ADOFAIRenderer/Util/FFmpeg/FFmpegNativeDecoder.cs

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using FFmpeg.AutoGen;
using MelonLoader;
using Unity.Collections;
using Unity.Collections.LowLevel.Unsafe;

namespace ADOFAIRenderer.Util.FFmpeg;

/// <summary>
/// High-performance video decoder that directly invokes FFmpeg native C libraries via P/Invoke.
/// Supports both GPU hardware-accelerated decoding (D3D11VA, DXVA2, QSV, CUDA, D3D12VA) and
/// CPU multi-threaded software decoding (FF_THREAD_FRAME) with automatic fallback.
///
/// ── Architecture &amp; Lifecycle Flow ──────────────────────────────────────────────
///
///   1. Construction &amp; Demuxing:
///      a) avformat_open_input: Open container file and parse stream headers.
///      b) avformat_find_stream_info: Inspect stream parameters, timebase, duration, FPS.
///      c) Identify the primary video stream (AVMEDIA_TYPE_VIDEO).
///
///   2. Codec &amp; Hardware Device Discovery:
///      a) avcodec_find_decoder: Locate codec implementation by codec ID.
///      b) avcodec_get_hw_config: Query supported hardware configurations for the decoder.
///      c) Prioritized HW device creation (D3D11VA → DXVA2 → QSV → CUDA → D3D12VA → VAAPI → VideoToolbox).
///      d) Attach hw_device_ctx and register custom get_format callback.
///      e) If HW initialization fails or is unsupported, fall back to CPU software decoding
///         with multi-threading enabled (thread_count = 0, FF_THREAD_FRAME).
///      f) avcodec_open2: Initialize codec context.
///
///   3. Frame Demuxing &amp; Decoding (TryRead*):
///      a) (Demuxing) av_read_frame reads packets and sends them to the decoder via avcodec_send_packet.
///      b) (Decoding) avcodec_receive_frame retrieves decoded frames.
///      c) (HW Transfer) For hardware frames, av_hwframe_transfer_data downloads GPU surface data
///         into system RAM as NV12 (av_frame_copy_props preserves timestamps).
///      d) (Output Dispatch):
///         - TryReadNV12Frame: Zero-copy direct memory transfer for NV12 frames, sws_scale fallback.
///         - TryReadYUV420PFrame: Zero-copy planar copy for YUV420P/YUVJ420P, sws_scale fallback.
///         - TryReadFrame: sws_scale conversion to RGBA32 with optional vertical flip (vflip).
///
///   4. Seeking &amp; Scrubbing (SeekToTime / SeekToFrame):
///      a) av_seek_frame: Fast backward keyframe seek.
///      b) avcodec_flush_buffers: Clear internal decoder state.
///      c) Forward decoding loop: Discards non-target frames without GPU transfer overhead;
///         transfers and caches the first matching frame in _pCachedFrame for immediate consumption.
///
///   5. Resource Disposal (Dispose):
///      a) Free scaler context (sws_freeContext).
///      b) Free all allocated frames and packets (av_frame_free, av_packet_free).
///      c) Free codec context (avcodec_free_context).
///      d) Release hardware device context (av_buffer_unref).
///      e) Close format context (avformat_close_input).
///
/// ── Reference Documentation ────────────────────────────────────────────────────
///   • FFmpeg Demuxing &amp; Decoding Overview: https://ffmpeg.org/doxygen/trunk/group__lavf__decoding.html
///   • Send/Receive Codec API: https://ffmpeg.org/doxygen/trunk/group__lavc__encdec.html
///   • Hardware Acceleration API: https://ffmpeg.org/doxygen/trunk/group__lavu__hwcontext.html
///   • FFmpeg HW Decoding Example: https://ffmpeg.org/doxygen/trunk/hw_decode_8c-example.html
///   • libswscale Scaling &amp; Conversion: https://ffmpeg.org/doxygen/trunk/group__libsws.html
/// </summary>
internal sealed unsafe class FFmpegNativeDecoder : IDisposable
{
    // ──────────────────────────────────────────────────────────────
    // Hardware Acceleration Priorities &amp; Candidate Structure
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Candidate hardware device configuration discovered via <c>avcodec_get_hw_config</c>.
    /// </summary>
    private record struct HwConfigCandidate
    {
        public AVHWDeviceType DeviceType;
        public AVPixelFormat PixelFormat;
    }

    /// <summary>
    /// Preferred hardware device types in priority order.
    /// Modern Windows platforms prioritize D3D11VA for wide driver support and direct integration,
    /// followed by DXVA2 (DirectX 9 / legacy), Intel QSV, NVIDIA CUDA, and DirectX 12 D3D12VA.
    /// Non-Windows systems fall back to VAAPI or VideoToolbox.
    /// </summary>
    private static readonly AVHWDeviceType[] PreferredHwDeviceTypes = new[]
    {
        AVHWDeviceType.AV_HWDEVICE_TYPE_D3D11VA,
        AVHWDeviceType.AV_HWDEVICE_TYPE_DXVA2,
        AVHWDeviceType.AV_HWDEVICE_TYPE_QSV,
        AVHWDeviceType.AV_HWDEVICE_TYPE_CUDA,
        AVHWDeviceType.AV_HWDEVICE_TYPE_D3D12VA,
        AVHWDeviceType.AV_HWDEVICE_TYPE_VAAPI,
        AVHWDeviceType.AV_HWDEVICE_TYPE_VIDEOTOOLBOX
    };

    // ──────────────────────────────────────────────────────────────
    // FFmpeg Core Struct Pointers
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Context managing container format demuxing (MP4, MKV, WebM, etc.).
    /// Used for packet demuxing and seeking.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVFormatContext.html
    /// </summary>
    private AVFormatContext* _pFormatContext = null;

    /// <summary>
    /// Context managing video decoder state and parameters.
    /// Configured with codec parameters, hardware contexts, and threading options.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVCodecContext.html
    /// </summary>
    private AVCodecContext* _pCodecContext = null;

    /// <summary>
    /// Reference-counted context for the GPU hardware device (D3D11, DXVA2, QSV, CUDA, etc.).
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVHWDeviceContext.html
    /// </summary>
    private AVBufferRef* _hwDeviceCtx = null;

    /// <summary>
    /// Frame structure holding raw decoded frames received directly from <c>avcodec_receive_frame</c>.
    /// In HW decoding mode, this contains GPU surface descriptors (e.g., ID3D11VideoDecoderOutputView).
    /// In SW decoding mode, this contains pixel buffers in system memory.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVFrame.html
    /// </summary>
    private AVFrame* _pDecodedFrame = null;

    /// <summary>
    /// Software frame structure in system RAM used to receive pixel data transferred from GPU surfaces
    /// via <c>av_hwframe_transfer_data</c>.
    /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__hwcontext.html
    /// </summary>
    private AVFrame* _pHwTransferFrame = null;

    /// <summary>
    /// Frame structure holding converted pixel data produced by <c>sws_scale</c> (e.g., RGBA or fallback YUV).
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVFrame.html
    /// </summary>
    private AVFrame* _pConvertedFrame = null;

