using System;
using System.Threading;
using System.Threading.Tasks;
using ytLive.Models;
using ytLive.Services.Compositor;

namespace ytLive.Services.Encoder;

/// <summary>
/// The live frame producer (TASK 4 ship step 5): the bridge between the capture
/// managers + compositor and the encoder. While live it snapshots the active
/// scene each tick, resolves every element to its latest frame, composites it
/// into the tier's output frame, and paces frames into the encoder at the tier's
/// FPS. All collaborators are constructor-injected seams (scene, resolver,
/// options, encoder factory, pacing delay) so the pump stays free of WPF and of
/// the capture managers and is fully hermetic in tests.
///
/// The RTMP URL comes from the options provider: until the live-stream create
/// flow lands (TASK 5) it yields null, so go-live runs the existing visual flow
/// without actually pushing.
/// </summary>
public sealed class FramePump : IDisposable
{
    private readonly Func<Scene?> _sceneProvider;
    private readonly Func<SceneElement, VideoFrame?> _frameResolver;
    private readonly Func<CompositorOptions> _compositorOptions;
    private readonly Func<EncoderOptions?> _encoderOptions;
    private readonly Func<IFfmpegEncoder> _encoderFactory;
    private readonly Action<string>? _log;
    private readonly Func<TimeSpan, CancellationToken, Task> _pacingDelay;
    private readonly Func<(VideoFrame? Frame, SocialBarPosition Position)>? _socialBar;
    private readonly TransitionService? _transition;
    private readonly SceneGraph? _sceneGraph;
    private readonly SceneCompositor _compositor = new();

    // Master-buffer scratch pool (take-4 starvation fix, slice 2 of 2): a fresh
    // 8.3MB byte[] every tick is ~500MB/s of LOH churn — GC stalls masquerading
    // as render cost. The pump pools ONLY buffers it handed out (reference-equality
    // set), so bake-cache / social-bar / static-cache frames are never touched;
    // SubmitFrameAsync copies the bytes to the encoder's stdin before returning,
    // so recycling after submit is safe (MyMistakes recipe).
    private readonly HashSet<byte[]> _ownedScratch = new(ReferenceEqualityComparer.Instance);
    private readonly List<byte[]> _freeScratch = new();
    private const int MaxScratchPooled = 4;

    private byte[] AcquireScratch(int size)
    {
        for (var i = _freeScratch.Count - 1; i >= 0; i--)
        {
            if (_freeScratch[i].Length != size) continue;
            var buffer = _freeScratch[i];
            _freeScratch.RemoveAt(i);
            return buffer;
        }
        var fresh = new byte[size];
        _ownedScratch.Add(fresh);
        return fresh;
    }

    private void ReleaseScratch(byte[]? buffer)
    {
        if (buffer == null || !_ownedScratch.Contains(buffer)) return;
        if (_freeScratch.Count >= MaxScratchPooled || _freeScratch.Contains(buffer)) return;
        _freeScratch.Add(buffer);
    }

    // Windows sleep quantum (take-9 finding, 2026-09-04): Task.Delay rounds every
    // request up to the system clock tick (~15.6ms default — learn.microsoft.com/en-us/
    // dotnet/api/system.threading.tasks.task.delay: "approximately 15 milliseconds on
    // Windows systems"), so a pacer requesting 3-15ms actually sleeps 15.6ms. Takes
    // 5-9 measured work ~25ms but period ~37ms: one padded wait per frame hid every
    // compositor improvement. Established media-app practice (game-loop/OBS canon —
    // stackoverflow.com/questions/5441464; and raise the resolution for the session —
    // learn.microsoft.com/en-us/windows/win32/api/timeapi/nf-timeapi-timebeginperiod):
    // timeBeginPeriod(1) while the pump runs, sleep only the BULK of the remainder,
    // and spin the last ~2ms across the deadline.
    [System.Runtime.InteropServices.DllImport("winmm.dll")]
    private static extern uint timeBeginPeriod(uint uMilliseconds);
    [System.Runtime.InteropServices.DllImport("winmm.dll")]
    private static extern uint timeEndPeriod(uint uMilliseconds);
    private static readonly long SpinTailTicks = System.Diagnostics.Stopwatch.Frequency * 2 / 1000; // 2ms

    private readonly object _gate = new();
    private IFfmpegEncoder? _encoder;
    private CancellationTokenSource? _cts;
    private Task? _pumpTask;
    private bool _started;

