import Foundation

extension Date {

    // MARK: Methods

    /// How long until the date, in a few words: `47 h left`, `2 days left`, `under an hour left`.
    /// - Parameter now: the moment counted from.
    func countdown(
        from now: Date
    ) -> String {
        let hours = Int(timeIntervalSince(now) / 3_600)

        switch hours {
        case ..<1: return "under an hour left"
        case ..<48: return "\(hours) h left"
        default: return "\(hours / 24) days left"
        }
    }

    /// The days an event runs over, e.g. `Sat 26 – Sun 27 Sep`.
    static func span(
        from start: Date,
        to end: Date
    ) -> String {
        let first = start.formatted(style.weekday(.abbreviated).day())
        let last = end.formatted(style.weekday(.abbreviated).day().month(.abbreviated))

        return calendar.isDate(start, inSameDayAs: end) ? last : "\(first) – \(last)"
    }

    /// Whether the date falls within the week from the given moment: an event "this weekend".
    func isWithinAWeek(
        of now: Date
    ) -> Bool {
        (0..<7 * 86_400).contains(timeIntervalSince(now)) || Self.calendar.isDate(self, inSameDayAs: now)
    }

    /// The date as a day on a record, e.g. `20 Sep`.
    var dayStamp: String {
        formatted(Self.style.day().month(.abbreviated))
    }

    /// The date as a stamp on a record: `today, 09:14`, `yesterday, 21:38`, `20 Sep, 14:05`, or with the year once it differs.
    ///
    /// Read on Amsterdam's clock: the marketplace is NL-first.
    /// - Parameter now: the moment "today" is judged from.
    func stamp(
        relativeTo now: Date
    ) -> String {
        let time = formatted(Self.style.hour(.twoDigits(amPM: .omitted)).minute(.twoDigits))

        if Self.calendar.isDate(self, inSameDayAs: now) {
            return "today, \(time)"
        }

        if
            let yesterday = Self.calendar.date(byAdding: .day, value: -1, to: now),
            Self.calendar.isDate(self, inSameDayAs: yesterday)
        {
            return "yesterday, \(time)"
        }

        let isSameYear = Self.calendar.isDate(self, equalTo: now, toGranularity: .year)
        let day = formatted(isSameYear ? Self.style.day().month(.abbreviated) : Self.style.day().month(.abbreviated).year())

        return "\(day), \(time)"
    }

}

// MARK: - Helpers

private extension Date {

    // MARK: Constants

    /// The calendar days are told apart by, on Amsterdam's clock.
    static let calendar: Calendar = {
        var calendar = Calendar(identifier: .gregorian)

        calendar.timeZone = timeZone

        return calendar
    }()

    /// The base format style: British English, on Amsterdam's clock.
    static let style = Date.FormatStyle(
        locale: .init(identifier: "en_GB"),
        calendar: calendar,
        timeZone: timeZone
    )

    static let timeZone = TimeZone(identifier: "Europe/Amsterdam") ?? .gmt

}