Refactor complex conditional statements in offer handling logic

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This PR refactors overly complex conditional statements in the supermarket offer handling logic to improve readability and maintainability. The changes address multiple issues found across Java and C implementations.

Issues Fixed

Critical Bug

Fixed a missing else keyword bug that caused FIVE_FOR_AMOUNT logic to execute inappropriately in both Java and C implementations:

// Before (bug)
} if (offer.offerType == SpecialOfferType.FIVE_FOR_AMOUNT) {
    x = 5;
}

// After (fixed)
} else if (offer.offerType == SpecialOfferType.FIVE_FOR_AMOUNT) {
    x = 5;
}

Complex Conditionals

Transformed complex nested conditionals with inline calculations into clean, readable code:

Before:

if (offer.offerType == SpecialOfferType.TWO_FOR_AMOUNT) {
    x = 2;
    if (quantityAsInt >= 2) {
        double total = offer.argument * (quantityAsInt / x) + quantityAsInt % 2 * unitPrice;
        double discountN = unitPrice * quantity - total;
        discount = new Discount(p, "2 for " + offer.argument, -discountN);
    }
}

After:

} else if (offer.offerType == SpecialOfferType.TWO_FOR_AMOUNT) {
    x = 2;
    if (isEligibleForTwoForAmount(quantityAsInt)) {
        discount = calculateTwoForAmountDiscount(p, offer, unitPrice, quantity, quantityAsInt, x);
    }
}

Improvements Made

Java Implementation

• Extracted boolean helper methods for eligibility checks (isEligibleForTwoForAmount(), isEligibleForThreeForTwo(), isEligibleForFiveForAmount())
• Extracted calculation methods for each discount type with descriptive names
• Separated concerns between eligibility checking and discount calculation

C Implementation

• Applied similar refactoring with static helper functions
• Used consistent naming conventions (is_eligible_for_*, calculate_*_discount)
• Maintained C idioms while improving structure

Benefits

• Readability: Complex conditions now have descriptive names
• Maintainability: Discount logic isolated in dedicated methods
• Testability: Individual components can be tested separately
• Bug Prevention: Clear separation reduces likelihood of logic errors

All existing tests pass, confirming that behavior is preserved while dramatically improving code quality.

Fixes #6.

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