How to format currency in Swift

Issue #355

final class CurrencyFormatter {
    func format(amount: UInt64, decimalCount: Int) -> String {
        let formatter = NumberFormatter()
        formatter.minimumFractionDigits = 0
        formatter.maximumFractionDigits = decimalCount
        formatter.numberStyle = .decimal

        let value = Double(amount) / pow(Double(10), Double(decimalCount))
        let fallback = String(format: "%.0f", value)
        return formatter.string(from: NSNumber(value: value)) ?? fallback
    }
}

class CurrencyFormatterTests: XCTestCase {
    func testFormat() {
        let formatter = CurrencyFormatter()
        // 120 USD
        XCTAssertEqual(formatter.format(amount: 120, decimalCount: 0), "120")

        // 12000 cents
        XCTAssertEqual(formatter.format(amount: 12000, decimalCount: 2), "120")

        // 12520 cents
        XCTAssertEqual(formatter.format(amount: 12520, decimalCount: 2), "125.2")
    }
}