    /// <summary>Forwards the encoder's parsed health — ship step 6 binds this to the bottom bar.</summary>
    public event EventHandler<StreamHealth>? HealthUpdated;

    /// <summary>Raised when the encoder cannot start or dies mid-stream. The pump stops itself.</summary>
    public event EventHandler<string>? Failed;

    public FramePump(
        Func<Scene?> sceneProvider,
        Func<SceneElement, VideoFrame?> frameResolver,
        Func<CompositorOptions> compositorOptions,
        Func<EncoderOptions?> encoderOptions,
        Func<IFfmpegEncoder> encoderFactory,
        Action<string>? log = null,
        Func<TimeSpan, CancellationToken, Task>? pacingDelay = null,
        Func<(VideoFrame? Frame, SocialBarPosition Position)>? socialBar = null,
        TransitionService? transition = null,
        SceneGraph? sceneGraph = null)
    {
        _sceneProvider = sceneProvider ?? throw new ArgumentNullException(nameof(sceneProvider));
        _frameResolver = frameResolver ?? throw new ArgumentNullException(nameof(frameResolver));
        _compositorOptions = compositorOptions ?? throw new ArgumentNullException(nameof(compositorOptions));
        _encoderOptions = encoderOptions ?? throw new ArgumentNullException(nameof(encoderOptions));
        _encoderFactory = encoderFactory ?? throw new ArgumentNullException(nameof(encoderFactory));
        _log = log;
        _pacingDelay = pacingDelay ?? ((delay, ct) => Task.Delay(delay, ct));
        _socialBar = socialBar;
        _transition = transition;
        _sceneGraph = sceneGraph;
    }

    public bool IsRunning { get; private set; }

    /// <summary>Never throws: failures are logged and surfaced via <see cref="Failed"/>,
    /// so the VM can fire-and-forget it from a sync command handler.</summary>
    public async Task StartAsync(CancellationToken cancellationToken = default)
    {
        lock (_gate)
        {
            if (_started) return;
            _started = true;
        }

        IFfmpegEncoder? encoder = null;
        try
        {
            var options = _encoderOptions();
            if (options == null)
            {
                _log?.Invoke("FramePump: no output configured (neither streaming nor recording) — encoder skipped");
                lock (_gate) _started = false;
                return;
            }

            encoder = _encoderFactory();
            encoder.HealthUpdated += OnHealthUpdated;
            encoder.ProcessFailed += OnProcessFailed;
            await encoder.StartAsync(options, cancellationToken);

            lock (_gate)
            {
                _encoder = encoder;
            }

            // IsRunning must be true before the loop starts: the loop reads it on
            // its first iteration, and with a completed-task delay it can run
            // synchronously on this thread before PumpAsync even returns.
            IsRunning = true;
            _cts = new CancellationTokenSource();
            // The loop runs on the thread pool ON PURPOSE (take-8 finding, 2026-09-04):
            // Task.Run installs no SynchronizationContext, so every await continuation
            // stays off the UI dispatcher. Before this, the pump inherited the UI
            // thread's sync context (StartAsync is fired from a command handler), so
            // "render 22ms, wait 10ms" was the producer sitting in the dispatcher
            // queue behind the live preview it is meant to be independent of — the
            // stats quantum fix made the wait VISIBLE; this removes its cause.
            // OBS's video threads are dedicated for exactly this reason.
            _pumpTask = Task.Run(() => PumpAsync(options, _cts.Token));
            _log?.Invoke($"FramePump started ({options.Width}×{options.Height} @ {options.Fps} fps)");
        }
        catch (Exception ex)
        {
            _log?.Invoke($"FramePump: start failed: {ex.Message}");
            if (encoder != null)
            {
                encoder.HealthUpdated -= OnHealthUpdated;
                encoder.ProcessFailed -= OnProcessFailed;
                try
                {
                    encoder.Dispose();
                }
                catch (Exception disposeEx)
                {
                    _log?.Invoke($"FramePump: disposing failed encoder: {disposeEx.Message}");
                }
            }
            lock (_gate)
            {
                _started = false;
                IsRunning = false;
            }
            Failed?.Invoke(this, ex.Message);
        }
    }

    public async Task StopAsync(CancellationToken cancellationToken = default)
    {
        IFfmpegEncoder? encoder;
        Task? pump;
        lock (_gate)
        {
            if (!_started && _encoder == null) return;
            _started = false;
            IsRunning = false;
            encoder = _encoder;
            pump = _pumpTask;
            _cts?.Cancel();
        }

