store

ProductPriceConverter.swift

import Foundation
import StoreKit

final class ProductPriceConverter {

    func convertPrice(
        _ price: NSDecimalNumber,
        from fromUnit: SKProduct.PeriodUnit,
        fromNumberOfUnits: Int,
        to toUnit: SKProduct.PeriodUnit,
        toNumberOfUnits: Int
    ) -> NSDecimalNumber {

        switch (fromUnit, toUnit) {
        case (.day, .day):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.day, .week):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 7))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.day, .month):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 30))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.day, .year):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 365))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.week, .day):
            return price.dividing(by: NSDecimalNumber(value: 7))
        case (.week, .week):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.week, .month):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 4.345))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.week, .year):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 52.143))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.month, .day):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .dividing(by: NSDecimalNumber(value: 30))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.month, .week):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .dividing(by: NSDecimalNumber(value: 4.345))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.month, .month):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.month, .year):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: 12))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.year, .day):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .dividing(by: NSDecimalNumber(value: 365))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.year, .week):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .dividing(by: NSDecimalNumber(value: 52.143))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.year, .month):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .dividing(by: NSDecimalNumber(value: 12))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        case (.year, .year):
            return price
                .dividing(by: NSDecimalNumber(value: fromNumberOfUnits))
                .multiplying(by: NSDecimalNumber(value: toNumberOfUnits))
        @unknown default:
            fatalError()
        }
    }

    func convertPrice(
        of product: SKProduct,
        to unit: SKProduct.PeriodUnit,
        numberOfUnits: Int
    ) -> NSDecimalNumber? {
        guard let subscriptionPeriod = product.subscriptionPeriod else { return nil }

        return convertPrice(
            product.price,
            from: subscriptionPeriod.unit,
            fromNumberOfUnits: subscriptionPeriod.numberOfUnits,
            to: unit,
            toNumberOfUnits: numberOfUnits)
    }

    func discount(
        of product: SKProduct,
        inRelationTo toProduct: SKProduct,
        unit: SKProduct.PeriodUnit
    ) -> NSDecimalNumber? {
        guard let price = convertPrice(
            of: product,
            to: .year,
            numberOfUnits: 1) else {
                return nil
        }

        guard let toPrice = convertPrice(
            of: toProduct,
            to: .year,
            numberOfUnits: 1) else {
                return nil
        }

        let decimalHandler = NSDecimalNumberHandler(
            roundingMode: .bankers,
            scale: 2,
            raiseOnExactness: false,
            raiseOnOverflow: false,
            raiseOnUnderflow: false,
            raiseOnDivideByZero: false)

        let ratio = price
            .dividing(by: toPrice)
            .rounding(accordingToBehavior: decimalHandler)

        return NSDecimalNumber(value: 1).subtracting(ratio)
    }
}