    /// <summary>
    /// Cached frame holding decoded system-memory pixel data after a seek or lookahead operation.
    /// Consumed by the next <c>TryRead*</c> invocation.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVFrame.html
    /// </summary>
    private AVFrame* _pCachedFrame = null;
    private bool _hasCachedFrame;

    /// <summary>
    /// Packet structure temporarily holding compressed demuxed data read from the input container.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVPacket.html
    /// </summary>
    private AVPacket* _pPacket = null;

    /// <summary>
    /// libswscale colorspace and format conversion context.
    /// Handles conversions such as NV12/YUV420P → RGBA32 and format adaptation.
    /// Reference: https://ffmpeg.org/doxygen/trunk/group__libsws.html
    /// </summary>
    private SwsContext* _pSwsContext = null;

    // ──────────────────────────────────────────────────────────────
    // Hardware Acceleration State &amp; Delegates
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Delegate instance stored in managed memory to prevent GC collection while referenced by unmanaged code.
    /// Passed to <c>_pCodecContext-&gt;get_format</c>.
    /// </summary>
    private AVCodecContext_get_format? _getFormatDelegate;

    /// <summary>Whether hardware-accelerated decoding is currently active.</summary>
    private bool _isHwDecoding;

    /// <summary>Hardware device type in use (e.g. D3D11VA, DXVA2, CUDA, QSV).</summary>
    private AVHWDeviceType _hwDeviceType = AVHWDeviceType.AV_HWDEVICE_TYPE_NONE;

    /// <summary>Pixel format of the GPU surface (e.g. AV_PIX_FMT_D3D11, AV_PIX_FMT_DXVA2_VLD).</summary>
    private AVPixelFormat _hwPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;

    /// <summary>
    /// Pixel format of the frame after downloading from GPU surface to system RAM (typically NV12).
    /// </summary>
    private AVPixelFormat _downloadedPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;

    // ──────────────────────────────────────────────────────────────
    // Playback, Stream &amp; Scaler Synchronization Fields
    // ──────────────────────────────────────────────────────────────

    private int _videoStreamIndex = -1;
    private long _currentFrameIndex;
    private AVRational _timeBase;
    private bool _isOpened;
    private bool _disposed;
    private bool _isDraining;

    /// <summary>Number of video packets skipped because the decoder rejected them as invalid data.</summary>
    private int _skippedPacketCount;

    /// <summary>Number of skipped packets for which a warning is logged.</summary>
    private const int MaxLoggedSkippedPackets = 5;

    private readonly object _lock = new();

    // Fields to detect SwsContext updates
    private AVPixelFormat _srcPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;
    private AVPixelFormat _dstPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;
    private int _srcWidth;
    private int _srcHeight;

    // ──────────────────────────────────────────────────────────────
    // Public Properties
    // ──────────────────────────────────────────────────────────────

    /// <summary>Video width in pixels.</summary>
    public int Width { get; private set; }

    /// <summary>Video height in pixels.</summary>
    public int Height { get; private set; }

    /// <summary>Frames per second.</summary>
    public double FPS { get; private set; }

    /// <summary>Total duration in seconds.</summary>
    public double Duration { get; private set; }

    /// <summary>Estimated total number of frames.</summary>
    public long TotalFrames { get; private set; }

    /// <summary>Index of the currently read frame.</summary>
    public long CurrentFrameIndex
    {
        get
        {
            lock (_lock)
            {
                return _currentFrameIndex;
            }
        }
    }

    /// <summary>Current playback time in seconds.</summary>
    public double CurrentTime
    {
        get
        {
            lock (_lock)
            {
                return FPS > 0 ? _currentFrameIndex / FPS : 0;
            }
        }
    }

    /// <summary>
    /// Pixel format of the video frames produced by the decoder.
    /// When hardware decoding is active, returns the pixel format of the downloaded software frame
    /// in system RAM (typically <see cref="AVPixelFormat.AV_PIX_FMT_NV12"/>) rather than the GPU surface format
    /// (e.g., D3D11VA, DXVA2, CUDA), allowing seamless binding to Compute Shader NV12 pipelines.
    /// In software mode, returns the codec context's pixel format (e.g., YUV420P).
    /// </summary>
    public AVPixelFormat PixelFormat
    {
        get
        {
            lock (_lock)
            {
                if (_isHwDecoding)
                {
                    return _downloadedPixFmt != AVPixelFormat.AV_PIX_FMT_NONE
                        ? _downloadedPixFmt
                        : AVPixelFormat.AV_PIX_FMT_NV12;
                }

                return _pCodecContext != null ? _pCodecContext->pix_fmt : AVPixelFormat.AV_PIX_FMT_NONE;
            }
        }
    }

    /// <summary>
    /// Color range of the video stream (MPEG limited [16-235] or JPEG full [0-255]).
    /// </summary>
    public AVColorRange ColorRange
    {
        get
        {
            lock (_lock)
            {
                if (_pCodecContext != null && _pCodecContext->color_range != AVColorRange.AVCOL_RANGE_UNSPECIFIED)
                {
                    return _pCodecContext->color_range;
                }
                if (_pFormatContext != null && _videoStreamIndex >= 0)
                {
                    return _pFormatContext->streams[_videoStreamIndex]->codecpar->color_range;
                }
                return AVColorRange.AVCOL_RANGE_UNSPECIFIED;
            }
        }
    }

    /// <summary>
    /// Gets a value indicating whether hardware-accelerated decoding is currently active.
    /// </summary>
    public bool IsHardwareAccelerated
    {
        get
        {
            lock (_lock)
            {
                return _isHwDecoding;
            }
        }
    }

    /// <summary>
    /// Gets the hardware device type in use for hardware acceleration, or null if software decoding.
    /// </summary>
    public AVHWDeviceType? HardwareDeviceType
    {
        get
        {
            lock (_lock)
            {
                return _isHwDecoding ? _hwDeviceType : null;
            }
        }
    }

    // ──────────────────────────────────────────────────────────────
    // Initialization &amp; Setup
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Initializes a new instance of <see cref="FFmpegNativeDecoder"/> and opens the specified video file.
    /// Automatically attempts hardware acceleration (D3D11VA, DXVA2, QSV, CUDA), falling back
    /// to multi-threaded software decoding if unavailable.
    /// </summary>
    /// <param name="videoPath">Absolute or relative file path to the video file.</param>
    public FFmpegNativeDecoder(string videoPath)
    {
        Open(videoPath);
    }

    /// <summary>
    /// Opens the video file, discovers streams, configures hardware/software decoding, and allocates frame buffers.
    /// </summary>
    /// <param name="videoPath">Path to video file.</param>
    private void Open(string videoPath)
    {
        lock (_lock)
        {
            if (_isOpened || _disposed) return;

            try
            {
                // 1. Open media container
                // Reference: https://ffmpeg.org/doxygen/trunk/group__lavf__decoding.html
                fixed (AVFormatContext** ppFormatCtx = &_pFormatContext)
                {
                    int ret = ffmpeg.avformat_open_input(ppFormatCtx, videoPath, null, null);
                    if (ret < 0)
                        throw new Exception($"Failed to open video file: {videoPath} ({GetFFmpegErrorString(ret)})");
                }

                // 2. Retrieve stream information
                if (ffmpeg.avformat_find_stream_info(_pFormatContext, null) < 0)
                    throw new Exception("Failed to retrieve stream info.");

