using System.Threading.Channels;
using Microsoft.Extensions.Logging;
using TeamsISO.Engine.Domain;
using TeamsISO.Engine.Interop;

namespace TeamsISO.Engine.Pipeline;

/// <summary>
/// Per-ISO unit. Owns one capture loop, one frame processor, one send loop, and the
/// supervisor that restarts the inner pipeline with exponential backoff on failure.
/// </summary>
public sealed class IsoPipeline : IAsyncDisposable
{
    private Func<CancellationToken, Task> _runInner;
    private readonly ExponentialBackoff _backoff;
    private readonly Func<TimeSpan, CancellationToken, Task> _delay;
    private readonly ILogger<IsoPipeline> _logger;

    private CancellationTokenSource? _cts;
    private Task? _supervisorTask;
    private int _consecutiveFailures;

    // Refs to the currently-live receiver, sender, and frame processor, set by the
    // inner loop on each restart. Reads via Volatile.Read are safe from any thread
    // (UI's stats poll).
    private NdiReceiver? _liveReceiver;
    private NdiSender? _liveSender;
    private FrameProcessor? _liveProcessor;

    // Last-frame metadata, snapshotted out of the RawFrame on capture so we don't
    // hold a reference to the frame's pixel buffer past its useful life. Two ints
    // and a DateTimeOffset are atomically writable on x64; we accept tearing on x86
    // (purely advisory stats display).
    private int _lastWidth;
    private int _lastHeight;
    private DateTimeOffset? _lastReceivedAt;

    // Ring buffer of the last 30 incoming-frame timestamps for live fps display.
    // Updated on the receiver's capture thread (single writer) and read by the UI
    // poll thread (single reader); we use a lock for the snapshot path because
    // an array-of-ticks can't be torn-read atomically.
    private readonly long[] _frameTimes = new long[30];
    private int _frameTimesHead;
    private int _frameTimesCount;
    private readonly object _frameTimesGate = new();

    public Guid ParticipantId { get; }
    public IsoState State { get; private set; } = IsoState.Idle;
    public int ConsecutiveFailures => _consecutiveFailures;

    /// <summary>
    /// Snapshot of the pipeline's current health. Safe to call from any thread; values
    /// are inherently a moment-in-time view and may change immediately. Returns
    /// <see cref="Domain.IsoHealthStats.Empty"/> when no inner pipeline is currently
    /// running (e.g. between supervisor restarts or after final failure).
    /// </summary>
    public Domain.IsoHealthStats GetStats()
    {
        var receiver = Volatile.Read(ref _liveReceiver);
        var sender = Volatile.Read(ref _liveSender);
        var processor = Volatile.Read(ref _liveProcessor);
        var w = Volatile.Read(ref _lastWidth);
        var h = Volatile.Read(ref _lastHeight);
        var lastAt = _lastReceivedAt;

        if (receiver is null || sender is null)
            return Domain.IsoHealthStats.Empty;

        // FrameProcessor.Stats already aggregates FramesDropped (older frames dropped
        // by the closest-frame strategy when the input channel had backlog) and
        // FramesDuplicated (last-frame re-emits when no new frame arrived this tick).
        var procStats = processor?.Stats;

        return new Domain.IsoHealthStats(
            FramesIn: receiver.FramesCaptured,
            FramesOut: sender.FramesSent,
            FramesDropped: procStats?.FramesDropped ?? 0,
            FramesDuplicated: procStats?.FramesDuplicated ?? 0,
            LastFrameAt: lastAt,
            IncomingFps: ComputeFps(),
            IncomingWidth: w,
            IncomingHeight: h);
    }

    /// <summary>
    /// Computes a moving-average incoming framerate from the last N frame timestamps.
    /// Rate = (count - 1) / (newest - oldest). Returns 0 if fewer than 2 frames are
    /// recorded or if the window is degenerate (clock skew, all-equal timestamps).
    /// </summary>
    private double ComputeFps()
    {
        long oldest, newest;
        int count;
        lock (_frameTimesGate)
        {
            count = _frameTimesCount;
            if (count < 2) return 0;
            // Oldest is at the slot AFTER head when buffer is full; otherwise at index 0.
            var oldestIdx = count < _frameTimes.Length
                ? 0
                : _frameTimesHead;
            var newestIdx = (_frameTimesHead - 1 + _frameTimes.Length) % _frameTimes.Length;
            oldest = _frameTimes[oldestIdx];
            newest = _frameTimes[newestIdx];
        }
        var deltaTicks = newest - oldest;
        if (deltaTicks <= 0) return 0;
        var seconds = deltaTicks / (double)TimeSpan.TicksPerSecond;
        return (count - 1) / seconds;
    }

    private void RecordFrameTimestamp(long ticks)
    {
        lock (_frameTimesGate)
        {
            _frameTimes[_frameTimesHead] = ticks;
            _frameTimesHead = (_frameTimesHead + 1) % _frameTimes.Length;
            if (_frameTimesCount < _frameTimes.Length) _frameTimesCount++;
        }
    }

    private void ResetFrameTimestamps()
    {
        lock (_frameTimesGate)
        {
            _frameTimesHead = 0;
            _frameTimesCount = 0;
        }
    }

