LeCountdown/LeCountdown/Views/Reusable/CalendarView.swift

//
//  CalendarView.swift
//  LeCountdown
//
//  Created by Laurent Morvillier on 12/04/2023.
//
import SwiftUI

fileprivate extension DateFormatter {
    static var month: DateFormatter {
        let formatter = DateFormatter()
        formatter.dateFormat = "MMMM"
        return formatter
    }

    static var monthAndYear: DateFormatter {
        let formatter = DateFormatter()
        formatter.dateFormat = "MMMM yyyy"
        return formatter
    }
}

fileprivate extension Calendar {
    func generateDates(
        inside interval: DateInterval,
        matching components: DateComponents
    ) -> [Date] {
        var dates: [Date] = []
        dates.append(interval.start)

        enumerateDates(
            startingAfter: interval.start,
            matching: components,
            matchingPolicy: .nextTime
        ) { date, _, stop in
            if let date = date {
                if date < interval.end {
                    dates.append(date)
                } else {
                    stop = true
                }
            }
        }
        return dates
    }
}

struct CalendarView<DateView>: View where DateView: View {
    
    let calendar: Calendar = Calendar.current

    let interval: DateInterval
    let showHeaders: Bool
    
    let content: (Date) -> DateView

    init(
        interval: DateInterval,
        showHeaders: Bool = true,
        @ViewBuilder content: @escaping (Date) -> DateView
    ) {
        self.interval = interval
        self.showHeaders = showHeaders
        self.content = content
        
        self.months = calendar.generateDates(
            inside: interval,
            matching: DateComponents(day: 1, hour: 0, minute: 0, second: 0)
        )
        
        for month in self.months {
            if showHeaders {
                self.monthHeaders += [month.formattedMonth]
            }
            self.monthToDays[month] = self.days(for: month)
        }
    }

    var body: some View {
        LazyVGrid(columns: Array(repeating: GridItem(), count: 7)) {
            ForEach(Array(months.enumerated().reversed()), id: \.offset) { (index,month) in
                Section(header: self.headerView(for: monthHeaders[index])) {
                    if let days = self.monthToDays[month] {
                        ForEach(days, id: \.self) { date in
                            if calendar.isDate(date, equalTo: month, toGranularity: .month) {
                                content(date).id(date)
                            } else {
                                content(date).hidden()
                            }
                        }
                    }
                }
            }
        }
    }

    private var months = [Date]()
    private var monthHeaders = [String]()
    private var monthToDays = [Date: [Date]]()
    
    // Important: dont add padding or any frame modifications to the headerView or you will get performance issues
    private func headerView(for month:String) -> some View {
        Text(month).font(.title)
    }

    private func header(for month: Date) -> String {
        let component = calendar.component(.month, from: month)
        let formatter = component == 1 ? DateFormatter.monthAndYear : .month
        return formatter.string(from: month)
    }

    private func days(for month: Date) -> [Date] {
        guard
            let monthInterval = calendar.dateInterval(of: .month, for: month),
            let monthFirstWeek = calendar.dateInterval(of: .weekOfMonth, for: monthInterval.start),
            let monthLastWeek = calendar.dateInterval(of: .weekOfMonth, for: monthInterval.end)
        else { return [] }
        return calendar.generateDates(
            inside: DateInterval(start: monthFirstWeek.start, end: monthLastWeek.end),
            matching: DateComponents(hour: 0, minute: 0, second: 0)
        )
    }
}

struct CalendarView_Previews: PreviewProvider {
    static var previews: some View {
        CalendarView(interval: DateInterval(start: Date(timeIntervalSinceNow: -2 * 31 * 24 * 3600), end: Date())) { date in
            Text(date.get(.day).formatted())
                .padding(8)
        }
    }
}