namespace HSchool.Simulation;

/// <summary>
/// In-game calendar of one school. Time only moves while <see cref="IsRunning"/> is set, and it
/// moves by whole fixed steps — never by wall-clock deltas — so the same tick count always
/// produces the same date.
/// </summary>
public sealed class GameClock
{
    /// <summary>Earliest date a school may start at; anything below is a typo, not a design choice.</summary>
    public static readonly DateTime MinStartDate = new(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc);

    public static readonly DateTime MaxStartDate = new(2999, 12, 31, 23, 59, 59, DateTimeKind.Utc);

    private int _speedIndex = ClockSpeed.DefaultIndex;

    public GameClock(DateTime startDate)
    {
        if (!IsValidStartDate(startDate))
        {
            throw new ArgumentOutOfRangeException(nameof(startDate), startDate, "Start date is outside the supported range.");
        }

        // The game calendar is not tied to a real time zone; UTC keeps serialization unambiguous.
        Time = DateTime.SpecifyKind(startDate, DateTimeKind.Utc);
    }

    public DateTime Time { get; private set; }

    /// <summary>
    /// Schools live on their own: a new calendar starts running and only the player's pause
    /// button stops it. Leaving for the menu does not.
    /// </summary>
    public bool IsRunning { get; set; } = true;

    /// <summary>Index into <see cref="ClockSpeed.Multipliers"/>; invalid values are ignored.</summary>
    public int SpeedIndex
    {
        get => _speedIndex;
        set
        {
            if (ClockSpeed.IsValid(value))
            {
                _speedIndex = value;
            }
        }
    }

    public double Multiplier => ClockSpeed.MultiplierAt(_speedIndex);

    public static bool IsValidStartDate(DateTime date) => date >= MinStartDate && date <= MaxStartDate;

    /// <summary>Empty-time skip. Not a tick — the calendar jumps to an instant already known to be legal.</summary>
    public void JumpTo(DateTime time)
    {
        if (!IsValidStartDate(time))
        {
            throw new ArgumentOutOfRangeException(nameof(time), time, "Jump target is outside the supported range.");
        }

        Time = DateTime.SpecifyKind(time, DateTimeKind.Utc);
    }

    /// <summary>
    /// Advances the calendar by one fixed step of <paramref name="realSeconds"/>, scaled by the
    /// base rate and the current speed. Does nothing while paused.
    /// </summary>
    /// <returns>Game minutes actually added; zero while paused.</returns>
    public double Advance(double realSeconds, double gameMinutesPerRealSecond)
    {
        if (!IsRunning)
        {
            return 0;
        }

        var gameMinutes = realSeconds * gameMinutesPerRealSecond * Multiplier;
        Time = Time.AddMinutes(gameMinutes);
        return gameMinutes;
    }
}