using CronExpressionDescriptor;

using Cronos;

using System;

using System.Linq; 

using System.Globalization;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var cronExpressions = new []

        {

            // MINUTE HOUR DAY_OF_MONTH MONTH DAY_OF_WEEK

            BuildCronExpression(), // Dynamic CRON Expression for the day

            "* 19 * JAN-DEC THU", // Assume service is down from 7PM to 8PM every thursday

            "* 0-6 * JAN-DEC MON", // Assume service is down every monday from 12AM to 6AM

            "*/2 8-16 18-19 JUN *", // Assume service is down this weekend only from 8AM to 5PM

            "* 0-19 7-9 OCT FRI,SAT,SUN", // Assume service is down only from friday to sunday from on October 7th to 9th from 12 AM to 7 PM every minute

            "* 4-7 2 DEC FRI", // Assume service is down only on friday only on December 2nd from 4am to 8am every minute

            "* 22-23 18 DEC SUN" // Assume service is down only on sunday only on December 18th from 10pm to 12am every minute

        };

        foreach (var cronExpression in cronExpressions)

        {

            Explain(cronExpression);

        }

        CronExpression expression = CronExpression.Parse("* * * * *");

        DateTimeOffset? next = expression.GetNextOccurrence(DateTimeOffset.Now, TimeZoneInfo.Local);

        var nextLocalTime = next?.DateTime;

        //Console.WriteLine(nextLocalTime);

        //https://github.com/HangfireIO/Cronos

        //occurances

        DateTime? newInput = DateTime.UtcNow;

        //Console.WriteLine(newInput);

        for(var i = 0; i < 5; i++) {

            CronExpression newExpression = CronExpression.Parse("1/5 * * * *");

            newInput = newExpression.GetNextOccurrence((DateTime)newInput);

            //Console.WriteLine(newInput);

        }

        //occurances

        DateTime? date = DateTimeOffset.UtcNow.ToUniversalTime().UtcDateTime;

        var expressions = (new string[] {"0 2 * * SAT", "0 2 1 * *"}).Select(e => CronExpression.Parse(e));

        do

        {

            date = GetNextOccurrence(expressions, date.Value);

            Console.WriteLine(date);

        } while(date <= new DateTime(2024,1,1));

    }

    private static void NextOccurrences(){

        DateTime date = DateTime.UtcNow;  

        DateTime end = DateTime.Now.AddDays(10);

​

        CronExpression expression = CronExpression.Parse("0 8 * * *");

​

        List<DateTime> dates = new List<DateTime>();

        while (date < end)

        {

            date = (DateTime)expression.GetNextOccurrence(date);

            dates.Add(date);

        }

​

        foreach(var a in dates){

            Console.WriteLine(a);

        }

   }

    private static void Explain(string cron)

    {

        var cronExpression = CronExpression.Parse(cron);

        var utcNow = DateTime.UtcNow; // DateTime.SpecifyKind(new DateTime(2022, 06, 16, 19, 57, 50), DateTimeKind.Utc);

        var nextOccurrence = cronExpression.GetNextOccurrence(utcNow, TimeZoneInfo.Utc);

        var expressionDescriptor = ExpressionDescriptor.GetDescription(cron);

        Console.WriteLine($"cronExpression: {cronExpression}");

        Console.WriteLine($"utcNow: {utcNow}");

        Console.WriteLine($"nextOccurrence: {nextOccurrence}");

        Console.WriteLine($"description: {expressionDescriptor}");

        IsDown(utcNow, nextOccurrence);

        Console.WriteLine(new string('-', 40));

    }

    private static void IsDown(DateTime utcNow, DateTime? nextOccurrence)

    {

        if (nextOccurrence.HasValue)

        {

            var secondsToNextRun = nextOccurrence.Value.Subtract(utcNow).TotalSeconds;

            Console.WriteLine($"Seconds to next run: {secondsToNextRun}");

            if (secondsToNextRun >= 0D && secondsToNextRun <= 60D)

            {

                Console.WriteLine($"IS DOWN");

            }

            else

            {

                Console.WriteLine($"IS UP");

            }

        }

    }

    public static DateTime? GetNextOccurrence(IEnumerable<CronExpression> expressions, DateTime start)

    {

        DateTime? nextOccurrence = DateTime.MaxValue;

​

        foreach (var expression in expressions)

        {

            var occurrence = expression.GetNextOccurrence(start);

            if (occurrence < nextOccurrence)

            {

                nextOccurrence = occurrence;

            }

        }

​

        return nextOccurrence;

    }

    private static string BuildCronExpression()

    {

        var utcNow = DateTime.UtcNow;

        const string everyMinute = "*";

        var fromHour = utcNow.Hour;

        var toHour = utcNow.AddHours(4).Hour;

        var fromDayOfMonth = utcNow.Day;

        var toDayOfMonth = utcNow.AddHours(4).Day;

        var fromMonth = utcNow.ToString("MMM").ToUpper(CultureInfo.InvariantCulture);

        var toMonth = utcNow.AddHours(4).ToString("MMM").ToUpper(CultureInfo.InvariantCulture);

        var fromDayOfWeek = utcNow.ToString("ddd").ToUpper(CultureInfo.InvariantCulture);

        var toDayOfWeek = utcNow.AddHours(4).ToString("ddd").ToUpper(CultureInfo.InvariantCulture);

        return $"{everyMinute} {fromHour}-{toHour} {fromDayOfMonth}-{toDayOfMonth} {fromMonth}-{toMonth} {fromDayOfWeek}-{toDayOfWeek}";

    }

}