feat(audio): v1 signature producer, bit-exact with the pipeline
`AudioSignature` is the DSP — band table, periodic Hann, radix-2 FFT, band-mean peak, energy class, packing — and `AudioSignatureService` the decode, running the FFmpeg binary `IMediaEncoder.EncoderPath` names. The plugin gained no dependency. The server specification's prose does not determine a byte stream, so the parameters it leaves open are pinned by the fixture shared with the extraction repo and restated at the top of `AudioSignature`: double throughout, whole frames only, periodic Hann, unnormalised FFT, band mean rather than sum, argmax ties to the lowest index. `fixtures/audio/` holds the extraction repo's three files byte-identically and the computed signature equals the recorded vector exactly. The binding check regenerates the fixture PCM from `make_fixture.py`'s arithmetic and verifies it against the recorded decode checksums, so it runs on a host with no codec at all and a decode divergence stays distinguishable from a DSP one; the two tests that drive real FFmpeg self-skip without a binary. The workflow named "Test Plugin" until now only compiled one. A test that is built and never run is not evidence, and a golden vector shared across two repos exists precisely so CI fails when they drift. TRACES: JR-042, JR-043 | SR-003
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using System;
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using System.Buffers.Binary;
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using System.Diagnostics;
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using System.Globalization;
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using System.IO;
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using System.Threading;
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using System.Threading.Tasks;
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using MediaBrowser.Controller.MediaEncoding;
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using Microsoft.Extensions.Logging;
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namespace Jellyfin.Plugin.JRay.Services;
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/// <summary>
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/// Computes the v1 audio signature for a media file, decoding its centre window
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/// with the FFmpeg binary Jellyfin already ships.
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/// </summary>
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/// <remarks>
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/// <b>No new dependency.</b> FFmpeg performs decode, downmix and resample; the
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/// plugin adds only the fixed FFT and bin-peak extraction in
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/// <see cref="AudioSignature"/>. The binary is reached through
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/// <see cref="IMediaEncoder.EncoderPath"/>, so an installation that can
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/// transcode can compute signatures, with nothing further to install and no
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/// second copy of FFmpeg to keep in step.
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/// <para>
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/// The pipeline computes the same signature for files it processes locally
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/// (extraction <c>IR-004</c>); this exists for the files it never sees. Both
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/// producers must therefore agree exactly, including on the decode: the command
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/// below is the CLI spelling of what the pipeline asks libswresample for — best
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/// audio stream, mono, 11025 Hz, 32-bit float — and the golden fixture pins the
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/// decoded PCM as well as the signature, so a codec-level divergence is
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/// distinguishable from a DSP-level one (JR-043).
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/// </para>
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/// <para>
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/// <b>Every failure degrades to no signature rather than to an error.</b> A
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/// signature is an enhancement to cut matching; a missing one costs a tier, and
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/// must never be able to break a fetch.
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/// </para>
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/// </remarks>
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// TRACES: JR-042 | SR-003
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public class AudioSignatureService
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{
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private readonly IMediaEncoder _encoder;
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private readonly ILogger<AudioSignatureService> _logger;
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/// <summary>
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/// Initializes a new instance of the <see cref="AudioSignatureService"/> class.
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/// </summary>
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/// <param name="encoder">Supplies the path of the FFmpeg binary Jellyfin ships.</param>
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/// <param name="logger">Logger.</param>
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public AudioSignatureService(IMediaEncoder encoder, ILogger<AudioSignatureService> logger)
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{
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_encoder = encoder;
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_logger = logger;
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}
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/// <summary>
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/// Computes the signature of the 120 s window centred on the media's
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/// midpoint.
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/// </summary>
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/// <remarks>
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/// The centre is used because the head and tail are the least
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/// content-specific parts of a release: logos and cold opens at one end,
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/// credits at the other.
