import AppKit

/// Which screens to keep, remove, or add when reconciling live bars against the
/// current `NSScreen.screens`.
///
/// Pure and identity-based (it never reads `NSScreen` properties), so it can be
/// unit-tested with any `[NSScreen]` without driving real window lifecycle.
enum PHScreenSync {
	struct Diff: Equatable {
		/// Screens that already have a bar and are still attached. Their window
		/// frame is recomputed since geometry may have changed.
		let kept: [NSScreen]
		/// Screens whose bar must be torn down (monitor disconnected).
		let removed: [NSScreen]
		/// Screens with no bar yet that need one (monitor connected).
		let added: [NSScreen]
	}

	/// Compare the screens now attached (`current`) against the screens currently
	/// occupied by bars (`occupied`). A disconnected monitor reappears as a new
	/// `NSScreen` instance, so it is reported as remove + add, never a keep.
	static func diff(current: [NSScreen], occupied: [NSScreen]) -> Diff {
		let currentIDs = Set(current.map(ObjectIdentifier.init))
		let occupiedIDs = Set(occupied.map(ObjectIdentifier.init))
		return Diff(
			kept: occupied.filter { currentIDs.contains(ObjectIdentifier($0)) },
			removed: occupied.filter { !currentIDs.contains(ObjectIdentifier($0)) },
			added: current.filter { !occupiedIDs.contains(ObjectIdentifier($0)) }
		)
	}
}