# Task 2.1: CostCalculationService Implementation Summary

## Overview
Successfully implemented the `CostCalculationService` with fare difference calculation, modification fee retrieval, total cost calculation, and wallet impact determination for the booking modification feature.

## Implementation Details

### 1. DTOs Created

#### ModificationCost.java
- **Location**: `src/main/java/com/flightticket/dto/modification/ModificationCost.java`
- **Purpose**: Represents the complete cost breakdown of a booking modification
- **Key Fields**:
  - `modificationFee`: Airline-imposed fee (always non-negative)
  - `fareDifference`: Difference between new and original fare (can be positive, negative, or zero)
  - `totalCost`: Sum of modification fee and fare difference
  - `transactionType`: DEBIT_MODIFICATION or CREDIT_MODIFICATION
  - `currency`: ISO 4217 currency code
  - `breakdown`: Detailed cost breakdown

#### CostBreakdown.java
- **Location**: `src/main/java/com/flightticket/dto/modification/CostBreakdown.java`
- **Purpose**: Detailed breakdown of modification costs
- **Key Fields**:
  - `originalFare`: Original booking fare
  - `newFare`: New flight fare
  - `airlineModificationFee`: Airline-imposed fee
  - `platformFee`: Platform fee (currently zero)
  - `taxes`: Tax differences (currently zero)

#### WalletImpact.java
- **Location**: `src/main/java/com/flightticket/dto/modification/WalletImpact.java`
- **Purpose**: Represents the impact of a modification on the wallet
- **Key Fields**:
  - `currentBalance`: Current wallet balance
  - `modificationCost`: Total modification cost
  - `newBalance`: Projected balance after modification
  - `transactionType`: DEBIT or CREDIT
  - `sufficientBalance`: Whether wallet has sufficient funds
  - `shortfall`: Amount needed if insufficient (only for debit)

### 2. Enum Updates

#### TransactionType.java
- **Added**:
  - `DEBIT_MODIFICATION`: For fare increases or modifications with net cost
  - `CREDIT_MODIFICATION`: For fare decreases exceeding modification fee

### 3. Service Interface

#### CostCalculationService.java
- **Location**: `src/main/java/com/flightticket/service/CostCalculationService.java`
- **Methods**:
  1. `calculateFareDifference(originalFare, newFare)`: Calculates fare difference (new - original)
  2. `getModificationFee(booking, modificationType)`: Retrieves modification fee from fare rules
  3. `calculateTotalCost(modificationFee, fareDifference)`: Calculates total cost (fee + difference)
  4. `calculateModificationCost(booking, newFare, modificationType)`: Complete cost calculation
  5. `calculateWalletImpact(currentBalance, modificationCost)`: Determines wallet impact

### 4. Service Implementation

#### CostCalculationServiceImpl.java
- **Location**: `src/main/java/com/flightticket/service/impl/CostCalculationServiceImpl.java`
- **Features**:
  - Implements all cost calculation methods
  - Uses default modification fees by type:
    - Passenger Details: $50.00
    - Flight Date: $150.00
    - Flight Route: $200.00
    - Cabin Class: $100.00
    - Combined: $250.00
  - Validates all inputs (null checks, non-negative fees)
  - Comprehensive logging for debugging and monitoring
  - Handles positive, negative, and zero fare differences
  - Correctly determines DEBIT vs CREDIT transactions
  - Calculates wallet balance sufficiency and shortfall

### 5. Unit Tests

#### CostCalculationServiceImplTest.java
- **Location**: `src/test/java/com/flightticket/service/impl/CostCalculationServiceImplTest.java`
- **Test Coverage**: 30 unit tests covering:
  - **Fare Difference Calculation** (Property 11):
    - Positive fare difference (fare increase)
    - Negative fare difference (fare decrease)
    - Zero fare difference (no change)
    - Null parameter validation
    - Very large fare differences
  
  - **Modification Fee Retrieval**:
    - All modification types (5 types)
    - Null parameter validation
  
  - **Total Cost Calculation** (Property 9):
    - Fare increase scenarios
    - Fare decrease scenarios
    - Zero fare change
    - Fare decrease exceeding fee (Property 10)
    - Negative fee validation
    - Null parameter validation
  
  - **Complete Modification Cost**:
    - Fare increase with debit
    - Fare decrease with debit
    - Fare decrease with credit
  
  - **Wallet Impact Calculation** (Properties 12, 13):
    - Debit with sufficient balance (Property 12)
    - Debit with insufficient balance (Property 13)
    - Debit with exact balance
    - Credit transaction (Property 10)
    - Null parameter validation
  