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/// <para>
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/// Media shorter than <see cref="AudioSignature.WindowSec"/> yields
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/// <c>null</c>: the window underflows, so there is no signature — the
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/// identical rule the extraction producer applies, since diverging here
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/// would break exactly the short items most likely to be misidentified
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/// (JR-044, whose remaining half — applying no sync offset — belongs with
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/// signature matching).
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/// </para>
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/// </remarks>
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/// <param name="path">Path of the media file.</param>
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/// <param name="runtimeSeconds">The item's runtime, as Jellyfin knows it.</param>
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/// <param name="cancellationToken">Cancellation token.</param>
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/// <returns>
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/// The <c>v1:</c>-prefixed signature, or <c>null</c> for short media, media
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/// with no usable audio, and any decode failure.
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/// </returns>
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public Task<string?> ComputeAsync(
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string path,
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double runtimeSeconds,
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CancellationToken cancellationToken)
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=> ComputeWithEncoderAsync(_encoder?.EncoderPath, path, runtimeSeconds, _logger, cancellationToken);
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/// <summary>
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/// <see cref="ComputeAsync"/> with the FFmpeg binary named explicitly.
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/// </summary>
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/// <remarks>
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/// Internal so the golden-fixture test can drive the real decode path with
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/// whatever FFmpeg the machine has, rather than standing up a fake
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/// <see cref="IMediaEncoder"/> — a stub of a thirty-member interface would
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/// be the larger risk of the two, and it is the decode that is under test.
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/// </remarks>
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/// <param name="encoderPath">Path of the FFmpeg binary to run.</param>
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/// <param name="path">Path of the media file.</param>
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/// <param name="runtimeSeconds">The item's runtime, as Jellyfin knows it.</param>
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/// <param name="logger">Logger.</param>
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/// <param name="cancellationToken">Cancellation token.</param>
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/// <returns>The <c>v1:</c>-prefixed signature, or <c>null</c>.</returns>
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internal static async Task<string?> ComputeWithEncoderAsync(
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string? encoderPath,
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string path,
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double runtimeSeconds,
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ILogger logger,
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CancellationToken cancellationToken)
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{
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if (string.IsNullOrEmpty(path) || runtimeSeconds < AudioSignature.WindowSec)
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{
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return null;
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}
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if (string.IsNullOrEmpty(encoderPath))
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{
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logger.LogDebug("No FFmpeg binary available; skipping the audio signature for {Path}", path);
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return null;
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}
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try
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{
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var samples = await DecodeCentreWindowAsync(encoderPath, path, runtimeSeconds, logger, cancellationToken)
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.ConfigureAwait(false);
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if (samples is null)
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{
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return null;
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}
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return AudioSignature.FromMonoSamples(samples);
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}
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catch (OperationCanceledException)
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{
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throw;
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}
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catch (Exception ex)
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{
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logger.LogDebug(ex, "Audio signature failed for {Path}; continuing without one", path);
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return null;
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}
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}
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/// <summary>
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/// Decodes the centre window as mono 32-bit float PCM at 11025 Hz.
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/// </summary>
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/// <remarks>
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/// The stream is truncated to exactly
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/// <see cref="AudioSignature.WindowSamples"/> samples, so the frame count is
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/// the same for every input rather than wobbling with seek granularity or a
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/// resampler tail.
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/// <para>
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/// No <c>-map</c> is given: FFmpeg's default audio selection is the same
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/// "best stream" choice the pipeline makes with
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/// <c>av_find_best_stream</c>, and naming <c>0:a:0</c> instead would pick a
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/// different track from the pipeline's on any file whose first audio stream
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/// is not its main one — a commentary track, say.
