import Foundation

public enum DubaiTime {
    private static let stampFormatter = LockedDateFormatter(dateFormat: "d MMM yyyy, HH:mm 'Dubai'")
    private static let fileStampFormatter = LockedDateFormatter(dateFormat: "yyyyMMdd-HHmmss")

    public static func stamp(_ date: Date) -> String {
        stampFormatter.string(from: date)
    }

    public static func fileStamp(_ date: Date) -> String {
        fileStampFormatter.string(from: date)
    }
}

/// DateFormatter is not Sendable. This holder is the only shared mutable state
/// around a formatter: every read is serialized by the lock, so concurrent
/// calls (one per PDF page) cannot race.
private final class LockedDateFormatter: @unchecked Sendable {
    private let lock = NSLock()
    private let formatter: DateFormatter

    init(dateFormat: String) {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .gregorian)
        formatter.locale = Locale(identifier: "en_GB")
        guard let dubai = TimeZone(identifier: "Asia/Dubai") else {
            preconditionFailure("The Asia/Dubai time zone is missing from this system.")
        }
        formatter.timeZone = dubai
        formatter.dateFormat = dateFormat
        self.formatter = formatter
    }

    func string(from date: Date) -> String {
        lock.lock()
        defer { lock.unlock() }
        return formatter.string(from: date)
    }
}