                // 3. Find primary video stream
                _videoStreamIndex = -1;
                for (int i = 0; i < _pFormatContext->nb_streams; i++)
                {
                    if (_pFormatContext->streams[i]->codecpar->codec_type == AVMediaType.AVMEDIA_TYPE_VIDEO)
                    {
                        _videoStreamIndex = i;
                        break;
                    }
                }

                if (_videoStreamIndex == -1)
                    throw new Exception("Failed to find video stream.");

                // Only the video stream is used. The demuxer does not need to return the packets of the other streams
                // (audio, subtitles, ...) once they are marked as discarded.
                // Reference: AVStream.discard - "Selects which packets can be discarded at will and do not need to be demuxed."
                // https://ffmpeg.org/doxygen/trunk/structAVStream.html
                for (int i = 0; i < _pFormatContext->nb_streams; i++)
                {
                    if (i != _videoStreamIndex)
                        _pFormatContext->streams[i]->discard = AVDiscard.AVDISCARD_ALL;
                }

                var stream = _pFormatContext->streams[_videoStreamIndex];
                _timeBase = stream->time_base;

                // 4. Calculate FPS
                if (stream->r_frame_rate.num > 0 && stream->r_frame_rate.den > 0)
                {
                    FPS = ffmpeg.av_q2d(stream->r_frame_rate);
                }
                else if (stream->avg_frame_rate.num > 0 && stream->avg_frame_rate.den > 0)
                {
                    FPS = ffmpeg.av_q2d(stream->avg_frame_rate);
                }
                else
                {
                    FPS = 30.0; // Fallback default
                }

                // 5. Find decoder by codec ID
                var codecId = stream->codecpar->codec_id;
                var decoder = ffmpeg.avcodec_find_decoder(codecId);
                if (decoder == null)
                    throw new Exception($"Unsupported video codec: {codecId}");

                // 6. Allocate codec context
                _pCodecContext = ffmpeg.avcodec_alloc_context3(decoder);
                if (_pCodecContext == null)
                    throw new Exception("Failed to allocate codec context.");

                if (ffmpeg.avcodec_parameters_to_context(_pCodecContext, stream->codecpar) < 0)
                    throw new Exception("Failed to copy codec parameters.");

                // 7. Attempt hardware decoding auto-discovery; fallback to multi-threaded CPU software decoding
                bool hwSuccess = TryInitializeHwDecoder(decoder, stream);
                if (!hwSuccess)
                {
                    Melon<RenderMod>.Logger.Msg("[FFmpegNativeDecoder] Hardware decoding unavailable. Falling back to multi-threaded CPU software decoding.");
                    _isHwDecoding = false;
                    _downloadedPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;

                    // Multi-threading settings for improved software decoding performance (auto thread count)
                    // Reference: https://ffmpeg.org/doxygen/trunk/structAVCodecContext.html
                    _pCodecContext->thread_count = 0;
                    _pCodecContext->thread_type = ffmpeg.FF_THREAD_FRAME;

                    int openRet = ffmpeg.avcodec_open2(_pCodecContext, decoder, null);
                    if (openRet < 0)
                        throw new Exception($"Failed to open software codec: {GetFFmpegErrorString(openRet)}");
                }

                Width = _pCodecContext->width > 0 ? _pCodecContext->width : stream->codecpar->width;
                Height = _pCodecContext->height > 0 ? _pCodecContext->height : stream->codecpar->height;

                // 8. Calculate total frames and duration
                if (stream->nb_frames > 0)
                {
                    TotalFrames = stream->nb_frames;
                }
                else if (stream->duration > 0)
                {
                    TotalFrames = (long)Math.Round(stream->duration * ffmpeg.av_q2d(stream->time_base) * FPS);
                }
                else if (_pFormatContext->duration > 0)
                {
                    TotalFrames = (long)Math.Round((_pFormatContext->duration / (double)ffmpeg.AV_TIME_BASE) * FPS);
                }
                else
                {
                    TotalFrames = 0;
                }

                // Duration of the video stream (AVStream.duration, in the stream time base).
                // When the stream does not carry a duration, the container duration (AVFormatContext.duration,
                // in AV_TIME_BASE units, the longest of all streams) is used.
                // Reference: https://ffmpeg.org/doxygen/trunk/structAVStream.html, https://ffmpeg.org/doxygen/trunk/structAVFormatContext.html
                Duration = stream->duration > 0
                    ? stream->duration * ffmpeg.av_q2d(stream->time_base)
                    : _pFormatContext->duration > 0 ? _pFormatContext->duration / (double)ffmpeg.AV_TIME_BASE : 0;

                // 9. Allocate frame and packet buffers
                // Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__frame.html
                _pDecodedFrame = ffmpeg.av_frame_alloc();
                _pHwTransferFrame = ffmpeg.av_frame_alloc();
                _pConvertedFrame = ffmpeg.av_frame_alloc();
                _pCachedFrame = ffmpeg.av_frame_alloc();
                _pPacket = ffmpeg.av_packet_alloc();

                _currentFrameIndex = 0;
                _isOpened = true;
                _isDraining = false;
            }
            catch (Exception)
            {
                Dispose();
                throw;
            }
        }
    }

    /// <summary>
    /// Searches for supported hardware device configurations via <c>avcodec_get_hw_config</c>,
    /// creates the appropriate hardware device context (D3D11VA, DXVA2, QSV, CUDA),
    /// registers the <c>get_format</c> callback, and attempts to open the hardware decoder.
    /// </summary>
    /// <param name="decoder">Target decoder.</param>
    /// <param name="stream">Source video stream.</param>
    /// <returns>True if hardware decoding was successfully configured and opened; false to fall back.</returns>
    private bool TryInitializeHwDecoder(AVCodec* decoder, AVStream* stream)
    {
        // ReSharper disable once InconsistentNaming
        // Collect all supported hardware configs from decoder
        const int AV_CODEC_HW_CONFIG_METHOD_HW_DEVICE_CTX = 0x01;
        var candidates = new List<HwConfigCandidate>();
        for (int i = 0; ; i++)
        {
            var config = ffmpeg.avcodec_get_hw_config(decoder, i);
            if (config == null) break;

            if ((config->methods & AV_CODEC_HW_CONFIG_METHOD_HW_DEVICE_CTX) != 0)
            {
                candidates.Add(new HwConfigCandidate
                {
                    DeviceType = config->device_type,
                    PixelFormat = config->pix_fmt
                });
            }
        }

        if (candidates.Count == 0)
        {
            return false;
        }

        // Order candidates according to preference
        var orderedCandidates = new List<HwConfigCandidate>();
        foreach (var preferred in PreferredHwDeviceTypes)
        {
            foreach (var candidate in candidates)
            {
                if (candidate.DeviceType == preferred && !orderedCandidates.Contains(candidate))
                {
                    orderedCandidates.Add(candidate);
                }
            }
        }

        // Add any remaining candidates not explicitly listed in PreferredHwDeviceTypes
        foreach (var candidate in candidates)
        {
            if (!orderedCandidates.Contains(candidate))
            {
                orderedCandidates.Add(candidate);
            }
        }

