import SwiftUI

struct MonthCalendarView: View {
    let habit: Habit
    let onToggle: (Date) -> Void

    @State private var monthStart = Calendar.german.startOfMonth(for: .now)

    private let weekdaySymbols = ["Mo", "Di", "Mi", "Do", "Fr", "Sa", "So"]
    private let columns = Array(repeating: GridItem(.flexible()), count: 7)

    private var calendar: Calendar { .german }
    private var today: Date { calendar.startOfDay(for: .now) }

    var body: some View {
        VStack(spacing: 12) {
            header

            LazyVGrid(columns: columns, spacing: 8) {
                ForEach(weekdaySymbols, id: \.self) { symbol in
                    Text(symbol)
                        .font(.caption2)
                        .foregroundStyle(.secondary)
                }

                ForEach(Array(dayCells.enumerated()), id: \.offset) { _, date in
                    if let date {
                        dayCell(for: date)
                    } else {
                        Color.clear.frame(height: 36)
                    }
                }
            }
        }
    }

    private var header: some View {
        HStack {
            Button {
                shiftMonth(by: -1)
            } label: {
                Image(systemName: "chevron.left")
            }

            Spacer()

            Text(monthTitle)
                .font(.headline)

            Spacer()

            Button {
                shiftMonth(by: 1)
            } label: {
                Image(systemName: "chevron.right")
            }
            .disabled(isCurrentMonthOrLater)
        }
        .buttonStyle(.borderless)
    }

    private func dayCell(for date: Date) -> some View {
        let isDone = habit.isCompleted(on: date, calendar: calendar)
        let isToday = calendar.isDate(date, inSameDayAs: today)
        let isFuture = date > today
        let color = Color(hex: habit.colorHex)

        return Text("\(calendar.component(.day, from: date))")
            .font(.callout)
            .frame(width: 36, height: 36)
            .background {
                if isDone {
                    Circle().fill(color)
                } else if isToday {
                    Circle().strokeBorder(color, lineWidth: 1.5)
                }
            }
            .foregroundStyle(isDone ? .white : (isFuture ? Color.secondary.opacity(0.4) : .primary))
            .contentShape(Circle())
            .onTapGesture {
                if !isFuture { onToggle(date) }
            }
    }

    private var monthTitle: String {
        let formatter = DateFormatter()
        formatter.calendar = calendar
        formatter.locale = Locale(identifier: "de_DE")
        formatter.dateFormat = "LLLL yyyy"
        return formatter.string(from: monthStart)
    }

    /// Leading nil padding for the weekday offset, then one date per day in the month.
    private var dayCells: [Date?] {
        guard let range = calendar.range(of: .day, in: .month, for: monthStart) else { return [] }
        let weekday = calendar.component(.weekday, from: monthStart)
        let leadingBlanks = (weekday - calendar.firstWeekday + 7) % 7

        var cells: [Date?] = Array(repeating: nil, count: leadingBlanks)
        for day in range {
            cells.append(calendar.date(byAdding: .day, value: day - 1, to: monthStart))
        }
        return cells
    }

    private var isCurrentMonthOrLater: Bool {
        monthStart >= calendar.startOfMonth(for: .now)
    }

    private func shiftMonth(by value: Int) {
        if let next = calendar.date(byAdding: .month, value: value, to: monthStart) {
            monthStart = next
        }
    }
}

extension Calendar {
    /// Gregorian calendar with Monday as the first weekday (German convention).
    static var german: Calendar {
        var calendar = Calendar(identifier: .gregorian)
        calendar.firstWeekday = 2
        return calendar
    }

    func startOfMonth(for date: Date) -> Date {
        let components = dateComponents([.year, .month], from: date)
        return self.date(from: components) ?? startOfDay(for: date)
    }
}