using System;
using Xunit;
using ytLive.Helpers;

namespace ytLive.Tests;

/// <summary>
/// 2026-11-05: a 1080p video layer played at 20.9 fps on a 30 fps clip because pacing slept a full
/// frame interval ON TOP of the frame's own work. These pin deadline pacing: sleep the remainder of
/// the interval, resync when work overruns, never burst to catch up.
/// </summary>
public class FramePacerTests
{
    private static readonly TimeSpan Frame = TimeSpan.FromMilliseconds(33);

    [Fact]
    public void FirstFrame_WaitsAFullInterval()
        => Assert.Equal(Frame, new FramePacer().DelayFor(Frame, TimeSpan.Zero));

    [Fact]
    public void WorkInsideTheInterval_IsSubtractedFromTheWait()
    {
        var pacer = new FramePacer();
        pacer.DelayFor(Frame, TimeSpan.Zero);                 // schedules the next frame at 33ms

        // The next frame is emitted 20ms later: only 13ms of waiting is left, not another 33ms.
        var wait = pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(20));

        Assert.Equal(TimeSpan.FromMilliseconds(13), wait);
    }

    [Fact]
    public void WorkOnSchedule_DoesNotAccumulateDrift()
    {
        var pacer = new FramePacer();
        Assert.Equal(Frame, pacer.DelayFor(Frame, TimeSpan.Zero));

        // A frame arriving exactly on its deadline waits not at all…
        Assert.Equal(TimeSpan.Zero, pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(33)));

        // …and a frame arriving 3ms early waits exactly the 3ms that is left, every time: the
        // deadline advances one interval per frame, so no drift accumulates.
        Assert.Equal(TimeSpan.FromMilliseconds(3), pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(63)));
        Assert.Equal(TimeSpan.FromMilliseconds(3), pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(96)));
    }

    [Fact]
    public void FallingBehind_ReturnsZeroAndResyncs()
    {
        var pacer = new FramePacer();
        pacer.DelayFor(Frame, TimeSpan.Zero);

        // The frame took 200ms: it is late, so no sleep — and the schedule restarts from now.
        var late = pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(200));
        Assert.Equal(TimeSpan.Zero, late);

        var next = pacer.DelayFor(Frame, TimeSpan.FromMilliseconds(200));
        Assert.Equal(Frame, next);
    }

    [Fact]
    public void ZeroFrameDuration_NeverWaits()
        => Assert.Equal(TimeSpan.Zero, new FramePacer().DelayFor(TimeSpan.Zero, TimeSpan.Zero));

    [Fact]
    public void Reset_RestartsTheSchedule()
    {
        var pacer = new FramePacer();
        pacer.DelayFor(Frame, TimeSpan.Zero);
        pacer.Reset();

        Assert.Equal(Frame, pacer.DelayFor(Frame, TimeSpan.FromSeconds(10)));
    }
}