import Foundation

/// 日清日历的展示模式。
enum OfflineCollectionCalendarMode: Sendable, Equatable {
    case week
    case month
}

/// 周/月日历日期运算状态，不依赖 UIKit，便于单元测试。
struct OfflineCollectionCalendarState: Sendable, Equatable {
    private let calendar: Calendar
    private(set) var selectedDate: Date
    private(set) var maximumDate: Date
    private(set) var mode: OfflineCollectionCalendarMode

    init(
        selectedDate: Date,
        maximumDate: Date,
        mode: OfflineCollectionCalendarMode = .week,
        calendar: Calendar = OfflineCollectionDate.calendar
    ) {
        self.calendar = calendar
        self.maximumDate = calendar.startOfDay(for: maximumDate)
        self.selectedDate = min(calendar.startOfDay(for: selectedDate), self.maximumDate)
        self.mode = mode
    }

    /// 切换周历和月历并保留选中日期。
    mutating func toggleMode() {
        mode = mode == .week ? .month : .week
    }

    /// 选择不晚于服务端今日的日期。
    mutating func select(_ date: Date) -> Bool {
        let value = calendar.startOfDay(for: date)
        guard value <= maximumDate else { return false }
        selectedDate = value
        return true
    }

    /// 横滑一个周或一个月，并返回新的选中日期。
    @discardableResult
    mutating func movePage(_ offset: Int) -> Date {
        let candidate: Date
        switch mode {
        case .week:
            candidate = calendar.date(byAdding: .day, value: offset * 7, to: selectedDate) ?? selectedDate
        case .month:
            let current = calendar.dateComponents([.year, .month, .day], from: selectedDate)
            let monthStart = calendar.date(from: DateComponents(year: current.year, month: current.month, day: 1)) ?? selectedDate
            let targetMonth = calendar.date(byAdding: .month, value: offset, to: monthStart) ?? monthStart
            let days = calendar.range(of: .day, in: .month, for: targetMonth)?.count ?? 1
            candidate = calendar.date(byAdding: .day, value: min(current.day ?? 1, days) - 1, to: targetMonth) ?? targetMonth
        }
        selectedDate = min(calendar.startOfDay(for: candidate), maximumDate)
        return selectedDate
    }

    /// 当前周的周一至周日。
    var weekDates: [Date] {
        let weekday = calendar.component(.weekday, from: selectedDate)
        let daysFromMonday = (weekday + 5) % 7
        let monday = calendar.date(byAdding: .day, value: -daysFromMonday, to: selectedDate) ?? selectedDate
        return (0 ..< 7).compactMap { calendar.date(byAdding: .day, value: $0, to: monday) }
    }

    /// 当前月份固定六行、周一开周的 42 个日期。
    var monthDates: [Date] {
        let parts = calendar.dateComponents([.year, .month], from: selectedDate)
        let first = calendar.date(from: parts) ?? selectedDate
        let weekday = calendar.component(.weekday, from: first)
        let leading = (weekday + 5) % 7
        let start = calendar.date(byAdding: .day, value: -leading, to: first) ?? first
        return (0 ..< 42).compactMap { calendar.date(byAdding: .day, value: $0, to: start) }
    }

    /// 当前选中月份标题。
    var monthTitle: String {
        let parts = calendar.dateComponents([.year, .month], from: selectedDate)
        return "\(parts.year ?? 0)年\(parts.month ?? 0)月"
    }
}