using LehrerApp.Core.Models;

namespace LehrerApp.Core.Services;

public enum SchoolYearPeriodKind { FullYear, H1, H2 }

/// <summary>
/// Gemeinsame Regeln für zeitlich begrenzte Gruppenzugehörigkeiten.
/// Eintritt und Austritt gelten unabhängig vom gewählten Halbjahresmodus.
/// </summary>
public static class GroupMembershipService
{
    public static bool IsActiveOn(GroupMembership membership, DateOnly date)
    {
        if (membership.JoinedAt is { } joinedAt && date < joinedAt) return false;
        if (membership.LeftAt is { } leftAt && date > leftAt) return false;

        return membership.Period switch
        {
            MembershipPeriod.H1Only => IsFirstSemester(date),
            MembershipPeriod.H2Only => !IsFirstSemester(date),
            _ => true,
        };
    }

    public static bool Overlaps(GroupMembership membership, DateOnly from, DateOnly to)
    {
        if (to < from) return false;

        var effectiveFrom = membership.JoinedAt is { } joinedAt && joinedAt > from ? joinedAt : from;
        var effectiveTo = membership.LeftAt is { } leftAt && leftAt < to ? leftAt : to;
        if (effectiveTo < effectiveFrom) return false;

        if (membership.Period is MembershipPeriod.FullYear or MembershipPeriod.Custom) return true;

        for (var month = new DateOnly(effectiveFrom.Year, effectiveFrom.Month, 1);
             month <= effectiveTo;
             month = month.AddMonths(1))
        {
            if (membership.Period == MembershipPeriod.H1Only && IsFirstSemester(month)) return true;
            if (membership.Period == MembershipPeriod.H2Only && !IsFirstSemester(month)) return true;
        }

        return false;
    }

    public static (DateOnly From, DateOnly To) SchoolYearPeriod(
        string schoolYear, SchoolYearPeriodKind period)
    {
        var slash = schoolYear.IndexOf('/');
        var startText = slash >= 0 ? schoolYear[..slash] : schoolYear;
        if (!int.TryParse(startText, out var startYear))
            throw new ArgumentException("Ungültiges Schuljahr.", nameof(schoolYear));

        return period switch
        {
            SchoolYearPeriodKind.H1 => (new DateOnly(startYear, 8, 1), new DateOnly(startYear + 1, 1, 31)),
            SchoolYearPeriodKind.H2 => (new DateOnly(startYear + 1, 2, 1), new DateOnly(startYear + 1, 7, 31)),
            _ => (new DateOnly(startYear, 8, 1), new DateOnly(startYear + 1, 7, 31)),
        };
    }

    private static bool IsFirstSemester(DateOnly date) => date.Month >= 8 || date.Month <= 1;
}