        // Try candidate hardware configurations in priority order
        foreach (var candidate in orderedCandidates)
        {
            AVBufferRef* hwDeviceCtx = null;
            int err;

            // Device creation strategy
            // Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__hwcontext.html
            if (candidate.DeviceType == AVHWDeviceType.AV_HWDEVICE_TYPE_QSV)
            {
                var isWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
                string childDeviceName = isWindows ? "d3d11va" : "vaapi";
                AVDictionary* opts = null;
                ffmpeg.av_dict_set(&opts, "child_device_type", childDeviceName, 0);

                err = ffmpeg.av_hwdevice_ctx_create(&hwDeviceCtx, AVHWDeviceType.AV_HWDEVICE_TYPE_QSV, null, opts, 0);
                ffmpeg.av_dict_free(&opts);

                if (err < 0)
                {
                    err = ffmpeg.av_hwdevice_ctx_create(&hwDeviceCtx, AVHWDeviceType.AV_HWDEVICE_TYPE_QSV, null, null, 0);
                }
            }
            else
            {
                err = ffmpeg.av_hwdevice_ctx_create(&hwDeviceCtx, candidate.DeviceType, null, null, 0);
            }

            if (err < 0 || hwDeviceCtx == null)
            {
                if (hwDeviceCtx != null)
                {
                    ffmpeg.av_buffer_unref(&hwDeviceCtx);
                    hwDeviceCtx = null;
                }
                Melon<RenderMod>.Logger.Msg($"[FFmpegNativeDecoder] Hardware device creation failed for {candidate.DeviceType}: {GetFFmpegErrorString(err)}. Trying next candidate...");
                continue;
            }

            // Assign hardware context and format selection callback
            _hwDeviceCtx = hwDeviceCtx;
            _hwPixFmt = candidate.PixelFormat;
            _hwDeviceType = candidate.DeviceType;
            _isHwDecoding = true;
            _downloadedPixFmt = AVPixelFormat.AV_PIX_FMT_NV12;

            _pCodecContext->hw_device_ctx = ffmpeg.av_buffer_ref(_hwDeviceCtx);
            if (_pCodecContext->hw_device_ctx == null)
            {
                Melon<RenderMod>.Logger.Warning($"[FFmpegNativeDecoder] Failed to reference HW device context for {candidate.DeviceType}. Retrying next candidate...");
                var pHw = _hwDeviceCtx;
                ffmpeg.av_buffer_unref(&pHw);
                _hwDeviceCtx = null;
                _isHwDecoding = false;
                continue;
            }

            _getFormatDelegate = GetFormat;
            _pCodecContext->get_format = _getFormatDelegate;

            // Hardware decoding requires single-threaded decoding (thread_count = 1).
            // Multi-threading (thread_count = 0 / FF_THREAD_FRAME) is incompatible with fixed-function
            // GPU decoders and causes surface pool allocation exhaustion or crashes in get_format.
            _pCodecContext->thread_count = 1;

            // Attempt opening codec context with hardware configuration
            int openRet = ffmpeg.avcodec_open2(_pCodecContext, decoder, null);
            if (openRet >= 0)
            {
                Melon<RenderMod>.Logger.Msg($"[FFmpegNativeDecoder] Hardware decoding enabled successfully: Device={candidate.DeviceType}, SurfaceFormat={candidate.PixelFormat}");
                return true;
            }

            // Failed to open codec with this hardware configuration, clean up and try next
            Melon<RenderMod>.Logger.Warning($"[FFmpegNativeDecoder] avcodec_open2 failed with HW device {candidate.DeviceType}: {GetFFmpegErrorString(openRet)}. Retrying next candidate...");

            if (_pCodecContext != null)
            {
                var pCodecCtx = _pCodecContext;
                ffmpeg.avcodec_free_context(&pCodecCtx);
                _pCodecContext = null;
            }

            if (_hwDeviceCtx != null)
            {
                var pHw = _hwDeviceCtx;
                ffmpeg.av_buffer_unref(&pHw);
                _hwDeviceCtx = null;
            }

            _isHwDecoding = false;
            _getFormatDelegate = null;

            // Reallocate fresh codec context for subsequent attempts
            _pCodecContext = ffmpeg.avcodec_alloc_context3(decoder);
            if (_pCodecContext == null)
                throw new Exception("Failed to re-allocate codec context during fallback.");

            if (ffmpeg.avcodec_parameters_to_context(_pCodecContext, stream->codecpar) < 0)
                throw new Exception("Failed to copy codec parameters during fallback.");
        }

        return false;
    }

    /// <summary>
    /// Callback invoked by libavcodec during decoding to negotiate the output pixel format.
    /// Confirms the hardware surface format (<see cref="_hwPixFmt"/>) if offered by the decoder,
    /// or falls back to software pixel formats via <c>avcodec_default_get_format</c>.
    /// Reference: https://ffmpeg.org/doxygen/trunk/structAVCodecContext.html
    /// </summary>
    private AVPixelFormat GetFormat(AVCodecContext* ctx, AVPixelFormat* pixFmts)
    {
        for (var p = pixFmts; *p != AVPixelFormat.AV_PIX_FMT_NONE; p++)
        {
            if (*p == _hwPixFmt)
            {
                return *p;
            }
        }

        Melon<RenderMod>.Logger.Warning($"[FFmpegNativeDecoder] Hardware surface format {_hwPixFmt} not offered by decoder for this stream. Falling back to software format.");
        _isHwDecoding = false;
        _downloadedPixFmt = AVPixelFormat.AV_PIX_FMT_NONE;
        return ffmpeg.avcodec_default_get_format(ctx, pixFmts);
    }

    // ──────────────────────────────────────────────────────────────
    // Demuxing &amp; Decoding Engine
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Reads packets from the container until a packet belonging to the video stream is found,
    /// then sends it to the decoder via <c>avcodec_send_packet</c>.
    /// A packet that the decoder rejects as invalid data (<c>AVERROR_INVALIDDATA</c>, e.g. a damaged packet)
    /// is skipped and the next packet is read. Any other decoder error is thrown.
    /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavc__encdec.html
    /// </summary>
    /// <returns>True if a video packet was sent; false on EOF.</returns>
    private bool ReadAndSendPacket()
    {
        while (true)
        {
            ffmpeg.av_packet_unref(_pPacket);
            int ret = ffmpeg.av_read_frame(_pFormatContext, _pPacket);
            if (ret < 0) return false; // EOF or read error

            if (_pPacket->stream_index == _videoStreamIndex)
            {
                ret = ffmpeg.avcodec_send_packet(_pCodecContext, _pPacket);
                ffmpeg.av_packet_unref(_pPacket);

                if (ret == ffmpeg.AVERROR_INVALIDDATA)
                {
                    // The warning is logged for the first few packets only (a damaged file can have many).
                    _skippedPacketCount++;
                    if (_skippedPacketCount <= MaxLoggedSkippedPackets)
                    {
                        Melon<RenderMod>.Logger.Warning($"[FFmpegNativeDecoder] Skipped an invalid video packet ({_skippedPacketCount}): {GetFFmpegErrorString(ret)}");
                    }
                    continue;
                }

                if (ret < 0)
                {
                    throw new Exception($"Error sending packet to decoder: {GetFFmpegErrorString(ret)}");
                }
                return true;
            }
        }
    }

