import AVFoundation
import SwiftUI

/// Camera QR scanner, presented as a sheet. On a simulator (no capture device)
/// it explains that instead of showing a dead viewfinder.
struct QRScannerView: UIViewControllerRepresentable {
  var onScan: (String) -> Void
  var onCancel: () -> Void

  func makeUIViewController(context: Context) -> ScannerViewController {
    let controller = ScannerViewController()
    controller.onScan = onScan
    controller.onCancel = onCancel
    return controller
  }

  func updateUIViewController(_ controller: ScannerViewController, context: Context) {}
}

final class ScannerViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
  var onScan: ((String) -> Void)?
  var onCancel: (() -> Void)?

  private let session = AVCaptureSession()
  private var previewLayer: AVCaptureVideoPreviewLayer?
  private var hasScanned = false
  private var messageLabel: UILabel?

  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .black

    let cancel = UIButton(type: .system)
    cancel.setTitle("Cancel", for: .normal)
    cancel.setTitleColor(.white, for: .normal)
    cancel.titleLabel?.font = .systemFont(ofSize: 16, weight: .semibold)
    cancel.addTarget(self, action: #selector(cancelTapped), for: .touchUpInside)
    cancel.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(cancel)
    NSLayoutConstraint.activate([
      cancel.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 12),
      cancel.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
    ])

    let label = UILabel()
    label.numberOfLines = 0
    label.textAlignment = .center
    label.textColor = .white
    label.font = .systemFont(ofSize: 14)
    label.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(label)
    NSLayoutConstraint.activate([
      label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
      label.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 32),
      label.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -32),
    ])
    messageLabel = label

    requestCamera()
  }

  override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    previewLayer?.frame = view.bounds
  }

  override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    if session.isRunning { session.stopRunning() }
  }

  @objc private func cancelTapped() {
    onCancel?()
  }

  private func requestCamera() {
    guard let device = AVCaptureDevice.default(for: .video) else {
      showMessage("This device has no camera. Use “Photo” to pick a QR from the library instead.")
      return
    }

    switch AVCaptureDevice.authorizationStatus(for: .video) {
    case .authorized:
      configure(device: device)
    case .notDetermined:
      AVCaptureDevice.requestAccess(for: .video) { [weak self] granted in
        DispatchQueue.main.async {
          if granted {
            self?.configure(device: device)
          } else {
            self?.showMessage("Camera access is off. Enable it in Settings, or use “Photo”.")
          }
        }
      }
    default:
      showMessage("Camera access is off. Enable it in Settings, or use “Photo”.")
    }
  }

  private func configure(device: AVCaptureDevice) {
    guard let input = try? AVCaptureDeviceInput(device: device), session.canAddInput(input) else {
      showMessage("The camera could not be started. Use “Photo” instead.")
      return
    }
    session.addInput(input)

    let output = AVCaptureMetadataOutput()
    guard session.canAddOutput(output) else {
      showMessage("The camera could not be started. Use “Photo” instead.")
      return
    }
    session.addOutput(output)
    output.setMetadataObjectsDelegate(self, queue: .main)
    output.metadataObjectTypes = [.qr]

    let layer = AVCaptureVideoPreviewLayer(session: session)
    layer.videoGravity = .resizeAspectFill
    layer.frame = view.bounds
    view.layer.insertSublayer(layer, at: 0)
    previewLayer = layer

    DispatchQueue.global(qos: .userInitiated).async { [weak self] in
      self?.session.startRunning()
    }
  }

  private func showMessage(_ text: String) {
    messageLabel?.text = text
  }

  func metadataOutput(_ output: AVCaptureMetadataOutput,
                      didOutput metadataObjects: [AVMetadataObject],
                      from connection: AVCaptureConnection) {
    guard !hasScanned else { return }
    guard let object = metadataObjects.first as? AVMetadataMachineReadableCodeObject,
          let value = object.stringValue, !value.isEmpty else { return }
    hasScanned = true
    session.stopRunning()
    onScan?(value)
  }
}