        // Stop the encoder BEFORE awaiting the pump: closing its stdin unblocks a
        // write stuck on pipe backpressure, otherwise the pump could await forever.
        if (encoder != null)
        {
            try
            {
                await encoder.StopAsync(cancellationToken);
            }
            catch (Exception ex)
            {
                _log?.Invoke($"FramePump: encoder stop failed: {ex.Message}");
            }
        }

        if (pump != null)
        {
            try
            {
                await pump;
            }
            catch (Exception ex)
            {
                _log?.Invoke($"FramePump: pump loop faulted during stop: {ex.Message}");
            }
        }

        if (encoder != null)
        {
            encoder.HealthUpdated -= OnHealthUpdated;
            encoder.ProcessFailed -= OnProcessFailed;
            try
            {
                encoder.Dispose();
            }
            catch (Exception ex)
            {
                _log?.Invoke($"FramePump: encoder dispose failed: {ex.Message}");
            }
        }

        lock (_gate)
        {
            _encoder = null;
            _cts = null;
            _pumpTask = null;
        }
        _log?.Invoke("FramePump stopped");
    }

    public void Dispose()
    {
        try
        {
            StopAsync().GetAwaiter().GetResult();
        }
        catch (Exception ex)
        {
            _log?.Invoke($"FramePump: dispose failed: {ex.Message}");
        }
        _freeScratch.Clear();
        _ownedScratch.Clear();
    }

    private async Task PumpAsync(EncoderOptions options, CancellationToken ct)
    {
        var interval = TimeSpan.FromSeconds(1d / Math.Max(1, options.Fps));
        var lastTick = System.Diagnostics.Stopwatch.StartNew();

        // Deadline pacing (2026-09-03, take-3 fix): the frame interval is a DEADLINE,
        // not an afterthought sleep — the OBS libobs video-io.c pattern (researched
        // before coding; see https://github.com/obsproject/obs-studio/blob/master/
        // libobs/media-io/video-io.c). The old loop slept the FULL interval after
        // each render, so period = render + submit + interval: at take-3's 258ms
        // render that was 3.6fps stamped into a 60fps container — rawvideo stamps by
        // arrival, so 30 wall-seconds muxed as a 2.1s time-lapse, no error anywhere.
        var intervalTicks = Math.Max(1, (long)Math.Round(interval.TotalSeconds * System.Diagnostics.Stopwatch.Frequency));
        var nextTick = System.Diagnostics.Stopwatch.GetTimestamp();

        // Stage timing (2026-09-01, take two): rawvideo carries no per-frame
        // timestamps — ffmpeg stamps frames by ARRIVAL at the declared fps. A producer
        // slower than the declared rate yields a time-lapsed, short file (observed:
        // 39 frames in 27 wall-seconds ≈ 30x at 60fps) with no error anywhere.
        // Log the render/submit split every 5s so the next take names the stage.
        var renderSw = new System.Diagnostics.Stopwatch();
        var submitSw = new System.Diagnostics.Stopwatch();
        var waitSw = new System.Diagnostics.Stopwatch();
        // Resolve-vs-composite split (2026-09-04, take-6 ambiguity): "render" was a
        // black box — the stats line now reports resolver time separately so a take
        // names the stage (get-frame vs blit) instead of feeding another guess.
        var resolveSw = new System.Diagnostics.Stopwatch();
        long renderTicks = 0, submitTicks = 0, resolveTicks = 0, waitTicks = 0, worstRender = 0;
        int statFrames = 0;
        var statsNext = DateTime.UtcNow + TimeSpan.FromSeconds(5);
        void ReportStats()
        {
            if (DateTime.UtcNow < statsNext) return;
            var target = 5d / interval.TotalSeconds; // frames expected per window
            _log?.Invoke(statFrames == 0
                ? "FramePump stats: NO frames produced in 5s (loop stalled?)"
                : $"FramePump stats: {statFrames}/{target:F0} frames per 5s, " +
                  $"avg render {renderTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms " +
                  $"(resolve {resolveTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}), " +
                  $"avg submit {submitTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms, "
                  + $"avg wait {waitTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms, "
                  + $"worst render {worstRender / (double)System.Diagnostics.Stopwatch.Frequency * 1000:F1}ms");
            worstRender = 0;
            renderTicks = submitTicks = resolveTicks = waitTicks = 0;
            statFrames = 0;
            statsNext = DateTime.UtcNow + TimeSpan.FromSeconds(5);
        }