    /// <summary>
    /// Test ctor. The caller supplies the inner runner directly so failures and lifetimes
    /// can be controlled from a unit test.
    /// </summary>
    internal IsoPipeline(
        Guid participantId,
        Func<CancellationToken, Task> runInner,
        ExponentialBackoff backoff,
        Func<TimeSpan, CancellationToken, Task> delay,
        ILoggerFactory loggerFactory)
    {
        ParticipantId = participantId;
        _runInner = runInner;
        _backoff = backoff;
        _delay = delay;
        _logger = loggerFactory.CreateLogger<IsoPipeline>();
    }

    /// <summary>
    /// Production ctor. Builds the inner runner from the engine dependencies.
    /// </summary>
    public IsoPipeline(
        IsoPipelineConfig config,
        INdiInterop interop,
        IFrameScaler scaler,
        IFrameClock frameClock,
        ExponentialBackoff backoff,
        Func<TimeSpan, CancellationToken, Task> delay,
        ILoggerFactory loggerFactory)
        : this(config.ParticipantId,
               // The inner-runner closure captures `this` so the receiver/sender
               // wired by RunInnerPipelineAsync can be published to instance fields
               // for stats reads.
               default(Func<CancellationToken, Task>)!,
               backoff,
               delay,
               loggerFactory)
    {
        _runInner = ct => RunInnerPipelineAsync(
            config, interop, scaler, frameClock, loggerFactory, ct,
            onLive: (recv, send, proc) =>
            {
                Volatile.Write(ref _liveReceiver, recv);
                Volatile.Write(ref _liveSender, send);
                Volatile.Write(ref _liveProcessor, proc);
                ResetFrameTimestamps(); // fresh window on every supervisor restart
            },
            onClear: () =>
            {
                Volatile.Write(ref _liveReceiver, null);
                Volatile.Write(ref _liveSender, null);
                Volatile.Write(ref _liveProcessor, null);
                ResetFrameTimestamps();
            },
            onFrame: frame =>
            {
                // Snapshot dimensions only — don't hold the RawFrame reference past
                // the channel write so the GC can reclaim it on schedule, and so a
                // late stats read can never resurrect a dropped frame's pixel buffer.
                Volatile.Write(ref _lastWidth, frame.Width);
                Volatile.Write(ref _lastHeight, frame.Height);
                var nowTicks = DateTimeOffset.UtcNow.UtcTicks;
                _lastReceivedAt = new DateTimeOffset(nowTicks, TimeSpan.Zero);
                RecordFrameTimestamp(nowTicks);
            });
    }

    /// <summary>Starts the supervisor. Returns immediately; pipeline runs in the background.</summary>
    public Task StartAsync()
    {
        if (_supervisorTask is not null)
            throw new InvalidOperationException("Pipeline already started.");
        _cts = new CancellationTokenSource();
        State = IsoState.Receiving;
        _supervisorTask = SupervisorLoopAsync(_cts.Token);
        return Task.CompletedTask;
    }

    /// <summary>Stops the pipeline and awaits supervisor completion.</summary>
    public async Task StopAsync()
    {
        if (_cts is null) return;
        _cts.Cancel();
        if (_supervisorTask is not null)
        {
            try { await _supervisorTask; }
            catch (OperationCanceledException) { /* expected */ }
        }
        State = IsoState.Idle;
        _cts.Dispose();
        _cts = null;
        _supervisorTask = null;
    }

    public async ValueTask DisposeAsync()
    {
        await StopAsync();
    }

    private async Task SupervisorLoopAsync(CancellationToken ct)
    {
        _consecutiveFailures = 0;
        while (!ct.IsCancellationRequested)
        {
            try
            {
                await _runInner(ct);
                // Inner exited normally (typically only on cancellation) — leave the loop.
                break;
            }
            catch (OperationCanceledException)
            {
                break;
            }
            catch (Exception ex)
            {
                _consecutiveFailures++;
                _logger.LogWarning(ex,
                    "Pipeline {ParticipantId} failed (consecutive failure #{N}).",
                    ParticipantId, _consecutiveFailures);

                if (_backoff.ShouldGiveUp(_consecutiveFailures))
                {
                    State = IsoState.Error;
                    _logger.LogError("Pipeline {ParticipantId} giving up after {N} consecutive failures.",
                        ParticipantId, _consecutiveFailures);
                    return;
                }

                var wait = _backoff.NextDelay(_consecutiveFailures);
                _logger.LogInformation("Pipeline {ParticipantId} retrying in {Delay}.", ParticipantId, wait);
                try
                {
                    await _delay(wait, ct);
                }
                catch (OperationCanceledException)
                {
                    break;
                }
                State = IsoState.Receiving;
            }
        }
    }

