LlamaCasty/Helpers/StripPool.cs

using System;
using System.Threading;

namespace ytLive.Helpers;

/// <summary>
/// A fixed pool of four DEDICATED background threads at <see cref="ThreadPriority.BelowNormal"/>
/// that split one composite into horizontal row strips. The compositor needs the split to get
/// render inside a 16.67ms slot, but it must not compete with the encoder, the WGC capture
/// callbacks, the WebView2 chat or the game the user is playing — hence the hard cap of four
/// and the demotion.
///
/// Research (2026-10-05, Unit 5): the demotion must NOT be done by poking
/// <c>Thread.CurrentThread.Priority</c> inside <c>Parallel.For</c> — the pool is shared, so the
/// demotion leaks into every unrelated work item that thread later runs
/// (https://stackoverflow.com/questions/524483/net-backgroundworker-objects-thread-priority:
/// "you have next to no control of which thread pool thread will actually run your task");
/// https://stackoverflow.com/questions/1578599/c-sharp-lower-thread-priority-in-thread-pool:
/// "You shouldn't mess with the priority of the regular ThreadPool, since you aren't the only
/// consumer ... write your own producer/consumer queue, using your own Thread(s) as the
/// consumer(s) — setting the thread priority yourself"). A real failed attempt confirmed it: a
/// full test run after demoting the pool threads broke an unrelated audio-mix timing test.
/// This class IS that producer/consumer queue; the caller thread is the producer and also runs
/// one strip itself, so four workers never means five-way oversubscription.
///
/// Contract: <see cref="For"/> blocks until every strip has run, each strip must write only its
/// own rows (the compositor's blits already partition by row), and it must not be called from
/// inside a strip (the blits never nest).
/// </summary>
internal static class StripPool
{
    /// <summary>Four workers: the measured sweet spot on this box. Fewer leaves render over
    /// budget; more steals cycles from the game and the encoder and made render worse, not better.</summary>
    public const int Workers = 4;

    /// <summary>Below this many rows the fan-out costs more than it saves — run it inline.</summary>
    private const int MinRowsForFanOut = 64;

    private static readonly object[] _queueLocks = CreateLocks();
    private static readonly Queue<Strip>[] _queues = CreateQueues();
    private static readonly Queue<Strip>[] _free = CreateQueues();
    private static readonly SemaphoreSlim[] _signals = CreateSignals();
    private static Thread[]? _threads;
    private static int _roundRobin = -1;

    private sealed class Strip
    {
        public int From;
        public int To;              // exclusive
        public Action<int>? Body;
        public Batch? Owner;
    }

    private sealed class Batch
    {
        public int Remaining;
        public Exception? Error;
    }

    /// <summary>Run <paramref name="body"/> for every row in [from, to), split across the pool.</summary>
    public static void For(int from, int to, Action<int> body)
    {
        if (from >= to) return;

        var rows = to - from;
        if (rows < MinRowsForFanOut || !EnsureStarted())
        {
            for (var y = from; y < to; y++) body(y);
            return;
        }

        // Strips are handed to whole workers (not per-row tasks) so the strip boundaries —
        // and therefore the destination row each strip owns — are fixed before any of them run.
        var strips = Math.Min(Workers, rows);
        var chunk = (rows + strips - 1) / strips;
        var batch = new Batch { Remaining = strips - 1 }; // the caller runs the last one

        var first = from;
        for (var k = 0; k < strips - 1; k++)
        {
            var end = Math.Min(first + chunk, to);
            var w = NextWorker();
            var strip = Rent(w);
            strip.From = first;
            strip.To = end;
            strip.Body = body;
            strip.Owner = batch;
            lock (_queueLocks[w]) _queues[w].Enqueue(strip);
            _signals[w].Release();
            first = end;
        }

        try
        {
            for (var y = first; y < to; y++) body(y);
        }
        catch (Exception ex)
        {
            batch.Error ??= ex;
        }

        if (batch.Remaining > 0)
        {
            lock (batch)
            {
                while (batch.Remaining > 0) Monitor.Wait(batch);
            }
        }

        if (batch.Error != null) throw new InvalidOperationException("A compositor strip failed.", batch.Error);
    }

    private static void Worker(int index)
    {
        var signal = _signals[index];
        while (true)
        {
            signal.Wait();
            Strip? strip = null;
            lock (_queueLocks[index])
            {
                if (_queues[index].Count > 0) strip = _queues[index].Dequeue();
            }
            if (strip == null) continue; // released for an already-drained queue
            var batch = strip.Owner!;
            try
            {
                for (var y = strip.From; y < strip.To; y++) strip.Body!(y);
            }
            catch (Exception ex)
            {
                batch.Error ??= ex;
            }
            finally
            {
                strip.Body = null;
                strip.Owner = null;
                lock (_queueLocks[index]) _free[index].Enqueue(strip);
                lock (batch)
                {
                    if (--batch.Remaining == 0) Monitor.PulseAll(batch);
                }
            }
        }
    }

    private static Strip Rent(int worker)
    {
        lock (_queueLocks[worker])
            return _free[worker].Count > 0 ? _free[worker].Dequeue() : new Strip();
    }

    private static int NextWorker()
    {
        var w = Interlocked.Increment(ref _roundRobin) % Workers;
        return w < 0 ? w + Workers : w;
    }

    /// <summary>Start the workers on first use, so a WPF cold start pays nothing for them.</summary>
    private static bool EnsureStarted()
    {
        if (_threads != null) return true;
        lock (_signals)
        {
            if (_threads != null) return true;
            var threads = new Thread[Workers];
            for (var i = 0; i < Workers; i++)
            {
                var index = i; // capture per-iteration: the loop variable would be Workers by the time the thread runs
                var t = new Thread(() => Worker(index), 256 * 1024)
                {
                    IsBackground = true,
                    Name = $"compositor-strip-{i}",
                    Priority = ThreadPriority.BelowNormal,
                };
                threads[i] = t;
                t.Start();
            }
            _threads = threads;
            return true;
        }
    }

    private static object[] CreateLocks()
    {
        var l = new object[Workers];
        for (var i = 0; i < Workers; i++) l[i] = new object();
        return l;
    }

    private static Queue<Strip>[] CreateQueues()
    {
        var q = new Queue<Strip>[Workers];
        for (var i = 0; i < Workers; i++) q[i] = new Queue<Strip>();
        return q;
    }

    private static SemaphoreSlim[] CreateSignals()
    {
        var s = new SemaphoreSlim[Workers];
        for (var i = 0; i < Workers; i++) s[i] = new SemaphoreSlim(0, int.MaxValue);
        return s;
    }
}