갚아야 할 돈)
+    private func calculatePaymentsToMake(
+        myBalance: Double,
+        memberBalances: [String: Double],
+        members: [TravelMember],
+        currentUserId: String?
+    ) -> [PaymentInfo] {
+        guard currentUserId != nil else { return [] }
+        guard myBalance < 0 else { return [] } // 내가 받을 돈이 있으면 지급할 것이 없음
+
+        // 양수 잔액을 가진 멤버들 (받을 돈이 있는 사람들)
+        let creditors = memberBalances
+            .filter { $0.value > 0 }
+            .sorted { $0.value > $1.value } // 많이 받을 사람부터
+
+        var payments: [PaymentInfo] = []
+        var remainingDebt = abs(myBalance)
+
+        for (memberId, creditAmount) in creditors {
+            guard remainingDebt > 0.01 else { break } // 소수점 오차 고려
+
+            let paymentAmount = min(remainingDebt, creditAmount)
+
+            if let member = members.first(where: { $0.id == memberId }) {
+                payments.append(PaymentInfo(
+                    id: UUID().uuidString,
+                    memberId: memberId,
+                    memberName: member.name,
+                    amount: paymentAmount
+                ))
+                remainingDebt -= paymentAmount
+            }
+        }
+
+        return payments
+    }
+
+    // 수령 예정 금액 (나에게 빚진 사람들로부터 받을 돈)
+    private func calculatePaymentsToReceive(
+        myBalance: Double,
+        memberBalances: [String: Double],
+        members: [TravelMember],
+        currentUserId: String?
+    ) -> [PaymentInfo] {
+        guard currentUserId != nil else { return [] }
+        guard myBalance > 0 else { return [] } // 내가 빚진 돈이 있으면 받을 것이 없음
+
+        // 음수 잔액을 가진 멤버들 (빚진 사람들)
+        let debtors = memberBalances
+            .filter { $0.value < 0 }
+            .sorted { $0.value < $1.value } // 많이 빚진 사람부터
+
+        var receipts: [PaymentInfo] = []
+        var remainingCredit = myBalance
+
+        for (memberId, debtAmount) in debtors {
+            guard remainingCredit > 0.01 else { break } // 소수점 오차 고려
+
+            let receiptAmount = min(remainingCredit, abs(debtAmount))
+
+            if let member = members.first(where: { $0.id == memberId }) {
+                receipts.append(PaymentInfo(
+                    id: UUID().uuidString,
+                    memberId: memberId,
+                    memberName: member.name,
+                    amount: receiptAmount
+                ))
+                remainingCredit -= receiptAmount
+            }
+        }
+
+        return receipts
+    }
+}
+
+// MARK: - DependencyKey
+public enum CalculateSettlementUseCaseDependencyKey: DependencyKey {
+    public static var liveValue: any CalculateSettlementUseCaseProtocol = CalculateSettlementUseCase()
+
+    public static var testValue: any CalculateSettlementUseCaseProtocol = CalculateSettlementUseCase()
+
+    public static var previewValue: any CalculateSettlementUseCaseProtocol = CalculateSettlementUseCase()
+}
+
+public extension DependencyValues {
+    var calculateSettlementUseCase: any CalculateSettlementUseCaseProtocol {
+        get { self[CalculateSettlementUseCaseDependencyKey.self] }
+        set { self[CalculateSettlementUseCaseDependencyKey.self] = newValue }
+    }
+}

Domain/Sources/UseCase/Settlement/CalculateSettlementUseCaseProtocol.swift

@@ -0,0 +1,68 @@
+//
+//  CalculateSettlementUseCaseProtocol.swift
+//  Domain
+//
+//  Created by 홍석현 on 12/10/25.
+//
+
+import Foundation
+
+// 정산 계산 결과
+public struct SettlementCalculation: Equatable {
+    public let totalExpenseAmount: Double // 총 지출 금액
+    public let myShareAmount: Double // 내가 부담해야 할 금액 (내가 참여한 지출들의 분담금 합계)
+    public let totalPersonCount: Int // 인원수
+    public let averagePerPerson: Double // 1인 평균 지출
+    public let myNetBalance: Double // 내 순 차액 (Pay - Owe)
+    public let memberBalances: [String: Double] // 각 멤버의 순 차액
+    public let paymentsToMake: [PaymentInfo] // 지급 예정 금액
+    public let paymentsToReceive: [PaymentInfo] // 수령 예정 금액
+
+    public init(
+        totalExpenseAmount: Double,
+        myShareAmount: Double,
+        totalPersonCount: Int,
+        averagePerPerson: Double,
+        myNetBalance: Double,
+        memberBalances: [String: Double],
+        paymentsToMake: [PaymentInfo],
+        paymentsToReceive: [PaymentInfo]
+    ) {
+        self.totalExpenseAmount = totalExpenseAmount
+        self.myShareAmount = myShareAmount
+        self.totalPersonCount = totalPersonCount
+        self.averagePerPerson = averagePerPerson
+        self.myNetBalance = myNetBalance
+        self.memberBalances = memberBalances
+        self.paymentsToMake = paymentsToMake
+        self.paymentsToReceive = paymentsToReceive
+    }
+}
+
+// 지급/수령 정보
+public struct PaymentInfo: Equatable, Identifiable {
+    public let id: String
+    public let memberId: String
+    public let memberName: String
+    public let amount: Double
+
+    public init(
+        id: String,
+        memberId: String,
+        memberName: String,
+        amount: Double
+    ) {
+        self.id = id
+        self.memberId = memberId
+        self.memberName = memberName
+        self.amount = amount
+    }
+}
+
+public protocol CalculateSettlementUseCaseProtocol {
+    func execute(
+        expenses: [Expense],
+        members: [TravelMember],
+        currentUserId: String?
+    ) -> SettlementCalculation
+}

Domain/Tests/ExpenseValidationTests.swift

@@ -17,7 +17,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "점심",
-            note: nil,
             amount: -1000,  // ❌ 음수
             currency: "KRW",
             convertedAmount: -1000,
@@ -40,7 +39,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "   ",  // ❌ 공백만
-            note: nil,
             amount: 1000,
             currency: "KRW",
             convertedAmount: 1000,
@@ -65,7 +63,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "점심",
-            note: nil,
             amount: 12_000,
             currency: "KRW",
             convertedAmount: 12_000,
@@ -88,7 +85,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "점심",
-            note: nil,
             amount: 12_000,
             currency: "KRW",
             convertedAmount: 12_000,
@@ -109,7 +105,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "점심",
-            note: nil,
             amount: 12_000,
             currency: "KRW",
             convertedAmount: 12_000,
@@ -133,7 +128,6 @@ struct ExpenseValidationTests {
         let expense = Expense(
             id: "1",
             title: "점심",
-            note: "회식",
             amount: 50_000,
             currency: "KRW",
             convertedAmount: 50_000,