namespace AcDream.Core.Net.Tests.Transport;

/// <summary>
/// Deterministic monotonic time source for transport tests. Exposes the same
/// (timestamp, frequency) shape as <c>System.Diagnostics.Stopwatch</c>
/// (<c>GetTimestamp()</c> + <c>Frequency</c>) so slice N1 can inject it
/// behind the production <c>TransportClock</c> without adapting call sites.
/// Time only moves when a test calls <see cref="Advance"/>.
///
/// <para>
/// Deliberately dependency-free (System only) and NOT tied to the machine's
/// <c>Stopwatch.Frequency</c>: a fixed 100 ns tick makes every gate
/// computation reproducible across platforms.
/// </para>
/// </summary>
internal sealed class VirtualClock
{
    /// <summary>
    /// Fixed tick rate: 100 ns ticks (10,000,000 per second), equal to
    /// <see cref="TimeSpan.TicksPerSecond"/> so <see cref="TimeSpan"/>
    /// arithmetic maps 1:1 onto clock ticks.
    /// </summary>
    public const long TicksPerSecond = TimeSpan.TicksPerSecond;

    private long _timestamp;

    public VirtualClock(long startTimestamp = 0) => _timestamp = startTimestamp;

    /// <summary><c>Stopwatch.Frequency</c> equivalent.</summary>
    public long Frequency => TicksPerSecond;

    /// <summary><c>Stopwatch.GetTimestamp()</c> equivalent.</summary>
    public long GetTimestamp() => _timestamp;

    /// <summary>
    /// Seconds since the clock's epoch as a double — the shape of the
    /// retail/ACE PortalYearTicks-style wall values written into TimeSync
    /// payloads and the packet header's 16-bit <c>Time</c> field.
    /// </summary>
    public double Seconds => (double)_timestamp / TicksPerSecond;

    /// <summary>Move time forward. The clock is monotonic — negative deltas throw.</summary>
    public void Advance(TimeSpan delta)
    {
        if (delta < TimeSpan.Zero)
        {
            throw new ArgumentOutOfRangeException(
                nameof(delta),
                "the clock is monotonic — it cannot go backwards");
        }

        _timestamp += delta.Ticks;
    }
}