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/// </para>
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/// </remarks>
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private static async Task<float[]?> DecodeCentreWindowAsync(
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string encoderPath,
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string path,
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double runtimeSeconds,
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ILogger logger,
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CancellationToken cancellationToken)
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{
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var start = (runtimeSeconds / 2.0) - (AudioSignature.WindowSec / 2.0);
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if (start < 0.0)
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{
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start = 0.0;
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}
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var startArgument = start.ToString("0.000", CultureInfo.InvariantCulture);
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var windowArgument = AudioSignature.WindowSec.ToString("0.000", CultureInfo.InvariantCulture);
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var rateArgument = AudioSignature.SampleRate.ToString(CultureInfo.InvariantCulture);
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var startInfo = new ProcessStartInfo
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{
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FileName = encoderPath,
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RedirectStandardOutput = true,
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RedirectStandardError = true,
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RedirectStandardInput = false,
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UseShellExecute = false,
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CreateNoWindow = true,
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};
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startInfo.ArgumentList.Add("-nostdin");
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startInfo.ArgumentList.Add("-v");
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startInfo.ArgumentList.Add("error");
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// Input seeking, so FFmpeg does not decode the whole file to reach the
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// middle of it. Accurate by default: it seeks to the preceding keyframe
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// and discards the excess, which is what the pipeline does by hand.
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startInfo.ArgumentList.Add("-ss");
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startInfo.ArgumentList.Add(startArgument);
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startInfo.ArgumentList.Add("-i");
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startInfo.ArgumentList.Add(path);
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startInfo.ArgumentList.Add("-t");
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startInfo.ArgumentList.Add(windowArgument);
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startInfo.ArgumentList.Add("-vn");
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startInfo.ArgumentList.Add("-sn");
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startInfo.ArgumentList.Add("-dn");
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startInfo.ArgumentList.Add("-ac");
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startInfo.ArgumentList.Add("1");
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startInfo.ArgumentList.Add("-ar");
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startInfo.ArgumentList.Add(rateArgument);
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startInfo.ArgumentList.Add("-f");
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startInfo.ArgumentList.Add("f32le");
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startInfo.ArgumentList.Add("-");
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using var process = new Process { StartInfo = startInfo };
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if (!process.Start())
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{
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return null;
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}
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try
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{
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// Read stderr concurrently: it is redirected, so leaving it unread
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// would deadlock the moment FFmpeg filled the pipe.
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var errorTask = process.StandardError.ReadToEndAsync(cancellationToken);
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var bytes = await ReadWindowAsync(process.StandardOutput.BaseStream, cancellationToken)
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.ConfigureAwait(false);
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var error = await errorTask.ConfigureAwait(false);
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if (bytes.Length < AudioSignature.FrameSize * sizeof(float))
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{
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// No audio stream, an unreadable file, or a runtime Jellyfin
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// knows but the container does not support seeking into.
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logger.LogDebug(
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"FFmpeg returned {Bytes} bytes of audio for {Path}: {Error}",
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bytes.Length,
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path,
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error);
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return null;
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}
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var samples = new float[bytes.Length / sizeof(float)];
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for (var i = 0; i < samples.Length; i++)
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{
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samples[i] = BinaryPrimitives.ReadSingleLittleEndian(bytes.AsSpan(i * sizeof(float)));
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}
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return samples;
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}
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finally
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{
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if (!process.HasExited)
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{
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try
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{
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process.Kill(entireProcessTree: true);
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}
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catch (InvalidOperationException)
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{
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// Exited between the check and the kill.
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}
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}
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}
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}
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private static async Task<byte[]> ReadWindowAsync(Stream stream, CancellationToken cancellationToken)
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{
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var wanted = AudioSignature.WindowSamples * sizeof(float);
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var buffer = new byte[wanted];
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var filled = 0;
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while (filled < wanted)
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{
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var read = await stream.ReadAsync(buffer.AsMemory(filled, wanted - filled), cancellationToken)
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.ConfigureAwait(false);
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if (read == 0)
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{
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break;
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}
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filled += read;
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}
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if (filled == wanted)
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{
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return buffer;
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}
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var truncated = new byte[filled];
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Array.Copy(buffer, truncated, filled);
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return truncated;
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}
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}
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