import Foundation

/// The university's time zone: dates and "now" never follow the phone's zone.
let minsk = TimeZone(identifier: "Europe/Minsk")!

/// A Gregorian calendar in Minsk time, weeks from Monday.
let minskCalendar: Calendar = {
    var c = Calendar(identifier: .gregorian)
    c.timeZone = minsk
    c.firstWeekday = 2
    c.locale = Locale(identifier: "ru_RU")
    return c
}()

/// A calendar date without a time, like java.time.LocalDate: "2026-10-18".
struct CivilDate: Hashable, Comparable, Codable, Identifiable, CustomStringConvertible {
    var year: Int
    var month: Int
    var day: Int

    init(year: Int, month: Int, day: Int) {
        self.year = year
        self.month = month
        self.day = day
    }

    init?(_ iso: String) {
        let parts = iso.prefix(10).split(separator: "-").compactMap { Int($0) }
        guard parts.count == 3 else { return nil }
        self.init(year: parts[0], month: parts[1], day: parts[2])
    }

    init(_ date: Date) {
        let c = minskCalendar.dateComponents([.year, .month, .day], from: date)
        self.init(year: c.year!, month: c.month!, day: c.day!)
    }

    static func today(_ now: Date = Date()) -> CivilDate { CivilDate(now) }

    /// Noon in Minsk, so adding days never trips over a daylight saving change.
    var date: Date {
        minskCalendar.date(from: DateComponents(year: year, month: month, day: day, hour: 12))!
    }

    var iso: String { String(format: "%04d-%02d-%02d", year, month, day) }
    var description: String { iso }
    var id: String { iso }

    func adding(days: Int) -> CivilDate {
        CivilDate(minskCalendar.date(byAdding: .day, value: days, to: date)!)
    }

    func adding(months: Int) -> CivilDate {
        CivilDate(minskCalendar.date(byAdding: .month, value: months, to: date)!)
    }

    /// 1 = Monday … 7 = Sunday, as in java.time.
    var weekday: Int {
        let w = minskCalendar.component(.weekday, from: date)  // 1 = Sunday
        return w == 1 ? 7 : w - 1
    }

    var monday: CivilDate { adding(days: 1 - weekday) }
    var firstOfMonth: CivilDate { CivilDate(year: year, month: month, day: 1) }

    func days(to other: CivilDate) -> Int {
        minskCalendar.dateComponents([.day], from: date, to: other.date).day ?? 0
    }

    func at(_ time: ClockTime) -> Date {
        minskCalendar.date(from: DateComponents(year: year, month: month, day: day, hour: time.hour, minute: time.minute))!
    }

    static func < (a: CivilDate, b: CivilDate) -> Bool {
        (a.year, a.month, a.day) < (b.year, b.month, b.day)
    }

    // Codable as the ISO string, the way the server and the Android cache write dates.
    init(from decoder: Decoder) throws {
        let s = try decoder.singleValueContainer().decode(String.self)
        guard let d = CivilDate(s) else {
            throw DecodingError.dataCorrupted(.init(codingPath: decoder.codingPath, debugDescription: "Не дата: \(s)"))
        }
        self = d
    }

    func encode(to encoder: Encoder) throws {
        var c = encoder.singleValueContainer()
        try c.encode(iso)
    }
}

/// A time of day in minutes since midnight: "08:30".
struct ClockTime: Hashable, Comparable, CustomStringConvertible {
    var minutes: Int

    init(minutes: Int) { self.minutes = minutes }

    init(hour: Int, minute: Int) { minutes = hour * 60 + minute }

    init?(_ hhmm: String) {
        let parts = hhmm.split(separator: ":").compactMap { Int($0) }
        guard parts.count >= 2 else { return nil }
        minutes = parts[0] * 60 + parts[1]
    }

    static func now(_ date: Date = Date()) -> ClockTime {
        let c = minskCalendar.dateComponents([.hour, .minute], from: date)
        return ClockTime(hour: c.hour!, minute: c.minute!)
    }

    var hour: Int { minutes / 60 }
    var minute: Int { minutes % 60 }
    var description: String { String(format: "%02d:%02d", hour, minute) }

    func adding(minutes m: Int) -> ClockTime { ClockTime(minutes: minutes + m) }

    static func < (a: ClockTime, b: ClockTime) -> Bool { a.minutes < b.minutes }
}

// MARK: - Russian labels

private let monthsGenitive = ["января", "февраля", "марта", "апреля", "мая", "июня", "июля", "августа", "сентября", "октября", "ноября", "декабря"]
private let monthsNominative = ["Январь", "Февраль", "Март", "Апрель", "Май", "Июнь", "Июль", "Август", "Сентябрь", "Октябрь", "Ноябрь", "Декабрь"]
private let monthsShort = ["Янв.", "Февр.", "Март", "Апр.", "Май", "Июнь", "Июль", "Авг.", "Сент.", "Окт.", "Нояб.", "Дек."]
private let weekdaysFull = ["Понедельник", "Вторник", "Среда", "Четверг", "Пятница", "Суббота", "Воскресенье"]
private let weekdaysShort = ["Пн", "Вт", "Ср", "Чт", "Пт", "Сб", "Вс"]

extension CivilDate {
    /// "18 октября"
    var longLabel: String { "\(day) \(monthsGenitive[month - 1])" }
    /// "Пн"
    var shortWeekday: String { weekdaysShort[weekday - 1] }
    /// "Понедельник"
    var weekdayName: String { weekdaysFull[weekday - 1] }
    /// "Октябрь 2026"
    var monthTitle: String { "\(monthsNominative[month - 1]) \(year)" }
    /// "Окт."
    var shortMonth: String { monthsShort[month - 1] }
    /// "18.10"
    var numeric: String { String(format: "%02d.%02d", day, month) }
}

/// "Понедельник" for 1…7.
func dayName(_ day: Int) -> String { weekdaysFull[max(0, min(6, day - 1))] }

/// "12–18 октября" or "28 сентября – 4 октября".
func weekSpan(_ start: CivilDate, _ end: CivilDate) -> String {
    start.month == end.month ? "\(start.day)–\(end.longLabel)" : "\(start.longLabel) – \(end.longLabel)"
}

/// Russian plural: 1 пара, 2 пары, 5 пар.
func plural(_ n: Int, _ one: String, _ few: String, _ many: String) -> String {
    let tail = n % 100
    if (11...14).contains(tail) { return many }
    switch n % 10 {
    case 1: return one
    case 2...4: return few
    default: return many
    }
}

/// "1 пара", "3 пары".
func pairs(_ n: Int) -> String { "\(n) \(plural(n, "пара", "пары", "пар"))" }

/// "45 мин", "1 ч 20 мин".
func minutesLabel(_ m: Int) -> String {
    if m < 60 { return "\(m) мин" }
    return "\(m / 60) ч" + (m % 60 > 0 ? " \(m % 60) мин" : "")
}

/// "Окт. · Неделя 7": the title of a shown week.
func weekTitle(_ date: CivilDate, _ n: Int?) -> String {
    n.map { "\(date.shortMonth) · Неделя \($0)" } ?? date.shortMonth
}