# Outlet-Based Tax Calculation Implementation

## Overview

This document describes the implementation of outlet-based tax calculation in the order processing system. The system now uses the tax rate configured for each outlet instead of a hardcoded tax rate.

## Feature Description

Previously, the system used a hardcoded 10% tax rate for all orders. Now, the tax calculation is based on the `tax_rate` field configured for each outlet, allowing for different tax rates across different locations.

## Implementation Details

### 1. Order Processor Changes

The `OrderProcessorImpl` has been updated to:

- Accept an `OutletRepository` dependency
- Fetch outlet information to get the tax rate
- Calculate tax using the outlet's specific tax rate
- Recalculate tax when adding items to existing orders

### 2. Tax Calculation Logic

```go
// Get outlet information for tax rate
outlet, err := p.outletRepo.GetByID(ctx, req.OutletID)
if err != nil {
    return nil, fmt.Errorf("outlet not found: %w", err)
}

// Calculate tax using outlet's tax rate
taxAmount := subtotal * outlet.TaxRate
totalAmount := subtotal + taxAmount
```

### 3. Database Schema

The `outlets` table includes:

- `tax_rate`: Decimal field (DECIMAL(5,4)) for tax rate as a decimal (e.g., 0.085 for 8.5%)
- Constraint: `CHECK (tax_rate >= 0 AND tax_rate <= 1)` to ensure valid percentage

### 4. Tax Rate Examples

| Tax Rate (Decimal) | Percentage | Example Calculation |
|-------------------|------------|-------------------|
| 0.0000 | 0% | No tax |
| 0.0500 | 5% | $100 × 0.05 = $5.00 tax |
| 0.0850 | 8.5% | $100 × 0.085 = $8.50 tax |
| 0.1000 | 10% | $100 × 0.10 = $10.00 tax |
| 0.1500 | 15% | $100 × 0.15 = $15.00 tax |

### 5. API Usage

The tax calculation is automatic and transparent to the API consumer. When creating orders or adding items, the system:

1. Fetches the outlet's tax rate
2. Calculates tax based on the current subtotal
3. Updates the order with the correct tax amount

```json
{
  "outlet_id": "uuid-of-outlet",
  "order_items": [
    {
      "product_id": "uuid-of-product",
      "quantity": 2
    }
  ]
}
```

The response will include the calculated tax amount:

```json
{
  "id": "order-uuid",
  "outlet_id": "outlet-uuid",
  "subtotal": 20.00,
  "tax_amount": 1.70,  // Based on outlet's tax rate
  "total_amount": 21.70
}
```

### 6. Business Scenarios

#### Scenario 1: Different Tax Rates by Location
- **Downtown Location**: 8.5% tax rate
- **Suburban Location**: 6.5% tax rate
- **Airport Location**: 10.0% tax rate

#### Scenario 2: Tax-Exempt Locations
- **Wholesale Outlet**: 0% tax rate
- **Export Zone**: 0% tax rate

#### Scenario 3: Seasonal Tax Changes
- **Holiday Period**: Temporary tax rate adjustments
- **Promotional Period**: Reduced tax rates

### 7. Validation

The system includes several validation checks:

1. **Outlet Existence**: Verifies the outlet exists
2. **Tax Rate Range**: Database constraint ensures 0% ≤ tax rate ≤ 100%
3. **Tax Calculation**: Ensures positive tax amounts

### 8. Error Handling

Common error scenarios:

- `outlet not found`: When an invalid outlet ID is provided
- Database constraint violations for invalid tax rates

### 9. Testing

The implementation includes unit tests to verify:

- Correct tax calculation with different outlet tax rates
- Proper error handling for invalid outlets
- Tax recalculation when adding items to existing orders

### 10. Migration

The feature uses existing database schema from migration `000002_create_outlets_table.up.sql` which includes the `tax_rate` column.

### 11. Configuration

Outlet tax rates can be configured through:

1. **Outlet Creation API**: Set initial tax rate
2. **Outlet Update API**: Modify tax rate for existing outlets
3. **Database Direct Update**: For bulk changes

### 12. Future Enhancements

Potential improvements:

1. **Tax Rate History**: Track tax rate changes over time
2. **Conditional Tax Rates**: Different rates based on order type or customer type
3. **Tax Exemptions**: Support for tax-exempt customers or items
4. **Multi-Tax Support**: Support for multiple tax types (state, local, etc.)
5. **Tax Rate Validation**: Integration with tax authority APIs for rate validation

### 13. Performance Considerations

- Outlet information is fetched once per order creation/modification
- Tax calculation is performed in memory for efficiency
- Consider caching outlet information for high-volume scenarios

### 14. Compliance

- Tax rates should comply with local tax regulations
- Consider implementing tax rate validation against official sources
- Maintain audit trails for tax rate changes