using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Channels;
using System.Threading.Tasks;
using AcDream.Core.World;

namespace AcDream.App.Streaming;

/// <summary>
/// Services landblock load/unload requests by invoking caller-supplied
/// factory delegates (the production instance wraps
/// <see cref="LandblockLoader.Load"/> for loading and
/// <see cref="AcDream.Core.Terrain.LandblockMesh.Build"/> for the terrain
/// mesh) and posting results to an outbox the render thread drains once
/// per OnUpdate.
///
/// <para>
/// <b>Thread model (Phase A.5 T11+):</b> <see cref="Start"/> spawns a
/// dedicated background worker thread. <see cref="EnqueueLoad"/> and
/// <see cref="EnqueueUnload"/> write non-blocking to the inbox
/// <see cref="Channel{T}"/>; the worker drains it and posts
/// <see cref="LandblockStreamResult"/> records to the outbox.
/// </para>
///
/// <para>
/// <b>DatCollection thread safety</b> is provided by the caller:
/// GameWindow's <c>_datLock</c> (Phase A.5 T10) serialises all
/// <c>DatCollection.Get&lt;T&gt;</c> calls. Both factory closures passed at
/// construction acquire that lock before reading dats. The worker never
/// touches <c>DatCollection</c> directly — it only calls the factories.
/// </para>
///
/// <para>
/// Unloads pass through the outbox as <see cref="LandblockStreamResult.Unloaded"/>
/// records so the render thread can release GPU state on the next drain —
/// the streamer never touches GPU resources directly.
/// </para>
///
/// <remarks>
/// Threading: <see cref="DrainCompletions"/> must be called from a single
/// consumer thread (the render thread in production). All other public
/// methods are thread-safe.
/// </remarks>
/// </summary>
public sealed class LandblockStreamer : IDisposable
{
    /// <summary>
    /// Default drain batch size. Tuned to cap GPU upload work the render
    /// thread does per frame while still draining a moderate backlog in a
    /// few frames. Callers can override on a per-call basis.
    /// </summary>
    public const int DefaultDrainBatchSize = 4;

    private readonly Func<uint, LoadedLandblock?> _loadLandblock;
    private readonly Func<uint, LoadedLandblock?, AcDream.Core.Terrain.LandblockMeshData?> _buildMeshOrNull;
    private readonly Channel<LandblockStreamJob> _inbox;
    private readonly Channel<LandblockStreamResult> _outbox;
    private readonly CancellationTokenSource _cancel = new();
    private Thread? _worker;
    private int _disposed;

    public LandblockStreamer(
        Func<uint, LoadedLandblock?> loadLandblock,
        Func<uint, LoadedLandblock?, AcDream.Core.Terrain.LandblockMeshData?>? buildMeshOrNull = null)
    {
        _loadLandblock = loadLandblock;
        // Default: no mesh build (returns null → Failed result). Production
        // wires in LandblockMesh.Build via the T12 construction site.
        _buildMeshOrNull = buildMeshOrNull ?? ((_, _) => null);
        _inbox  = Channel.CreateUnbounded<LandblockStreamJob>(
            new UnboundedChannelOptions { SingleReader = true, SingleWriter = false });
        _outbox = Channel.CreateUnbounded<LandblockStreamResult>(
            new UnboundedChannelOptions { SingleReader = true, SingleWriter = true });
    }

    /// <summary>
    /// Activate the dedicated background worker thread. Idempotent and
    /// thread-safe: concurrent callers will only spawn one worker; subsequent
    /// calls are no-ops. Atomic via <see cref="Interlocked.CompareExchange{T}(ref T, T, T)"/>.
    /// </summary>
    public void Start()
    {
        if (System.Threading.Volatile.Read(ref _disposed) != 0)
            throw new ObjectDisposedException(nameof(LandblockStreamer));

        // A.5 T10-T12 follow-up: atomically install the worker so concurrent
        // Start() callers don't both pass the null check and spawn duplicate
        // threads. Construct the candidate; CAS it into _worker; if we lost
        // the race, the candidate goes unstarted and is GCed.
        var candidate = new Thread(WorkerLoop)
        {
            IsBackground = true,
            Name = "acdream.streaming.worker",
        };
        if (Interlocked.CompareExchange(ref _worker, candidate, null) == null)
            candidate.Start();
        // else: another caller won the race; their thread is running.
    }