        // One wrapper shared by every render of the run — resolve time accumulates
        // inside the render measurement, and the stats line reports the split.
        timeBeginPeriod(1); // pairs with timeEndPeriod in the finally — see field note
        var previousGcMode = System.Runtime.GCSettings.LatencyMode;
        System.Runtime.GCSettings.LatencyMode = System.Runtime.GCLatencyMode.SustainedLowLatency;
        VideoFrame? TimedResolver(SceneElement element)
        {
            resolveSw.Restart();
            var frame = _frameResolver(element);
            resolveSw.Stop();
            resolveTicks += resolveSw.ElapsedTicks;
            return frame;
        }

        try
        {
            while (!ct.IsCancellationRequested)
            {
                var scene = _sceneProvider();
                if (scene != null)
                {
                    var compositorOptions = _compositorOptions();
                    VideoFrame? socialBarFrame = null;
                    var socialBarTop = 0;
                    if (_socialBar != null)
                    {
                        var (barFrame, position) = _socialBar();
                        socialBarFrame = barFrame;
                        if (barFrame != null)
                            socialBarTop = position == SocialBarPosition.Top
                                ? 0
                                : compositorOptions.SourceRectHeight - barFrame.Height;
                    }

                    VideoFrame frame;
                    var scratchSize = compositorOptions.SourceRectWidth
                        * compositorOptions.SourceRectHeight * 4;
                    // The transition "from" frame lives in TransitionService.FromFrame
                    // (captured at Start by the VM). The old per-tick fromScene render
                    // here was dead weight — a full extra scene composite every
                    // transition tick that BlendFrame never read; removed with the
                    // pooling change because its buffer's only consumer was its own release.
                    renderSw.Restart();
                    var scratch = AcquireScratch(scratchSize);
                    frame = RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop, scratch, TimedResolver);
                    if (_transition is { Active: true } transition)
                    {
                        frame = transition.BlendFrame(frame);
                        transition.Tick(lastTick.Elapsed.TotalMilliseconds);
                    }
                    // Restarted EVERY frame (transition or not): a transition begun
                    // after idle must not inherit a giant ElapsedMs and complete
                    // instantly on its first Tick.
                    lastTick.Restart();
                    renderSw.Stop();
                    renderTicks += renderSw.ElapsedTicks;
                    if (renderSw.ElapsedTicks > worstRender) worstRender = renderSw.ElapsedTicks;

                    IFfmpegEncoder? encoder;
                    lock (_gate) encoder = _encoder;
                    if (encoder == null) break;

                    // Count-based CFR emission (libobs video-io.c — the frame interval
                    // is a DEADLINE and the output stream holds its declared rate):
                    // one frame per interval slot, whatever the render cost. When the
                    // renderer falls behind, the SAME fresh composite is written again
                    // for every slot that ticked past, so a slow render expresses as
                    // duplicated footage (judder) — never as a skipped timestamp. The
                    // deadline counter is NEVER reset to wall-now: the old rebase
                    // erased every missed slot, so a 43fps reality was authored into a
                    // 60fps container and every recording played ~1.4x fast (rawvideo
                    // carries no timestamps — muxed duration is pure frame count).
                    submitSw.Restart();
                    while (!ct.IsCancellationRequested
                           && System.Diagnostics.Stopwatch.GetTimestamp() >= nextTick)
                    {
                        await encoder.SubmitFrameAsync(frame, ct);
                        statFrames++;
                        nextTick += intervalTicks;
                    }
                    submitSw.Stop();
                    submitTicks += submitSw.ElapsedTicks;

