Fast implementation for ISO8601 date formatter. Around 30 times faster than using (cached) DateFormatter

ISO8601DateFormatter.swift

class ISO8601DateFormatter {

	private static var cachedCalendar: Calendar = {
		var calendar = Calendar(identifier: .gregorian)
		calendar.timeZone = TimeZone.gmt
		return calendar
	}()
	
	static func format(_ date: Date) -> String {
		let components = cachedCalendar.dateComponents([.year, .month, .day, .hour, .minute, .second], from: date)
	
		return withVaList([components.year ?? 1970,
						   components.month ?? 1,
						   components.day ?? 1,
						   components.hour ?? 0,
						   components.minute ?? 0,
						   components.second ?? 0], { pointer in
			let chars = UnsafeMutablePointer<CChar>.allocate(capacity: 21)
			vsnprintf(chars, 21, "%d-%02d-%02dT%02d:%02d:%02dZ", pointer)
			return String(cString: chars)
		})
	}
	
}