import CryptoKit
import Foundation

enum OneTimePasscodeGenerator {
    static func code(for pairingCode: String, at date: Date) -> String {
        let timeSlot = Int(date.timeIntervalSince1970 / 30)
        let digest = SHA256.hash(data: Data("\(pairingCode)|\(timeSlot)".utf8))
        let value = digest.prefix(4).reduce(UInt32(0)) { partialResult, byte in
            (partialResult << 8) | UInt32(byte)
        }

        return String(format: "%06d", locale: Locale(identifier: "en_US_POSIX"), Int(value % 1_000_000))
    }

    static func renewalCountdown(at date: Date) -> Int {
        let elapsed = Int(date.timeIntervalSince1970) % 30
        return elapsed == 0 ? 30 : 30 - elapsed
    }
}