import EventKit
import Foundation

/// The production adapter is intentionally read-only and non-interactive.
/// Permission prompts belong to an explicit Settings flow, never tool
/// execution, so an agent run cannot surprise the user with a system prompt.
actor EventKitCalendarReader: SystemCalendarReading {
  private let eventStore: EKEventStore

  init(eventStore: EKEventStore = EKEventStore()) {
    self.eventStore = eventStore
  }

  func authorizationStatus() -> SystemCalendarAuthorization {
    Self.authorizationStatus(
      from: EKEventStore.authorizationStatus(for: .event)
    )
  }

  /// Called only from the explicit Settings button. Tool execution uses
  /// `readEvents` and therefore can never trigger an iOS permission prompt.
  func requestFullAccess() async throws -> SystemCalendarAuthorization {
    _ = try await eventStore.requestFullAccessToEvents()
    return authorizationStatus()
  }

  func readEvents(
    in interval: DateInterval,
    limit: Int
  ) throws -> SystemCalendarEventPage {
    let authorization = authorizationStatus()
    guard authorization == .fullAccess else {
      throw SystemCalendarReaderError.fullAccessRequired(authorization)
    }

    let safeLimit = min(max(limit, 1), 11)
    let predicate = eventStore.predicateForEvents(
      withStart: interval.start,
      end: interval.end,
      calendars: nil
    )
    let matchingEvents = eventStore.events(matching: predicate)
      .filter { event in
        event.startDate < interval.end && event.endDate > interval.start
      }
      .sorted { first, second in
        if first.startDate != second.startDate {
          return first.startDate < second.startDate
        }
        if first.endDate != second.endDate {
          return first.endDate < second.endDate
        }
        return (first.title ?? "").localizedStandardCompare(
          second.title ?? ""
        ) == .orderedAscending
      }
    let visibleEvents = matchingEvents.prefix(safeLimit).map { event in
      SystemCalendarEvent(
        title: event.title ?? "Untitled event",
        startsAt: event.startDate,
        endsAt: max(event.startDate, event.endDate),
        isAllDay: event.isAllDay
      )
    }
    return SystemCalendarEventPage(
      events: Array(visibleEvents),
      hasMore: matchingEvents.count > safeLimit
    )
  }

  private static func authorizationStatus(
    from status: EKAuthorizationStatus
  ) -> SystemCalendarAuthorization {
    switch status {
    case .fullAccess:
      .fullAccess
    case .notDetermined:
      .notDetermined
    case .denied:
      .denied
    case .restricted:
      .restricted
    case .writeOnly:
      .writeOnly
    @unknown default:
      .restricted
    }
  }
}