Refactor discount and receipt systems using Strategy and Factory patterns

[Copilot]

This PR addresses the code organization and flexibility issues in the supermarket receipt system by implementing well-established design patterns across two key areas of the codebase.

Problem

The original codebase suffered from several code smells:

1. Long Method: ShoppingCart.handleOffers() contained 40+ lines of complex conditional logic
2. Feature Envy: Extensive access to external objects within discount calculations
3. Open/Closed Principle Violations: Adding new discount types or receipt formats required modifying existing code
4. Lack of HTML Receipt Support: Only plain text receipts were available

Solution

1. Strategy Pattern for Discount Calculations

Extracted the complex discount calculation logic from ShoppingCart.handleOffers() into focused strategy classes:

// Before: Complex nested conditionals
if (offer.offerType == SpecialOfferType.THREE_FOR_TWO) {
    // 8+ lines of calculation logic
} else if (offer.offerType == SpecialOfferType.TWO_FOR_AMOUNT) {
    // 6+ lines of calculation logic
} // ... more conditionals

// After: Clean strategy pattern
DiscountStrategy strategy = DiscountStrategyFactory.createStrategy(offer.offerType);
Discount discount = strategy.calculateDiscount(p, quantity, unitPrice, offer.argument);

Concrete Strategies Implemented:

• ThreeForTwoDiscountStrategy
• TenPercentDiscountStrategy
• TwoForAmountDiscountStrategy
• FiveForAmountDiscountStrategy

2. Factory Pattern for Strategy Creation

DiscountStrategyFactory centralizes the creation of discount strategies, eliminating conditional logic and making it easy to add new discount types.

3. Strategy Pattern for Receipt Formatting

Implemented a flexible receipt formatting system that supports multiple output formats:

// Text receipts (default)
ReceiptPrinter textPrinter = new ReceiptPrinter(new TextReceiptFormatter());

// HTML receipts (new feature)
ReceiptPrinter htmlPrinter = new ReceiptPrinter(new HtmlReceiptFormatter());

The HtmlReceiptFormatter includes proper HTML escaping and structured table output, addressing the HTML receipt feature request from the README.

Results

Code Quality Improvements:

• Reduced ShoppingCart.handleOffers() from 40+ lines to 16 lines
• Eliminated complex nested conditionals
• Improved separation of concerns
• Enhanced testability with focused, isolated classes

Functionality Enhancements:

• Added HTML receipt generation capability
• Maintained 100% backward compatibility
• All existing functionality preserved

Testing:

• 17 comprehensive tests covering all strategies and integration scenarios
• Individual strategy testing for better isolation
• End-to-end verification of discount calculations and receipt formatting

The refactoring demonstrates how multiple design patterns can work together to create a more maintainable, extensible, and robust codebase while preserving all existing functionality.

Fixes #7.

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