    /// <summary>
    /// Default inner pipeline: spins up receiver → processor → sender on bounded channels
    /// and awaits all three. Throws if any of them throws.
    ///
    /// The optional <paramref name="onLive"/> / <paramref name="onClear"/> / <paramref name="onFrame"/>
    /// callbacks let the outer <see cref="IsoPipeline"/> publish references to the live
    /// receiver and sender (so it can read counters from any thread for health stats)
    /// and observe the most recent received frame (so source resolution / last-seen-at
    /// can be surfaced in the UI). All three are no-ops by default.
    /// </summary>
    private static async Task RunInnerPipelineAsync(
        IsoPipelineConfig config,
        INdiInterop interop,
        IFrameScaler scaler,
        IFrameClock frameClock,
        ILoggerFactory loggerFactory,
        CancellationToken ct,
        Action<NdiReceiver, NdiSender, FrameProcessor>? onLive = null,
        Action? onClear = null,
        Action<RawFrame>? onFrame = null)
    {
        var rawChannel = Channel.CreateBounded<RawFrame>(new BoundedChannelOptions(config.RawChannelCapacity)
        {
            FullMode = BoundedChannelFullMode.DropOldest,
            SingleReader = true,
            SingleWriter = true,
        });
        var processedChannel = Channel.CreateBounded<ProcessedFrame>(new BoundedChannelOptions(config.ProcessedChannelCapacity)
        {
            FullMode = BoundedChannelFullMode.DropOldest,
            SingleReader = true,
            SingleWriter = true,
        });

        // Tap the raw frames as they flow into the channel so the host can show "last
        // frame at" / source resolution without us re-implementing a probe.
        var rawWriter = onFrame is null
            ? rawChannel.Writer
            : new TappedChannelWriter<RawFrame>(rawChannel.Writer, onFrame);

        using var receiver = new NdiReceiver(
            interop, config.SourceName, rawWriter, loggerFactory.CreateLogger<NdiReceiver>());
        using var sender = new NdiSender(
            interop, config.OutputName, processedChannel.Reader, loggerFactory.CreateLogger<NdiSender>(),
            config.OutputGroups);

        var processor = new FrameProcessor(
            config.Settings, scaler, new SolidFrameRenderer(),
            frameClock, rawChannel.Reader, processedChannel.Writer,
            config.SlateThreshold, loggerFactory.CreateLogger<FrameProcessor>());

        onLive?.Invoke(receiver, sender, processor);

        var receiverTask = receiver.RunAsync(ct);
        var senderTask = sender.RunAsync(ct);
        var processorTask = ProcessorLoopAsync(processor, frameClock, ct);

        try
        {
            await Task.WhenAll(receiverTask, senderTask, processorTask);
        }
        finally
        {
            rawChannel.Writer.TryComplete();
            processedChannel.Writer.TryComplete();
            onClear?.Invoke();
        }
    }

    /// <summary>
    /// Channel-writer wrapper that fires a callback on every successful write but
    /// otherwise behaves identically to the inner writer. Used to tap the raw-frame
    /// stream for stats without entangling the receiver with the stats API.
    /// </summary>
    private sealed class TappedChannelWriter<T> : ChannelWriter<T>
    {
        private readonly ChannelWriter<T> _inner;
        private readonly Action<T> _onWrite;
        public TappedChannelWriter(ChannelWriter<T> inner, Action<T> onWrite) { _inner = inner; _onWrite = onWrite; }
        public override bool TryWrite(T item)
        {
            if (_inner.TryWrite(item)) { _onWrite(item); return true; }
            return false;
        }
        public override ValueTask<bool> WaitToWriteAsync(CancellationToken ct = default)
            => _inner.WaitToWriteAsync(ct);
        public override bool TryComplete(Exception? error = null) => _inner.TryComplete(error);
    }

    private static async Task ProcessorLoopAsync(FrameProcessor processor, IFrameClock clock, CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            var advanced = await clock.WaitForNextTickAsync(ct);
            if (!advanced) break;
            await processor.ProcessOnceAsync(ct);
        }
    }
}