    /// <summary>
    /// Receives the next decoded video frame from the codec context.
    /// If the frame is decoded on a GPU hardware surface (D3D11VA, DXVA2, CUDA, QSV),
    /// it is automatically downloaded to system RAM via <c>av_hwframe_transfer_data</c> into <c>_pHwTransferFrame</c>,
    /// preserving presentation timestamps via <c>av_frame_copy_props</c>.
    /// </summary>
    /// <param name="outFrame">The software frame containing pixel data in system RAM.</param>
    /// <returns>True if a frame was successfully decoded and available; false on EOF or draining.</returns>
    private bool TryReceiveFrame(out AVFrame* outFrame)
    {
        outFrame = null;

        while (true)
        {
            int ret = ffmpeg.avcodec_receive_frame(_pCodecContext, _pDecodedFrame);
            if (ret == 0)
            {
                // Check if the received frame is on a hardware surface
                if (_isHwDecoding && _pDecodedFrame->format == (int)_hwPixFmt)
                {
                    ffmpeg.av_frame_unref(_pHwTransferFrame);
                    int transferRet = ffmpeg.av_hwframe_transfer_data(_pHwTransferFrame, _pDecodedFrame, 0);
                    if (transferRet < 0)
                    {
                        ffmpeg.av_frame_unref(_pDecodedFrame);
                        throw new Exception($"Failed to transfer hardware frame to system memory: {GetFFmpegErrorString(transferRet)}");
                    }

                    // Copy presentation timestamp and metadata properties
                    ffmpeg.av_frame_copy_props(_pHwTransferFrame, _pDecodedFrame);
                    ffmpeg.av_frame_unref(_pDecodedFrame);

                    _downloadedPixFmt = (AVPixelFormat)_pHwTransferFrame->format;
                    outFrame = _pHwTransferFrame;
                    return true;
                }

                // Frame is in system memory (CPU decoding or software fallback format)
                outFrame = _pDecodedFrame;
                return true;
            }

            if (ret == ffmpeg.AVERROR(ffmpeg.EAGAIN))
            {
                if (_isDraining) return false;

                if (!ReadAndSendPacket())
                {
                    _isDraining = true;
                    ffmpeg.avcodec_send_packet(_pCodecContext, null);
                }
            }
            else if (ret == ffmpeg.AVERROR_EOF)
            {
                return false;
            }
            else
            {
                throw new Exception($"Error during decoding: {GetFFmpegErrorString(ret)}");
            }
        }
    }

    /// <summary>
    /// Lazily initializes or updates the <see cref="SwsContext"/> when source/destination formats or dimensions change.
    /// Reference: https://ffmpeg.org/doxygen/trunk/group__libsws.html
    /// </summary>
    private void InitOrUpdateSwsContext(AVPixelFormat srcFormat, int srcWidth, int srcHeight, AVPixelFormat dstFormat)
    {
        if (_pSwsContext != null && _srcPixFmt == srcFormat && _srcWidth == srcWidth && _srcHeight == srcHeight && _dstPixFmt == dstFormat)
        {
            return;
        }

        if (_pSwsContext != null)
        {
            ffmpeg.sws_freeContext(_pSwsContext);
            _pSwsContext = null;
        }

        _srcPixFmt = srcFormat;
        _srcWidth = srcWidth;
        _srcHeight = srcHeight;
        _dstPixFmt = dstFormat;

        _pSwsContext = ffmpeg.sws_getContext(
            srcWidth, srcHeight, srcFormat,
            Width, Height, dstFormat,
            (int)SwsFlags.SWS_BILINEAR, null, null, null
        );

        if (_pSwsContext == null)
        {
            throw new Exception($"Failed to initialize SwsContext (src: {srcFormat} {srcWidth}x{srcHeight} -> dst: {dstFormat} {Width}x{Height}).");
        }
    }

    /// <summary>
    /// Computes presentation timestamp (PTS) in seconds from frame metadata without modifying decoder state.
    /// Used by seeking to inspect candidate frames without falsely incrementing frame indices.
    /// </summary>
    private double GetFrameTimestamp(AVFrame* frame)
    {
        long ptsValue = frame->best_effort_timestamp;
        if (ptsValue == ffmpeg.AV_NOPTS_VALUE)
        {
            ptsValue = frame->pts;
        }

        if (ptsValue == ffmpeg.AV_NOPTS_VALUE)
        {
            return FPS > 0 ? _currentFrameIndex / FPS : 0;
        }

        return ptsValue * ffmpeg.av_q2d(_timeBase);
    }

    /// <summary>
    /// Computes playback presentation timestamp (PTS) in seconds from frame metadata and updates the frame counter.
    /// </summary>
    private double GetTimestampAndUpdateIndex(AVFrame* frame)
    {
        double timestamp = GetFrameTimestamp(frame);
        long ptsValue = frame->best_effort_timestamp != ffmpeg.AV_NOPTS_VALUE
            ? frame->best_effort_timestamp
            : frame->pts;

        if (ptsValue == ffmpeg.AV_NOPTS_VALUE)
        {
            _currentFrameIndex++;
        }
        else
        {
            _currentFrameIndex = (long)Math.Round(timestamp * FPS);
        }
        return timestamp;
    }

    /// <summary>
    /// Resets all data plane pointers and strides in <see cref="_pConvertedFrame"/> to zero.
    /// Prevents stale or dangling pointers from previous conversions being retained across frames.
    /// </summary>
    private void ClearConvertedFramePointers()
    {
        if (_pConvertedFrame == null) return;
        for (uint i = 0; i < 8; i++)
        {
            _pConvertedFrame->data[i] = null;
            _pConvertedFrame->linesize[i] = 0;
        }
    }

    // ──────────────────────────────────────────────────────────────
    // Frame Reading APIs: RGBA (CPU Fallback)
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Reads the next video frame and copies it in 32-bit RGBA format using libswscale.
    /// Safely processes both hardware-downloaded frames (NV12) and software-decoded frames (YUV420P).
    /// </summary>
    /// <param name="outBuffer">Pointer to destination memory storing RGBA data.</param>
    /// <param name="outStride">Byte stride per row of destination memory (Width * 4).</param>
    /// <param name="vflip">Whether to vertically flip the output to match Unity texture coordinates.</param>
    /// <param name="timestamp">Timestamp of the output frame in seconds.</param>
    /// <returns>True if a frame was successfully retrieved; false on EOF.</returns>
    public bool TryReadFrame(byte* outBuffer, int outStride, bool vflip, out double timestamp)
    {
        timestamp = 0;
        lock (_lock)
        {
            if (!_isOpened || _disposed || outBuffer == null) return false;

            AVFrame* frame;
            if (_hasCachedFrame)
            {
                frame = _pCachedFrame;
                _hasCachedFrame = false;
            }
            else
            {
                if (!TryReceiveFrame(out frame)) return false;
            }

            if (frame == null) return false;

            timestamp = GetTimestampAndUpdateIndex(frame);
            InitOrUpdateSwsContext((AVPixelFormat)frame->format, frame->width, frame->height, AVPixelFormat.AV_PIX_FMT_RGBA);

