LehrerApp/LehrerApp.Core/Services/SchoolYearService.cs

namespace LehrerApp.Core.Services;

public class SchoolYearService
{
    /// <param name="today">Nur für Tests — Standard ist das heutige Datum.</param>
    public string CurrentSchoolYear(DateOnly? today = null) =>
        FormatSchoolYear(StartYearFor(today ?? DateOnly.FromDateTime(DateTime.Today)));

    public string FormatSchoolYear(int startYear) =>
        $"{startYear}/{(startYear + 1) % 100:D2}";

    public DateOnly SchoolYearStart(string sy) =>
        new(int.Parse(sy.Split('/')[0]), 8, 1);

    public DateOnly SchoolYearEnd(string sy) =>
        new(int.Parse(sy.Split('/')[0]) + 1, 7, 31);

    /// <param name="today">Nur für Tests — Standard ist das heutige Datum.</param>
    public List<string> RecentSchoolYears(int count = 5, DateOnly? today = null)
    {
        var cur = StartYearFor(today ?? DateOnly.FromDateTime(DateTime.Today));
        return Enumerable.Range(0, count).Select(i => FormatSchoolYear(cur - i)).ToList();
    }

    // Schuljahreswechsel liegt am 1. August: ab diesem Tag zählt das neue Schuljahr.
    private static int StartYearFor(DateOnly date) => date.Month >= 8 ? date.Year : date.Year - 1;
}