GazpromApp/Utils/DateCalculator.cs

using System;
using System.Linq;
using GazpromApp.Enums;
using GazpromApp.Models;
using GazpromApp.ViewModels;

namespace GazpromApp.Utils;

public static class DateCalculator
{
    public static DateTime[] GetNearestDates(this InspectionViewModel inspection)
    {
        var today = DateTime.Today;

        var firstFutureDate = AddPeriod(inspection.InspectionDate.DateTime, inspection.PeriodType, inspection.PeriodValue);

        if (firstFutureDate > today)
        {
            return [inspection.InspectionDate.DateTime, firstFutureDate];
        }

        DateTime futureDate = Enumerable.Range(1, int.MaxValue)
            .Select(i => AddPeriod(inspection.InspectionDate.DateTime, inspection.PeriodType, inspection.PeriodValue * i))
            .First(d => d > today);

        DateTime pastDate = SubtractPeriod(futureDate, inspection.PeriodType, inspection.PeriodValue);

        return [pastDate, futureDate];
    }

    private static DateTime AddPeriod(DateTime date, PeriodType periodType, float value)
    {
        return periodType switch
        {
            PeriodType.Day => date.AddDays(value),
            PeriodType.Week => date.AddDays(value * 7),
            PeriodType.Month => date.AddDays(value * 30.436875), // Avg days/month
            PeriodType.Year => date.AddDays(value * 365.2425), // Avg days/year (incl. leap years)
            _ => throw new ArgumentException("Invalid period type")
        };
    }

    private static DateTime SubtractPeriod(DateTime date, PeriodType periodType, float value)
    {
        return AddPeriod(date, periodType, -value);
    }
}