A Practical Guide to Building Software That Can Change

public BigDecimal priceFor(String plan, boolean annual) {
  BigDecimal monthly = switch (plan) {
    case "BASIC" -> new BigDecimal("9.99");
    case "PRO"   -> new BigDecimal("19.99");
    default -> throw new IllegalArgumentException(
        "Unknown plan: " + plan);
  };
  if (annual) {
    return monthly.multiply(BigDecimal.valueOf(12))
        .multiply(new BigDecimal("0.80"))
        .setScale(2, RoundingMode.HALF_UP);
  }
  return monthly;
}

public BigDecimal amountDue(Subscription subscription) {
  BigDecimal monthly = subscription.getPlan().equals("PRO")
      ? new BigDecimal("19.99")
      : new BigDecimal("9.99");   // unknown plan? Basic
  if (subscription.isAnnual()) {
    return monthly.multiply(new BigDecimal("12"))
        .multiply(new BigDecimal("0.8"))
        .setScale(2, RoundingMode.HALF_UP);
  }
  return monthly;
}

public class PricingPolicy {
  private static final BigDecimal ANNUAL_DISCOUNT =
      new BigDecimal("0.15");
  private static final int MONTHS_PER_YEAR = 12;

  public BigDecimal priceFor(Plan plan, BillingCycle cycle) {
    if (cycle == BillingCycle.MONTHLY) {
      return plan.monthlyPrice();
    }
    BigDecimal fullYear = plan.monthlyPrice()
        .multiply(BigDecimal.valueOf(MONTHS_PER_YEAR));
    return fullYear
        .multiply(BigDecimal.ONE.subtract(ANNUAL_DISCOUNT))
        .setScale(2, RoundingMode.HALF_UP);
  }
}

public BigDecimal priceFor(Plan plan, BillingCycle cycle) {
  return pricing.priceFor(plan, cycle);
}

public BigDecimal amountDue(Subscription subscription) {
  return pricing.priceFor(subscription.plan(),
                          subscription.cycle());
}