            ClearConvertedFramePointers();
            if (vflip)
            {
                _pConvertedFrame->data[0] = outBuffer + outStride * (Height - 1);
                _pConvertedFrame->linesize[0] = -outStride;
            }
            else
            {
                _pConvertedFrame->data[0] = outBuffer;
                _pConvertedFrame->linesize[0] = outStride;
            }

            ffmpeg.sws_scale(
                _pSwsContext,
                frame->data, frame->linesize,
                0, frame->height,
                _pConvertedFrame->data, _pConvertedFrame->linesize
            );

            ClearConvertedFramePointers();
            ffmpeg.av_frame_unref(frame);
            return true;
        }
    }

    /// <summary>
    /// Reads the next video frame and copies it in RGBA format to an unmanaged memory pointer.
    /// </summary>
    public bool TryReadFrame(IntPtr outBuffer, int outStride, bool vflip, out double timestamp)
    {
        return TryReadFrame((byte*)outBuffer.ToPointer(), outStride, vflip, out timestamp);
    }

    /// <summary>
    /// Reads the next video frame and copies it in RGBA format to a Unity NativeArray.
    /// </summary>
    public bool TryReadFrame(NativeArray<byte> outBuffer, bool vflip, out double timestamp)
    {
        if (!outBuffer.IsCreated)
        {
            throw new ArgumentException("Output buffer is not created or already disposed.", nameof(outBuffer));
        }
        if (outBuffer.Length < (long)Width * Height * 4)
        {
            throw new ArgumentException("Output buffer size is smaller than video resolution.", nameof(outBuffer));
        }
        int stride = Width * 4;
        return TryReadFrame((byte*)outBuffer.GetUnsafePtr(), stride, vflip, out timestamp);
    }

    // ──────────────────────────────────────────────────────────────
    // Frame Reading APIs: NV12 (Compute Shader Pipeline)
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Reads the next video frame and copies its Y and interleaved UV channel data separately in NV12 format.
    /// When hardware decoding is active (producing NV12 in RAM), CPU color conversion is completely bypassed,
    /// enabling maximum memory copy throughput directly into Unity textures for Compute Shader processing.
    /// Other formats or mismatched dimensions are automatically converted and scaled to NV12 via libswscale.
    /// </summary>
    /// <param name="yBuffer">Pointer to destination buffer for Y plane (Width * Height bytes).</param>
    /// <param name="yStride">Row stride in bytes for Y plane.</param>
    /// <param name="uvBuffer">Pointer to destination buffer for interleaved UV plane (Width * (Height / 2) bytes).</param>
    /// <param name="uvStride">Row stride in bytes for UV plane.</param>
    /// <param name="timestamp">Timestamp of the output frame in seconds.</param>
    /// <returns>True if a frame was successfully retrieved; false on EOF.</returns>
    // ReSharper disable once InconsistentNaming
    public bool TryReadNV12Frame(
        byte* yBuffer, int yStride,
        byte* uvBuffer, int uvStride,
        out double timestamp)
    {
        timestamp = 0;
        lock (_lock)
        {
            if (!_isOpened || _disposed || yBuffer == null || uvBuffer == null) return false;

            AVFrame* frame;
            if (_hasCachedFrame)
            {
                frame = _pCachedFrame;
                _hasCachedFrame = false;
            }
            else
            {
                if (!TryReceiveFrame(out frame)) return false;
            }

            if (frame == null) return false;

            timestamp = GetTimestampAndUpdateIndex(frame);

            if (frame->format != (int)AVPixelFormat.AV_PIX_FMT_NV12 || frame->width != Width || frame->height != Height)
            {
                InitOrUpdateSwsContext((AVPixelFormat)frame->format, frame->width, frame->height, AVPixelFormat.AV_PIX_FMT_NV12);

                ClearConvertedFramePointers();
                _pConvertedFrame->data[0] = yBuffer;
                _pConvertedFrame->data[1] = uvBuffer;
                _pConvertedFrame->linesize[0] = yStride;
                _pConvertedFrame->linesize[1] = uvStride;

                ffmpeg.sws_scale(
                    _pSwsContext,
                    frame->data, frame->linesize,
                    0, frame->height,
                    _pConvertedFrame->data, _pConvertedFrame->linesize
                );
                ClearConvertedFramePointers();
            }
            else
            {
                int w = Width;
                int h = Height;
                int uvH = (h + 1) / 2;
                int uvRowBytes = (w / 2) * 2;

                // Copy Y plane (row by row to safely skip linesize GPU padding)
                for (int y = 0; y < h; y++)
                {
                    Buffer.MemoryCopy(
                        frame->data[0] + y * frame->linesize[0],
                        yBuffer + y * yStride,
                        yStride, w
                    );
                }

                // Copy UV plane (interleaved U and V samples)
                for (int y = 0; y < uvH; y++)
                {
                    Buffer.MemoryCopy(
                        frame->data[1] + y * frame->linesize[1],
                        uvBuffer + y * uvStride,
                        uvStride, uvRowBytes
                    );
                }
            }

            ffmpeg.av_frame_unref(frame);
            return true;
        }
    }

    /// <summary>
    /// Reads the next video frame and copies its NV12 Y and interleaved UV channel data to unmanaged memory pointers.
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public bool TryReadNV12Frame(
        IntPtr yBuffer, int yStride,
        IntPtr uvBuffer, int uvStride,
        out double timestamp)
    {
        return TryReadNV12Frame(
            (byte*)yBuffer.ToPointer(), yStride,
            (byte*)uvBuffer.ToPointer(), uvStride,
            out timestamp
        );
    }

    /// <summary>
    /// Reads the next video frame and copies its NV12 Y and interleaved UV channel data into Unity NativeArrays.
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public bool TryReadNV12Frame(
        NativeArray<byte> yBuffer,
        NativeArray<byte> uvBuffer,
        out double timestamp)
    {
        if (!yBuffer.IsCreated || !uvBuffer.IsCreated)
        {
            throw new ArgumentException("Input NativeArray buffer is not created or already disposed.");
        }

        int w = Width;
        int h = Height;
        int uvH = (h + 1) / 2;
        int uvRowBytes = (w / 2) * 2;

        if (yBuffer.Length < (long)w * h || uvBuffer.Length < (long)uvRowBytes * uvH)
        {
            throw new ArgumentException("Buffer size is smaller than video NV12 plane resolution.");
        }

        return TryReadNV12Frame(
            (byte*)yBuffer.GetUnsafePtr(), w,
            (byte*)uvBuffer.GetUnsafePtr(), uvRowBytes,
            out timestamp
        );
    }

    // ──────────────────────────────────────────────────────────────
    // Frame Reading APIs: YUV420P (Compute Shader Pipeline)
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Reads the next video frame and copies its planar Y, U, and V channel data separately.
    /// For native YUV420P sources, color conversion is bypassed and memory is copied directly.
    /// Non-YUV420P frames (including NV12 from HW decoding) or mismatched dimensions are converted via libswscale.
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public bool TryReadYUV420PFrame(
        byte* yBuffer, int yStride,
        byte* uBuffer, int uStride,
        byte* vBuffer, int vStride,
        out double timestamp)
    {
        timestamp = 0;
        lock (_lock)
        {
            if (!_isOpened || _disposed || yBuffer == null || uBuffer == null || vBuffer == null) return false;