                    // SubmitFrameAsync copied the bytes on every write above — the
                    // tick's buffers are recyclable once each due slot consumed them.
                    // The free-list Contains guard keeps the Cut path (BlendFrame
                    // returns toFrame itself, aliasing scratch) safe.
                    ReleaseScratch(frame.BgraPixels);
                    ReleaseScratch(scratch);

                    // Sleep the BULK of the remainder, SPIN the 2ms tail — never hand
                    // a sub-tick remainder to the sleep quantum (the timeBeginPeriod
                    // note). If the renderer already ate the budget there is nothing
                    // left to sleep and the loop renders the next frame straight away.
                    waitSw.Restart();
                    var ahead = nextTick - System.Diagnostics.Stopwatch.GetTimestamp();
                    if (ahead > SpinTailTicks)
                        await _pacingDelay(TimeSpan.FromSeconds(
                            (ahead - SpinTailTicks) / (double)System.Diagnostics.Stopwatch.Frequency), ct);
                    while (!ct.IsCancellationRequested
                           && System.Diagnostics.Stopwatch.GetTimestamp() < nextTick)
                        Thread.SpinWait(400);
                    waitSw.Stop();
                    waitTicks += waitSw.ElapsedTicks;
                    ReportStats();
                }
            }
        }
        catch (OperationCanceledException)
        {
            // normal stop
        }
        catch (Exception ex)
        {
            // A failure while the pump is supposed to run (encoder died under us,
            // scene provider faulted, ...) stops the pump and surfaces once.
            if (ct.IsCancellationRequested)
            {
                _log?.Invoke($"FramePump: pump exited during stop: {ex.Message}");
            }
            else
            {
                _log?.Invoke($"FramePump: pump loop faulted: {ex.Message}");
                Failed?.Invoke(this, ex.Message);
            }
        }
        finally
        {
            System.Runtime.GCSettings.LatencyMode = previousGcMode;
            timeEndPeriod(1);
            lock (_gate) IsRunning = false;
        }
    }

    private void OnHealthUpdated(object? sender, StreamHealth health) => HealthUpdated?.Invoke(this, health);

    /// <summary>Render a scene, using the baked-crust optimization when a <see cref="SceneGraph"/>
    /// is wired in: bake/cache the static layers below the split point, then composite the
    /// dynamic/above-split layers per frame. Without a SceneGraph, falls back to a full render
    /// (identical output — see SceneCompositorTests). <paramref name="scratch"/> is a pooled
    /// master buffer when supplied; the fully-static path returns the bake-cache frame itself
    /// (never pooled — the release side checks owned-by-reference).</summary>
    private VideoFrame RenderScene(
        Scene scene,
        CompositorOptions options,
        VideoFrame? socialBarFrame,
        int socialBarTop,
        byte[]? scratch = null,
        Func<SceneElement, VideoFrame?>? resolver = null)
    {
        // per-tick composites use the (timed) resolver; the rare bake uses the raw one
        // so bake cost lands in "render" but not "resolve".
        resolver ??= _frameResolver;
        if (_sceneGraph == null)
            return _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);

        var split = _sceneGraph.GetSplitPoint(scene);
        if (split == scene.Elements.Count)
        {
            // Fully static scene: bake once, reuse.
            var baked = _sceneGraph.GetBakedBase(scene, _frameResolver, _compositorOptions);
            if (baked != null)
                return StretchMath.BilinearScale(baked, options.OutputWidth, options.OutputHeight);
        }

        var baseFrame = _sceneGraph.GetBakedBase(scene, _frameResolver, _compositorOptions);
        if (baseFrame != null)
        {
            return SceneCompositor.CompositeLayers(
                baseFrame, scene, split, resolver, options, socialBarFrame, socialBarTop, scratch: scratch);
        }

        // No static base (first layer is dynamic or empty scene) — full render.
        return _compositor.Render(scene, resolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
    }

    private void OnProcessFailed(object? sender, string message)
    {
        _log?.Invoke($"FramePump: encoder process failed: {message}");
        Failed?.Invoke(this, message);
        _ = StopAsync();
    }
}