import SwiftUI

/// The contribution calendar: 53 weeks of columns, Sunday at the top,
/// ending on the current week — the same grid the web draws.
struct ActivityGraph: View {

    let days: [ProfileViewModel.Profile.Day]

    private static let calendar: Calendar = {
        var calendar = Calendar(identifier: .gregorian)
        calendar.timeZone = TimeZone(identifier: "UTC")!
        return calendar
    }()

    private static let formatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.calendar = calendar
        formatter.timeZone = calendar.timeZone
        formatter.dateFormat = "yyyy-MM-dd"
        return formatter
    }()

    /// Weeks of 7 days; nil is a day outside the window (before the start
    /// or after today), drawn as empty space rather than a zero cell.
    private var weeks: [[Cell?]] {
        let counts = Dictionary(days.map { ($0.date, $0.count) }, uniquingKeysWith: +)
        let today = Self.calendar.startOfDay(for: Date())
        // End on the Saturday of the current week, matching the server's
        // window so both surfaces show the same year.
        let weekday = Self.calendar.component(.weekday, from: today) // 1 = Sunday
        guard let end = Self.calendar.date(byAdding: .day, value: 7 - weekday, to: today),
              let start = Self.calendar.date(byAdding: .day, value: -53 * 7 + 1, to: end) else {
            return []
        }

        var result: [[Cell?]] = []
        var cursor = start
        while cursor <= end {
            var week: [Cell?] = []
            for _ in 0..<7 {
                if cursor > today {
                    week.append(nil)
                } else {
                    let key = Self.formatter.string(from: cursor)
                    week.append(Cell(date: key, count: counts[key] ?? 0))
                }
                cursor = Self.calendar.date(byAdding: .day, value: 1, to: cursor) ?? cursor
            }
            result.append(week)
        }
        return result
    }

    private struct Cell: Hashable {
        let date: String
        let count: Int

        /// The same five buckets activityLevel uses server-side.
        var level: Int {
            switch count {
            case 0: 0
            case 1...2: 1
            case 3...5: 2
            case 6...9: 3
            default: 4
            }
        }
    }

    var body: some View {
        ScrollView(.horizontal, showsIndicators: false) {
            HStack(alignment: .top, spacing: 3) {
                ForEach(Array(weeks.enumerated()), id: \.offset) { _, week in
                    VStack(spacing: 3) {
                        ForEach(Array(week.enumerated()), id: \.offset) { _, cell in
                            RoundedRectangle(cornerRadius: 2)
                                .fill(color(for: cell))
                                .frame(width: 11, height: 11)
                        }
                    }
                }
            }
            .padding(.vertical, 4)
        }
        .defaultScrollAnchor(.trailing)
    }

    /// The stylesheet's ramp: an empty day is --faint, and the rest mix
    /// --accent toward --bg at 25/50/75/100 percent. A day outside the
    /// window draws nothing at all.
    private func color(for cell: Cell?) -> Color {
        guard let cell else { return .clear }
        return switch cell.level {
        case 0: Color.gbFaint
        case 1: Color.gbAccent.mix(with: .gbBackground, by: 0.75)
        case 2: Color.gbAccent.mix(with: .gbBackground, by: 0.50)
        case 3: Color.gbAccent.mix(with: .gbBackground, by: 0.25)
        default: Color.gbAccent
        }
    }
}