// EventKit has no Linux equivalent. See the note in ContactsReader.swift.
#if canImport(EventKit)

import EventKit
import Foundation

/// A calendar the user can choose to export.
public struct CalendarSource: Sendable, Identifiable, Equatable, Hashable {
    public var id: String
    public var title: String
    public var accountName: String

    public init(id: String, title: String, accountName: String) {
        self.id = id
        self.title = title
        self.accountName = accountName
    }
}

/// Reads calendar events into `IPodEvent` values.
///
/// Events come back from EventKit already expanded — a weekly standup returns
/// one event per week, not one event plus a rule — which is why nothing
/// downstream ever has to emit an `RRULE` the device may not understand.
public actor CalendarReader {
    private let store = EKEventStore()

    public init() {}

    public nonisolated static var authorizationStatus: EKAuthorizationStatus {
        EKEventStore.authorizationStatus(for: .event)
    }

    /// Prompts for full calendar access, returning whether it was granted.
    ///
    /// Full rather than write-only access: the app's entire job is reading
    /// events out.
    public func requestAccess() async -> Bool {
        (try? await store.requestFullAccessToEvents()) ?? false
    }

    public func calendars() -> [CalendarSource] {
        store.calendars(for: .event)
            .map {
                CalendarSource(
                    id: $0.calendarIdentifier,
                    title: $0.title,
                    accountName: $0.source?.title ?? ""
                )
            }
            .sorted { ($0.accountName, $0.title) < ($1.accountName, $1.title) }
    }

    /// Fetches occurrences in `[start, end)` from the named calendars.
    public func events(
        calendarIdentifiers: [String],
        from start: Date,
        to end: Date
    ) -> [IPodEvent] {
        let selected = store.calendars(for: .event)
            .filter { calendarIdentifiers.contains($0.calendarIdentifier) }
        guard !selected.isEmpty, start < end else { return [] }

        var collected: [String: IPodEvent] = [:]

        // EventKit rejects a predicate spanning more than four years, so long
        // ranges are fetched in windows. Events straddling a window boundary
        // come back twice, hence keying by UID.
        for (windowStart, windowEnd) in Self.windows(from: start, to: end) {
            let predicate = store.predicateForEvents(
                withStart: windowStart,
                end: windowEnd,
                calendars: selected
            )
            for event in store.events(matching: predicate) {
                let converted = Self.convert(event)
                collected[converted.uid] = converted
            }
        }

        return collected.values.sorted { $0.start < $1.start }
    }

    /// Splits a range into windows comfortably inside EventKit's four-year limit.
    static func windows(from start: Date, to end: Date) -> [(Date, Date)] {
        let calendar = Calendar(identifier: .gregorian)
        var windows: [(Date, Date)] = []
        var cursor = start

        while cursor < end {
            let next = calendar.date(byAdding: .year, value: 2, to: cursor) ?? end
            windows.append((cursor, min(next, end)))
            cursor = next
        }

        return windows
    }

    static func convert(_ event: EKEvent) -> IPodEvent {
        IPodEvent(
            id: event.eventIdentifier ?? UUID().uuidString,
            uid: uid(for: event),
            summary: event.title ?? "",
            location: event.location ?? "",
            notes: event.notes ?? "",
            start: event.startDate,
            end: event.endDate,
            isAllDay: event.isAllDay,
            calendarTitle: event.calendar?.title ?? ""
        )
    }

    /// A UID unique per *occurrence*.
    ///
    /// Every occurrence of a recurring event shares one external identifier, so
    /// the start instant is folded in — otherwise a weekly meeting would emit
    /// fifty-two events all claiming the same UID, and importers are entitled to
    /// treat those as one event.
    static func uid(for event: EKEvent) -> String {
        let base = event.calendarItemExternalIdentifier
            ?? event.eventIdentifier
            ?? UUID().uuidString

        let safe = base.unicodeScalars
            .map { CharacterSet.alphanumerics.contains($0) || $0 == "-" ? Character($0) : "-" }
        let stamp = Int(event.startDate.timeIntervalSince1970)

        return "\(String(safe).prefix(64))-\(stamp)@ipod-sync"
    }
}

#endif