            AVFrame* frame;
            if (_hasCachedFrame)
            {
                frame = _pCachedFrame;
                _hasCachedFrame = false;
            }
            else
            {
                if (!TryReceiveFrame(out frame)) return false;
            }

            if (frame == null) return false;

            timestamp = GetTimestampAndUpdateIndex(frame);

            if ((frame->format != (int)AVPixelFormat.AV_PIX_FMT_YUV420P && frame->format != (int)AVPixelFormat.AV_PIX_FMT_YUVJ420P) || frame->width != Width || frame->height != Height)
            {
                InitOrUpdateSwsContext((AVPixelFormat)frame->format, frame->width, frame->height, AVPixelFormat.AV_PIX_FMT_YUV420P);

                ClearConvertedFramePointers();
                _pConvertedFrame->data[0] = yBuffer;
                _pConvertedFrame->data[1] = uBuffer;
                _pConvertedFrame->data[2] = vBuffer;
                _pConvertedFrame->linesize[0] = yStride;
                _pConvertedFrame->linesize[1] = uStride;
                _pConvertedFrame->linesize[2] = vStride;

                ffmpeg.sws_scale(
                    _pSwsContext,
                    frame->data, frame->linesize,
                    0, frame->height,
                    _pConvertedFrame->data, _pConvertedFrame->linesize
                );
                ClearConvertedFramePointers();
            }
            else
            {
                int w = Width;
                int h = Height;
                int uvW = (w + 1) / 2;
                int uvH = (h + 1) / 2;

                // Copy Y plane
                for (int y = 0; y < h; y++)
                {
                    Buffer.MemoryCopy(
                        frame->data[0] + y * frame->linesize[0],
                        yBuffer + y * yStride,
                        yStride, w
                    );
                }

                // Copy U plane
                for (int y = 0; y < uvH; y++)
                {
                    Buffer.MemoryCopy(
                        frame->data[1] + y * frame->linesize[1],
                        uBuffer + y * uStride,
                        uStride, uvW
                    );
                }

                // Copy V plane
                for (int y = 0; y < uvH; y++)
                {
                    Buffer.MemoryCopy(
                        frame->data[2] + y * frame->linesize[2],
                        vBuffer + y * vStride,
                        vStride, uvW
                    );
                }
            }

            ffmpeg.av_frame_unref(frame);
            return true;
        }
    }

    /// <summary>
    /// Reads the next video frame and copies its planar Y, U, and V channel data to unmanaged memory pointers.
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public bool TryReadYUV420PFrame(
        IntPtr yBuffer, int yStride,
        IntPtr uBuffer, int uStride,
        IntPtr vBuffer, int vStride,
        out double timestamp)
    {
        return TryReadYUV420PFrame(
            (byte*)yBuffer.ToPointer(), yStride,
            (byte*)uBuffer.ToPointer(), uStride,
            (byte*)vBuffer.ToPointer(), vStride,
            out timestamp
        );
    }

    /// <summary>
    /// Reads the next video frame and copies its planar Y, U, and V channel data into Unity NativeArrays.
    /// </summary>
    // ReSharper disable once InconsistentNaming
    public bool TryReadYUV420PFrame(
        NativeArray<byte> yBuffer,
        NativeArray<byte> uBuffer,
        NativeArray<byte> vBuffer,
        out double timestamp)
    {
        if (!yBuffer.IsCreated || !uBuffer.IsCreated || !vBuffer.IsCreated)
        {
            throw new ArgumentException("Input NativeArray buffer is not created or already disposed.");
        }

        int w = Width;
        int h = Height;
        int uvW = (w + 1) / 2;
        int uvH = (h + 1) / 2;

        if (yBuffer.Length < (long)w * h || uBuffer.Length < (long)uvW * uvH || vBuffer.Length < (long)uvW * uvH)
        {
            throw new ArgumentException("Buffer size is smaller than video YUV plane resolution.");
        }

        return TryReadYUV420PFrame(
            (byte*)yBuffer.GetUnsafePtr(), w,
            (byte*)uBuffer.GetUnsafePtr(), uvW,
            (byte*)vBuffer.GetUnsafePtr(), uvW,
            out timestamp
        );
    }

    // ──────────────────────────────────────────────────────────────
    // Seeking &amp; Skipping
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Maximum distance in frames over which <see cref="SeekToTime"/> moves forward by decoding.
    /// Decoding forward needs no keyframe seek and no decoder flush, which is cheaper than seeking for short distances.
    /// </summary>
    private const int MaxForwardDecodeFrames = 60;

    /// <summary>
    /// Decodes forward from the current decoder position until the first frame whose presentation time is at least
    /// <paramref name="timeSeconds"/> minus half a frame, and keeps that frame in <see cref="_pCachedFrame"/>
    /// for the next <c>TryRead*</c> call.
    /// Frames before the target are discarded without GPU-to-host memory transfer; only the target frame is transferred.
    /// When the end of the stream is reached before the target, no frame is cached.
    /// </summary>
    /// <param name="timeSeconds">Target time in seconds.</param>
    private void DecodeToTarget(double timeSeconds)
    {
        while (true)
        {
            int ret = ffmpeg.avcodec_receive_frame(_pCodecContext, _pDecodedFrame);
            if (ret == 0)
            {
                double currentPts = GetFrameTimestamp(_pDecodedFrame);

                // Check with half-frame margin to accommodate floating point rounding
                if (currentPts >= timeSeconds - (0.5 / FPS))
                {
                    // Cache the first matching frame in system RAM for immediate consumption
                    if (_isHwDecoding && _pDecodedFrame->format == (int)_hwPixFmt)
                    {
                        ffmpeg.av_frame_unref(_pCachedFrame);
                        int transferRet = ffmpeg.av_hwframe_transfer_data(_pCachedFrame, _pDecodedFrame, 0);
                        if (transferRet < 0)
                        {
                            ffmpeg.av_frame_unref(_pDecodedFrame);
                            throw new Exception($"Failed to transfer cached hardware frame during seek: {GetFFmpegErrorString(transferRet)}");
                        }

                        ffmpeg.av_frame_copy_props(_pCachedFrame, _pDecodedFrame);
                        ffmpeg.av_frame_unref(_pDecodedFrame);

                        _downloadedPixFmt = (AVPixelFormat)_pCachedFrame->format;
                    }
                    else
                    {
                        ffmpeg.av_frame_unref(_pCachedFrame);
                        ffmpeg.av_frame_move_ref(_pCachedFrame, _pDecodedFrame);
                    }

                    _hasCachedFrame = true;
                    return;
                }

                // Discard non-matching intermediate frame without transferring to CPU
                GetTimestampAndUpdateIndex(_pDecodedFrame);
                ffmpeg.av_frame_unref(_pDecodedFrame);
                continue;
            }

            if (ret == ffmpeg.AVERROR(ffmpeg.EAGAIN))
            {
                if (_isDraining) return;

                if (!ReadAndSendPacket())
                {
                    _isDraining = true;
                    ffmpeg.avcodec_send_packet(_pCodecContext, null);
                }
            }
            else if (ret == ffmpeg.AVERROR_EOF)
            {
                return;
            }
            else
            {
                throw new Exception($"Error during seek decoding: {GetFFmpegErrorString(ret)}");
            }
        }
    }

