using System.Globalization;

namespace Nonemm.Contests.Udc;

/// `MultipleSessions = 0000/30`: the contest runs in sessions, the first one
/// starting at 00:00 UTC and each lasting 30 minutes. A station worked in an
/// earlier session may be worked again, and `ResetMultsEverySession` and
/// `ResetSNEverySession` say whether the multipliers and the serial numbers
/// start over as well.
///
/// N1MM writes the length as minutes up to two digits and as hours and minutes
/// above that: `30` is half an hour, `200` is two hours. The shortest session
/// N1MM takes is ten minutes.
public sealed class UdcSessions
{
    private static readonly TimeSpan Shortest = TimeSpan.FromMinutes(10);

    private readonly TimeSpan start;
    private readonly TimeSpan length;

    public UdcSessions(UdcFile file)
    {
        ResetsMultipliers = file.Flag("ResetMultsEverySession", false);
        ResetsSerialNumbers = file.Flag("ResetSNEverySession", false);
        string[] parts = file.Text("MultipleSessions").Split('/', StringSplitOptions.TrimEntries);
        if (parts.Length != 2)
        {
            return;
        }
        TimeSpan? first = HoursAndMinutes(parts[0]);
        TimeSpan? each = HoursAndMinutes(parts[1]);
        if (first is null || each is null || each < Shortest)
        {
            return;
        }
        start = first.Value;
        length = each.Value;
        IsDefined = true;
    }

    public bool IsDefined { get; }

    public bool ResetsMultipliers { get; }

    public bool ResetsSerialNumbers { get; }

    /// Which session a contact belongs to. The sessions are counted off a
    /// fixed date so that two days of a contest never share a number, which is
    /// what N1MM's own stepping from the start time gives. Every contact is in
    /// session 0 for a contest without sessions.
    public int SessionOf(DateTime utc) =>
        IsDefined ? (int)((utc - (DateTime.UnixEpoch + start)).Ticks / length.Ticks) : 0;

    /// `30` is thirty minutes, `200` two hours, `0000` midnight. A value that
    /// is not a number at all leaves the setting off.
    private static TimeSpan? HoursAndMinutes(string value)
    {
        if (!int.TryParse(value, NumberStyles.None, CultureInfo.InvariantCulture, out int number))
        {
            return null;
        }
        return value.Length > 2
            ? TimeSpan.FromMinutes((number / 100 * 60) + (number % 100))
            : TimeSpan.FromMinutes(number);
    }
}