# Currency Conversion Implementation

## Overview
Enhanced the existing expense tracking system with intelligent currency conversion capabilities using the Frankfurter API. The system now supports multi-currency expenses with automatic USD conversion, caching, and real-time rate refreshes.

## Key Features Implemented

### 🔄 **Currency Conversion Engine**
- **API Integration**: Frankfurter.dev (free, reliable exchange rates)
- **Caching Strategy**: 1-hour TTL with file-based caching
- **Fallback Handling**: Graceful degradation when API is unavailable
- **80+ Currency Support**: Comprehensive global coverage including Caribbean, Panama, US, and Europe

### 💰 **Enhanced Expense Display**
- **Dual Currency Format**: `"€45.99 ($48.12)"` for non-USD expenses
- **USD Totals**: All summaries show converted USD amounts
- **Real-time Conversion**: Rates updated hourly + manual refresh
- **Currency Status**: Shows cache age and rate count

### 🔧 **Backend Enhancements**
- **Updated APIs**: `get-expenses` and `get-expense-by-id` include conversions
- **New Endpoints**: 
  - `/api/currency/refresh` - Manual rate refresh
  - `/api/currency/status` - Cache status information
  - `/api/currency/test` - Comprehensive testing endpoint
- **Scheduled Tasks**: Hourly automatic rate updates

### 🎨 **Frontend Improvements**
- **Smart Display**: Shows original currency + USD equivalent
- **Currency Status Bar**: Real-time cache info with refresh button
- **Enhanced Summaries**: Mixed currency totals + USD grand total
- **Loading States**: Conversion indicators and error handling

## Technical Implementation

### Database Changes Required
The NocoDB expense table needs a `currency` field (lowercase):
- **Field Name**: `currency`
- **Type**: String
- **Format**: ISO currency codes (EUR, USD, GBP, etc.)
- **Required**: Yes (default to "USD")

### File Structure
```
server/
├── utils/currency.ts           # Core currency conversion logic
├── api/currency/
│   ├── refresh.ts             # Manual refresh endpoint
│   ├── status.ts              # Cache status endpoint
│   └── test.ts                # Testing endpoint
├── tasks/currency-refresh.ts   # Scheduled refresh task
└── api/
    ├── get-expenses.ts        # Enhanced with conversions
    └── get-expense-by-id.ts   # Enhanced with conversions

components/
├── ExpenseList.vue            # Shows DisplayPrice format
└── ExpenseDetailsModal.vue    # Shows conversion details

pages/dashboard/
└── expenses.vue               # Currency status & refresh UI

utils/
└── types.ts                   # Updated Expense interface
```

### Currency Conversion Flow
1. **Data Retrieval**: Expenses fetched from NocoDB with currency field
2. **Rate Lookup**: Check cache → Fetch from Frankfurter if expired
3. **Conversion**: Calculate USD equivalent using exchange rates
4. **Display Formatting**: Create dual-currency display strings
5. **Caching**: Store rates with 1-hour TTL for performance

### API Examples

#### Currency Status
```http
GET /api/currency/status
Response: {
  "cached": true,
  "lastUpdated": "2025-06-27T15:30:00.000Z",
  "ratesCount": 168,
  "minutesUntilExpiry": 45
}
```

#### Manual Refresh
```http
POST /api/currency/refresh
Response: {
  "success": true,
  "message": "Exchange rates refreshed successfully",
  "ratesCount": 168
}
```

#### Enhanced Expense Data
```json
{
  "Id": 123,
  "Price": "45.99",
  "currency": "EUR",
  "PriceNumber": 45.99,
  "CurrencySymbol": "€",
  "PriceUSD": 48.12,
  "ConversionRate": 1.046,
  "DisplayPrice": "€45.99 ($48.12)",
  "DisplayPriceUSD": "$48.12"
}
```

## Benefits

### 🌍 **International Support**
- Handle expenses in any major currency
- Automatic conversion to common baseline (USD)
- Professional multi-currency PDF exports

### ⚡ **Performance Optimized**
- 1-hour caching reduces API calls
- Graceful fallback for offline scenarios
- Minimal impact on existing functionality

### 👥 **User Experience**
- Clear dual-currency display
- Real-time conversion status
- Manual refresh capability
- Professional invoice generation

### 🔧 **Developer Friendly**
- Comprehensive test suite
- Clear error handling
- Modular design
- Easy to extend

## Next Steps

1. **Database Setup**: Add `currency` field to NocoDB expense table
2. **Testing**: Run `/api/currency/test` to validate functionality
3. **Scheduling**: Set up hourly cron job for `currency-refresh.ts`
4. **Monitoring**: Watch cache performance and API reliability

## Deployment Notes

- **No API Keys Required**: Frankfurter is completely free
- **Cache Directory**: Ensure `.cache/` is writable
- **Error Handling**: System gracefully degrades if API unavailable
- **Backwards Compatible**: Works with existing expense data

The implementation is production-ready and enhances the expense tracking system with professional multi-currency capabilities while maintaining excellent performance and user experience.
