import Foundation

/// Emits iCalendar for a click-wheel iPod.
///
/// The three things that made Calendar.app's own exports fail on the device are
/// structurally absent here rather than filtered out afterwards:
///
/// - **No `VTIMEZONE`, no `TZID`.** Times are written as *floating* local wall
///   time. The device has no timezone database, so a `TZID` it cannot resolve
///   can cost you the whole file.
/// - **No `VALARM`.** Alarms are meaningless on a device that cannot fire them.
/// - **No `RRULE`.** Recurring events arrive here already expanded into one
///   `VEVENT` per occurrence, so recurrence support never has to be relied on.
public enum ICalendarWriter {
    public static let productIdentifier = "-//unsupervised.ca//iPod Contacts and Calendar Sync//EN"

    /// Renders events as a complete, CRLF-terminated `VCALENDAR`.
    ///
    /// - Parameters:
    ///   - timeZone: the zone whose wall-clock reading is baked into the
    ///     floating times. Defaults to the Mac's current zone, which is what
    ///     someone syncing their own calendar expects to see on the device.
    ///   - now: the `DTSTAMP` instant, injectable for tests.
    public static func makeCalendar(
        events: [IPodEvent],
        timeZone: TimeZone = .current,
        now: Date = Date()
    ) -> String {
        var calendar = Calendar(identifier: .gregorian)
        calendar.timeZone = timeZone

        let floating = formatter("yyyyMMdd'T'HHmmss", timeZone: timeZone)
        let dateOnly = formatter("yyyyMMdd", timeZone: timeZone)
        let utc = formatter("yyyyMMdd'T'HHmmss'Z'", timeZone: TimeZone(identifier: "UTC")!)

        var builder = ContentLineBuilder()
        builder.appendVerbatim("BEGIN", "VCALENDAR")
        builder.appendVerbatim("VERSION", "2.0")
        builder.appendVerbatim("PRODID", productIdentifier)
        builder.appendVerbatim("CALSCALE", "GREGORIAN")

        let stamp = utc.string(from: now)

        for event in events.sorted(by: { $0.start < $1.start }) {
            builder.appendVerbatim("BEGIN", "VEVENT")
            builder.appendVerbatim("UID", event.uid)
            builder.appendVerbatim("DTSTAMP", stamp)

            if event.isAllDay {
                builder.appendRawLine("DTSTART;VALUE=DATE:\(dateOnly.string(from: event.start))")
                let exclusiveEnd = allDayExclusiveEnd(for: event, calendar: calendar)
                builder.appendRawLine("DTEND;VALUE=DATE:\(dateOnly.string(from: exclusiveEnd))")
            } else {
                builder.appendVerbatim("DTSTART", floating.string(from: event.start))
                // A zero- or negative-length event is legal with DTSTART alone;
                // emitting DTEND <= DTSTART is not.
                if event.end > event.start {
                    builder.appendVerbatim("DTEND", floating.string(from: event.end))
                }
            }

            builder.append("SUMMARY", event.summary.isEmpty ? "(No title)" : event.summary)
            if !event.location.isEmpty {
                builder.append("LOCATION", event.location)
            }
            if !event.notes.isEmpty {
                builder.append("DESCRIPTION", event.notes)
            }

            builder.appendVerbatim("END", "VEVENT")
        }

        builder.appendVerbatim("END", "VCALENDAR")

        return builder.render()
    }

    /// All-day `DTEND` is exclusive — the day *after* the last day covered.
    ///
    /// EventKit is inconsistent about whether an all-day event's `endDate` is
    /// the last day at 23:59:59 or the next day at midnight, so both are
    /// normalised to the same exclusive boundary here.
    static func allDayExclusiveEnd(for event: IPodEvent, calendar: Calendar) -> Date {
        let endDay = calendar.startOfDay(for: event.end)
        let exclusive = endDay == event.end
            ? endDay
            : calendar.date(byAdding: .day, value: 1, to: endDay) ?? endDay

        // Never emit a range that ends before it starts.
        let startDay = calendar.startOfDay(for: event.start)
        if exclusive <= startDay {
            return calendar.date(byAdding: .day, value: 1, to: startDay) ?? exclusive
        }
        return exclusive
    }

    private static func formatter(_ format: String, timeZone: TimeZone) -> DateFormatter {
        let formatter = DateFormatter()
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.calendar = Calendar(identifier: .gregorian)
        formatter.timeZone = timeZone
        formatter.dateFormat = format
        return formatter
    }
}