import AppKit
import ShotdeckCore

@MainActor
public final class RegionPickerController {
    private var windows: [RegionPickerWindow] = []
    private var monitor: Any?
    private var completion: (@MainActor (CaptureRegion?) -> Void)?
    private var dragStart: NSPoint?
    private var startScreen: NSScreen?
    private var cursorPushed = false

    public init() {}

    /// Presents one overlay per attached screen. A pick already in progress is cancelled
    /// (its completion called with nil) before the new one starts — never leaves a caller
    /// waiting forever on a dropped re-entrant call.
    public func pick(completion: @escaping @MainActor (CaptureRegion?) -> Void) {
        if self.completion != nil {
            finish(region: nil)
        }
        self.completion = completion
        presentOverlays()
    }

    private func presentOverlays() {
        let screens = NSScreen.screens
        guard !screens.isEmpty else {
            finish(region: nil)
            return
        }

        NSCursor.crosshair.push()
        cursorPushed = true

        windows = screens.map { RegionPickerWindow(screen: $0) }
        for window in windows {
            window.orderFrontRegardless()
        }
        // Nonactivating panel can become key without activating the app; needed so
        // Escape reaches the local monitor immediately, before any mouse click.
        let mouse = NSEvent.mouseLocation
        let keyWindow = windows.first(where: { $0.coveringScreen.frame.contains(mouse) })
            ?? windows[0]
        keyWindow.makeKey()

        monitor = NSEvent.addLocalMonitorForEvents(
            matching: [.leftMouseDown, .leftMouseDragged, .leftMouseUp, .keyDown]
        ) { [weak self] event in
            guard let self else { return event }
            // NSEvent is not Sendable; lift the two Sendable fields the handler
            // needs. Mouse geometry is read from NSEvent.mouseLocation on the
            // main thread inside the isolated methods, per spec.
            let type = event.type
            let keyCode = event.keyCode
            let consume = MainActor.assumeIsolated {
                self.handle(type: type, keyCode: keyCode)
            }
            return consume ? nil : event
        }
    }

    /// Returns `true` when the event should be swallowed.
    private func handle(type: NSEvent.EventType, keyCode: UInt16) -> Bool {
        switch type {
        case .keyDown:
            if keyCode == 53 { // kVK_Escape
                finish(region: nil)
                return true
            }
            return false
        case .leftMouseDown:
            handleMouseDown()
            return false
        case .leftMouseDragged:
            handleMouseDragged()
            return false
        case .leftMouseUp:
            handleMouseUp()
            return false
        default:
            return false
        }
    }

    private func handleMouseDown() {
        let point = NSEvent.mouseLocation
        dragStart = point
        startScreen = NSScreen.screens.first(where: { $0.frame.contains(point) })
            ?? NSScreen.screens.first
        guard let startScreen else { return }
        let local = Self.localRect(
            from: CGRect(origin: point, size: .zero).intersection(startScreen.frame),
            on: startScreen
        )
        window(for: startScreen)?.updateSelection(localRect: local, sizeText: "0 x 0")
        for window in windows where window.coveringScreen !== startScreen {
            window.updateSelection(localRect: nil, sizeText: nil)
        }
    }

    private func handleMouseDragged() {
        guard dragStart != nil, startScreen != nil else { return }
        applyLiveSelection()
    }

    private func handleMouseUp() {
        guard let startScreen else {
            dragStart = nil
            return
        }
        guard let rect = currentClampedRect(), rect.width >= 8, rect.height >= 8 else {
            // Mis-click: reset drag state, leave every window open and fully dimmed.
            dragStart = nil
            self.startScreen = nil
            for window in windows {
                window.updateSelection(localRect: nil, sizeText: nil)
            }
            return
        }
        let region = CaptureRegion.fromAppKit(rect: rect, on: startScreen)
        finish(region: region)
    }

    private func applyLiveSelection() {
        guard let startScreen, let rect = currentClampedRect() else { return }
        let local = Self.localRect(from: rect, on: startScreen)
        let text = "\(Int(rect.width)) x \(Int(rect.height))"
        window(for: startScreen)?.updateSelection(localRect: local, sizeText: text)
        for window in windows where window.coveringScreen !== startScreen {
            window.updateSelection(localRect: nil, sizeText: nil)
        }
    }

    /// Normalize the drag (so bottom-right → up-left is not misjudged) then clamp
    /// to the screen the gesture started on — never selects across displays.
    private func currentClampedRect() -> CGRect? {
        guard let dragStart, let startScreen else { return nil }
        let current = NSEvent.mouseLocation
        let normalized = CGRect(
            x: min(dragStart.x, current.x),
            y: min(dragStart.y, current.y),
            width: abs(current.x - dragStart.x),
            height: abs(current.y - dragStart.y)
        )
        return normalized.intersection(startScreen.frame)
    }

    private func window(for screen: NSScreen) -> RegionPickerWindow? {
        windows.first { $0.coveringScreen === screen }
    }

    private static func localRect(from rect: CGRect, on screen: NSScreen) -> CGRect {
        CGRect(
            x: rect.minX - screen.frame.minX,
            y: rect.minY - screen.frame.minY,
            width: rect.width,
            height: rect.height
        )
    }

    private func finish(region: CaptureRegion?) {
        if let monitor {
            NSEvent.removeMonitor(monitor)
            self.monitor = nil
        }
        if cursorPushed {
            NSCursor.pop()
            cursorPushed = false
        }
        for window in windows {
            window.orderOut(nil)
        }
        windows.removeAll()
        dragStart = nil
        startScreen = nil
        let done = completion
        completion = nil
        done?(region)
    }
}