Explorer-Workbench/src/Explorer.Application/ThumbnailScheduler.cs

namespace Explorer.Application;

public readonly record struct ThumbnailKey(string Path, long ModifiedTicks, int PixelWidth)
{
    public static ThumbnailKey From(string path, DateTimeOffset? modifiedUtc, int pixelWidth)
        => new(path, modifiedUtc?.UtcTicks ?? 0, pixelWidth);
}

public readonly record struct ThumbnailJob(
    ThumbnailKey Key,
    int Rank,
    int Generation);

public readonly record struct ThumbnailSchedulerStats(
    int Generation,
    int Window,
    int Pending,
    int InFlight,
    int Requested,
    int CacheHits,
    int Generated,
    int Cancelled,
    int Failed);

public sealed class ThumbnailScheduler
{
    public const int DefaultConcurrency = 2;
    public const int DefaultPixelWidth = 128;
    public const int MaxPending = 96;
    public const int PrefetchItems = 24;

    private readonly object _gate = new();
    private readonly List<ThumbnailJob> _window = [];
    private readonly HashSet<ThumbnailKey> _windowKeys = [];
    private readonly HashSet<ThumbnailKey> _inFlight = [];
    private readonly HashSet<ThumbnailKey> _cached = [];
    private int _generation;
    private bool _enabled;
    private int _requested;
    private int _cacheHits;
    private int _generated;
    private int _cancelled;
    private int _failed;

    public int Generation
    {
        get { lock (_gate) return _generation; }
    }

    public int InFlight
    {
        get { lock (_gate) return _inFlight.Count; }
    }

    public void Reset(int generation)
    {
        lock (_gate)
        {
            _cancelled += _window.Count + _inFlight.Count;
            _generation = generation;
            _window.Clear();
            _windowKeys.Clear();
            _inFlight.Clear();
        }
    }

    public void SetEnabled(bool enabled)
    {
        lock (_gate)
        {
            _enabled = enabled;
            if (enabled)
            {
                return;
            }

            _cancelled += _window.Count + _inFlight.Count;
            _window.Clear();
            _windowKeys.Clear();
            _inFlight.Clear();
        }
    }

    public bool IsCached(ThumbnailKey key)
    {
        lock (_gate)
        {
            return _cached.Contains(key);
        }
    }

    public void MarkCached(ThumbnailKey key)
    {
        lock (_gate)
        {
            _cached.Add(key);
        }
    }

    public void RecordCacheHit()
    {
        lock (_gate)
        {
            _cacheHits++;
            _requested++;
        }
    }

    public IReadOnlyList<ThumbnailJob> SetWindow(int generation, IReadOnlyList<ThumbnailJob> jobs)
    {
        ArgumentNullException.ThrowIfNull(jobs);
        lock (_gate)
        {
            if (generation != _generation || !_enabled)
            {
                _cancelled += jobs.Count;
                return [];
            }

            var next = jobs
                .Where(j => j.Generation == _generation)
                .GroupBy(j => j.Key)
                .Select(g => g.OrderBy(j => j.Rank).First())
                .OrderBy(j => j.Rank)
                .Take(MaxPending)
                .ToList();

            var nextKeys = next.Select(j => j.Key).ToHashSet();
            foreach (var previous in _window)
            {
                if (!nextKeys.Contains(previous.Key))
                {
                    _cancelled++;
                }
            }

            foreach (var key in _inFlight.ToArray())
            {
                if (!nextKeys.Contains(key))
                {
                    _inFlight.Remove(key);
                    _cancelled++;
                }
            }

            _window.Clear();
            _window.AddRange(next);
            _windowKeys.Clear();
            foreach (var job in next)
            {
                _windowKeys.Add(job.Key);
            }

            _requested += next.Count(j => !_cached.Contains(j.Key) && !_inFlight.Contains(j.Key));
            return next;
        }
    }

    public bool TryTake(out ThumbnailJob job)
    {
        lock (_gate)
        {
            if (!_enabled)
            {
                job = default;
                return false;
            }

            foreach (var candidate in _window)
            {
                if (_cached.Contains(candidate.Key) || _inFlight.Contains(candidate.Key))
                {
                    continue;
                }

                _inFlight.Add(candidate.Key);
                job = candidate;
                return true;
            }
        }

        job = default;
        return false;
    }

    public bool IsCurrent(ThumbnailJob job)
    {
        lock (_gate)
        {
            return _enabled
                   && job.Generation == _generation
                   && _windowKeys.Contains(job.Key);
        }
    }

    public void Complete(ThumbnailJob job, bool generated, bool failed)
    {
        lock (_gate)
        {
            _inFlight.Remove(job.Key);
            if (failed)
            {
                _failed++;
                _cached.Add(job.Key);
                return;
            }

            if (generated)
            {
                _generated++;
                _cached.Add(job.Key);
            }
        }
    }

    public ThumbnailSchedulerStats Snapshot()
    {
        lock (_gate)
        {
            var pending = _window.Count(j => !_cached.Contains(j.Key) && !_inFlight.Contains(j.Key));
            return new ThumbnailSchedulerStats(
                _generation,
                _window.Count,
                pending,
                _inFlight.Count,
                _requested,
                _cacheHits,
                _generated,
                _cancelled,
                _failed);
        }
    }
}