    /// <summary>
    /// Moves the playback position to a time in seconds.
    /// The next <c>TryRead*</c> call returns the first frame whose presentation time is at least
    /// <paramref name="timeSeconds"/> minus half a frame (the frame nearest to the target time).
    /// Both paths below return that same frame:
    ///   - Forward decode: the target is ahead of the current position by less than <see cref="MaxForwardDecodeFrames"/> frames.
    ///     Frames are decoded and discarded up to the target.
    ///   - Keyframe seek: all other cases (target before the current position, far ahead, or the target frame already read).
    ///     A backward keyframe seek is followed by a forward decode up to the target.
    /// The target frame is transferred and preserved in <see cref="_pCachedFrame"/>; when it is already there, nothing is decoded.
    /// </summary>
    /// <param name="timeSeconds">Target time in seconds to seek to.</param>
    public void SeekToTime(double timeSeconds)
    {
        lock (_lock)
        {
            if (!_isOpened || _disposed) return;

            if (timeSeconds < 0) timeSeconds = 0;

            // Earliest presentation time that satisfies the target (target minus half a frame).
            double threshold = timeSeconds - (0.5 / FPS);

            // Index of the first frame that satisfies the target. The epsilon keeps a frame that lies exactly on the
            // margin from being excluded by floating point rounding.
            double targetFrameIndex = Math.Ceiling(timeSeconds * FPS - 0.5 - 1e-6);

            if (_hasCachedFrame)
            {
                // A frame is already positioned for the next read (e.g. by a previous SeekToTime call).
                double cachedPts = GetFrameTimestamp(_pCachedFrame);
                if (cachedPts >= threshold)
                {
                    // The cached frame is the first frame that satisfies the target.
                    // When the target lies before it, the frames in between are no longer available and a keyframe seek is required.
                    if (cachedPts - threshold < 1.0 / FPS) return;
                }
                else if (targetFrameIndex - _currentFrameIndex < MaxForwardDecodeFrames)
                {
                    // The cached frame lies before the target: drop it and continue decoding forward.
                    GetTimestampAndUpdateIndex(_pCachedFrame);
                    ffmpeg.av_frame_unref(_pCachedFrame);
                    _hasCachedFrame = false;
                    DecodeToTarget(timeSeconds);
                    return;
                }
            }
            else if (targetFrameIndex > _currentFrameIndex && targetFrameIndex - _currentFrameIndex < MaxForwardDecodeFrames)
            {
                // The target frame has not been read yet and is close: decode forward instead of seeking.
                DecodeToTarget(timeSeconds);
                return;
            }

            if (_hasCachedFrame)
            {
                ffmpeg.av_frame_unref(_pCachedFrame);
                _hasCachedFrame = false;
            }

            long targetPts = (long)Math.Round(timeSeconds / ffmpeg.av_q2d(_timeBase));
            int ret = ffmpeg.av_seek_frame(_pFormatContext, _videoStreamIndex, targetPts, ffmpeg.AVSEEK_FLAG_BACKWARD);
            if (ret < 0)
            {
                // Fallback to beginning of file on seek failure
                ret = ffmpeg.av_seek_frame(_pFormatContext, _videoStreamIndex, 0, ffmpeg.AVSEEK_FLAG_BACKWARD);
                if (ret < 0)
                {
                    throw new Exception($"Seek failed: {GetFFmpegErrorString(ret)}");
                }
            }

            // Flush decoder internal buffers
            // Reference: https://ffmpeg.org/doxygen/trunk/group__lavc__encdec.html
            ffmpeg.avcodec_flush_buffers(_pCodecContext);
            _isDraining = false;

            // Decode frames until reaching the target presentation timestamp
            DecodeToTarget(timeSeconds);
        }
    }

    /// <summary>
    /// Seeks to a specific frame index.
    /// </summary>
    /// <param name="frameIndex">Target frame index.</param>
    public void SeekToFrame(long frameIndex)
    {
        SeekToTime(frameIndex / FPS);
    }

    // ──────────────────────────────────────────────────────────────
    // Error Handling &amp; Disposal
    // ──────────────────────────────────────────────────────────────

    /// <summary>
    /// Translates an FFmpeg integer error code to a readable string using <c>av_strerror</c>.
    /// Reference: https://ffmpeg.org/doxygen/trunk/group__lavu__error.html
    /// </summary>
    private string GetFFmpegErrorString(int errnum)
    {
        const ulong bufferSize = 1024;
        byte* buffer = stackalloc byte[(int)bufferSize];
        if (ffmpeg.av_strerror(errnum, buffer, bufferSize) >= 0)
        {
            return Marshal.PtrToStringAnsi((IntPtr)buffer) ?? $"Unknown error (Code: {errnum})";
        }
        return $"Unknown error (Code: {errnum})";
    }

    /// <summary>
    /// Releases all native FFmpeg decoder resources, hardware contexts, frame buffers, and scaler structures.
    /// </summary>
    public void Dispose()
    {
        lock (_lock)
        {
            if (_disposed) return;
            _disposed = true;

            // 1. Release scaler context
            if (_pSwsContext != null)
            {
                ffmpeg.sws_freeContext(_pSwsContext);
                _pSwsContext = null;
            }

            // 2. Release frame buffers
            if (_pDecodedFrame != null)
            {
                var pFrame = _pDecodedFrame;
                ffmpeg.av_frame_free(&pFrame);
                _pDecodedFrame = null;
            }

            if (_pHwTransferFrame != null)
            {
                var pFrame = _pHwTransferFrame;
                ffmpeg.av_frame_free(&pFrame);
                _pHwTransferFrame = null;
            }

            if (_pCachedFrame != null)
            {
                var pFrame = _pCachedFrame;
                ffmpeg.av_frame_free(&pFrame);
                _pCachedFrame = null;
            }

            if (_pConvertedFrame != null)
            {
                var pFrame = _pConvertedFrame;
                ffmpeg.av_frame_free(&pFrame);
                _pConvertedFrame = null;
            }

            // 3. Release packet buffer
            if (_pPacket != null)
            {
                var pPacket = _pPacket;
                ffmpeg.av_packet_free(&pPacket);
                _pPacket = null;
            }

            // 4. Release codec context
            if (_pCodecContext != null)
            {
                var pCodecCtx = _pCodecContext;
                ffmpeg.avcodec_free_context(&pCodecCtx);
                _pCodecContext = null;
            }

            // 5. Release hardware device context
            if (_hwDeviceCtx != null)
            {
                var pHw = _hwDeviceCtx;
                ffmpeg.av_buffer_unref(&pHw);
                _hwDeviceCtx = null;
            }

            // 6. Close input file and release format context
            if (_pFormatContext != null)
            {
                var pFormatCtx = _pFormatContext;
                ffmpeg.avformat_close_input(&pFormatCtx);
                _pFormatContext = null;
            }

            _getFormatDelegate = null;
            _isOpened = false;
            _isHwDecoding = false;
            GC.SuppressFinalize(this);
        }
    }

    ~FFmpegNativeDecoder()
    {
        Dispose();
    }
}