    /// <summary>
    /// Non-blocking enqueue. The worker drains the inbox and posts a
    /// <see cref="LandblockStreamResult.Loaded"/> (or
    /// <see cref="LandblockStreamResult.Failed"/>) to the outbox.
    /// </summary>
    public void EnqueueLoad(uint landblockId, LandblockStreamJobKind kind = LandblockStreamJobKind.LoadNear)
    {
        if (System.Threading.Volatile.Read(ref _disposed) != 0)
            throw new ObjectDisposedException(nameof(LandblockStreamer));
        _inbox.Writer.TryWrite(new LandblockStreamJob.Load(landblockId, kind));
    }

    /// <summary>
    /// Non-blocking enqueue. The worker posts a
    /// <see cref="LandblockStreamResult.Unloaded"/> to the outbox.
    /// </summary>
    public void EnqueueUnload(uint landblockId)
    {
        if (System.Threading.Volatile.Read(ref _disposed) != 0)
            throw new ObjectDisposedException(nameof(LandblockStreamer));
        _inbox.Writer.TryWrite(new LandblockStreamJob.Unload(landblockId));
    }

    /// <summary>
    /// Drain up to <paramref name="maxBatchSize"/> completed results.
    /// Non-blocking. Call from the render thread once per OnUpdate.
    /// </summary>
    /// <remarks>
    /// Must be called from a single consumer thread. The outbox channel is
    /// configured with SingleReader = true and will throw on concurrent reads.
    /// </remarks>
    public IReadOnlyList<LandblockStreamResult> DrainCompletions(int maxBatchSize = DefaultDrainBatchSize)
    {
        var batch = new List<LandblockStreamResult>(maxBatchSize);
        while (batch.Count < maxBatchSize && _outbox.Reader.TryRead(out var result))
            batch.Add(result);
        return batch;
    }

    private void WorkerLoop()
    {
        try
        {
            // Safe to block: this is a dedicated worker thread with no
            // SynchronizationContext, so .Result/.GetResult cannot deadlock
            // against any captured continuation. Using the sync pattern
            // here keeps the loop linear; an async-enumerable alternative
            // would force WorkerLoop to be async Task and lose the
            // simple thread-start shape.
            while (!_cancel.Token.IsCancellationRequested)
            {
                if (!_inbox.Reader.WaitToReadAsync(_cancel.Token).AsTask().GetAwaiter().GetResult())
                    break;

                while (_inbox.Reader.TryRead(out var job))
                {
                    if (_cancel.Token.IsCancellationRequested) return;
                    HandleJob(job);
                }
            }
        }
        catch (OperationCanceledException) { /* graceful shutdown */ }
        catch (Exception ex)
        {
            // Last-ditch: surface via outbox so the caller at least sees
            // something. We never retry a crashed worker.
            _outbox.Writer.TryWrite(new LandblockStreamResult.WorkerCrashed(ex.ToString()));
        }
        finally
        {
            _outbox.Writer.TryComplete();
        }
    }

    private void HandleJob(LandblockStreamJob job)
    {
        switch (job)
        {
            case LandblockStreamJob.Load load:
                // TODO(A.5 T16): route by load.Kind. LoadFar will skip
                // LandBlockInfo + scenery generation; PromoteToNear will skip
                // mesh build (terrain already on GPU). Today every Kind takes
                // the full-load path via _loadLandblock, which matches today's
                // single-tier semantics.
                try
                {
                    var lb = _loadLandblock(load.LandblockId);
                    if (lb is null)
                    {
                        _outbox.Writer.TryWrite(new LandblockStreamResult.Failed(
                            load.LandblockId, "LandblockLoader.Load returned null"));
                        break;
                    }
                    var mesh = _buildMeshOrNull(load.LandblockId, lb);
                    if (mesh is null)
                    {
                        _outbox.Writer.TryWrite(new LandblockStreamResult.Failed(
                            load.LandblockId, "buildMeshOrNull returned null"));
                        break;
                    }
                    var tier = load.Kind == LandblockStreamJobKind.LoadFar
                        ? LandblockStreamTier.Far : LandblockStreamTier.Near;
                    _outbox.Writer.TryWrite(new LandblockStreamResult.Loaded(
                        load.LandblockId, tier, lb, mesh));
                }
                catch (Exception ex)
                {
                    _outbox.Writer.TryWrite(new LandblockStreamResult.Failed(
                        load.LandblockId, ex.ToString()));
                }
                break;

            case LandblockStreamJob.Unload unload:
                _outbox.Writer.TryWrite(new LandblockStreamResult.Unloaded(unload.LandblockId));
                break;
        }
    }

    public void Dispose()
    {
        if (System.Threading.Interlocked.Exchange(ref _disposed, 1) != 0) return;
        _cancel.Cancel();
        _inbox.Writer.TryComplete();
        _worker?.Join(TimeSpan.FromSeconds(2));
        _cancel.Dispose();
    }
}