  - **Edge Cases**:
    - Very large fare differences
    - Very large negative fare differences

### 6. Test Results
- **Total Tests**: 30
- **Passed**: 30
- **Failed**: 0
- **Status**: ✅ All tests passing

## Requirements Validated

### Requirement 6.1: Modification Cost Calculation
✅ **Property 9**: Total cost equals modification fee plus fare difference
- Implemented in `calculateTotalCost()` method
- Validated with multiple test scenarios

### Requirement 6.2: Credit Calculation for Fare Decreases
✅ **Property 10**: Credit amount correctly calculated when fare decrease exceeds fee
- Implemented in `calculateModificationCost()` method
- Returns negative totalCost for credit scenarios
- Validated with test `testCalculateModificationCost_CreditScenario()`

### Requirement 3.2: Fare Difference Calculation
✅ **Property 11**: Fare difference calculated as new_fare - original_fare
- Implemented in `calculateFareDifference()` method
- Preserves sign (positive/negative/zero)
- Validated with multiple test scenarios

### Requirement 6.3: Wallet Balance Validation
✅ **Property 12**: Wallet balance validated before processing
- Implemented in `calculateWalletImpact()` method
- Checks if balance >= totalCost for debit transactions
- Validated with test `testCalculateWalletImpact_DebitSufficientBalance()`

### Requirement 6.4: Insufficient Balance Handling
✅ **Property 13**: Shortfall calculated when balance insufficient
- Implemented in `calculateWalletImpact()` method
- Calculates shortfall = totalCost - currentBalance
- Validated with test `testCalculateWalletImpact_DebitInsufficientBalance()`

## Key Design Decisions

1. **Default Modification Fees**: Used hardcoded default fees for now. In production, these would be retrieved from Amadeus API fare rules.

2. **Transaction Type Determination**: 
   - `DEBIT_MODIFICATION` when totalCost >= 0
   - `CREDIT_MODIFICATION` when totalCost < 0

3. **Wallet Impact Calculation**:
   - For debit: checks balance sufficiency and calculates shortfall
   - For credit: always sufficient (adding money to wallet)

4. **BigDecimal Precision**: All monetary calculations use BigDecimal with proper scale handling to avoid floating-point errors.

5. **Comprehensive Validation**: All public methods validate inputs and throw IllegalArgumentException for invalid parameters.

## Integration Points

### Dependencies
- `Booking` entity: Source of original fare and currency
- `ModificationType` enum: Determines modification fee
- `TransactionType` enum: Extended with modification-specific types

### Used By (Future Tasks)
- `ModificationService`: Will use this service for cost calculations
- `WalletService`: Will use wallet impact for transaction processing
- `ModificationController`: Will expose cost calculation endpoints

## Files Created/Modified

### Created Files
1. `src/main/java/com/flightticket/dto/modification/ModificationCost.java`
2. `src/main/java/com/flightticket/dto/modification/CostBreakdown.java`
3. `src/main/java/com/flightticket/dto/modification/WalletImpact.java`
4. `src/main/java/com/flightticket/service/CostCalculationService.java`
5. `src/main/java/com/flightticket/service/impl/CostCalculationServiceImpl.java`
6. `src/test/java/com/flightticket/service/impl/CostCalculationServiceImplTest.java`

### Modified Files
1. `src/main/java/com/flightticket/model/enums/TransactionType.java` - Added DEBIT_MODIFICATION and CREDIT_MODIFICATION

## Next Steps

The following tasks depend on this implementation:

1. **Task 2.2**: Write property test for cost calculation (Property 9)
2. **Task 2.3**: Write property test for credit calculation (Property 10)
3. **Task 2.4**: Write property test for fare difference calculation (Property 11)
4. **Task 2.5**: Write unit tests for cost calculation edge cases

The CostCalculationService is now ready to be integrated with:
- EligibilityService (Task 3.1)
- WalletService extensions (Task 6.1)
- ModificationService orchestration (Task 10.1)

## Verification

### Compilation
```bash
mvn compile -q
```
✅ Success - No compilation errors

### Unit Tests
```bash
mvn test -Dtest=CostCalculationServiceImplTest -q
```
✅ Success - All 30 tests passing

## Notes

- The implementation follows the design document specifications exactly
- All required methods from the task details are implemented
- Comprehensive error handling and validation
- Detailed logging for debugging and monitoring
- Ready for integration with other services
- Property-based tests (Tasks 2.2-2.4) will provide additional validation across the input space

## Task Status
✅ **COMPLETED** - All requirements met, all tests passing, ready for integration