WakewordModel: port openwakeword streaming pipeline (mel→emb→cls)
Ports openwakeword 0.6.0's AudioFeatures._streaming_features() and Model.predict() to C# / Microsoft.ML.OnnxRuntime 1.26.0. Bug fixed vs plan: actual melspectrogram.onnx output shape is (1, 1, n_frames, 32) — n_frames lives at Dimensions[2], not [1]. Access pattern corrected to melOut[0, 0, f, b]. Smoke test on Pi: 25/50 non-zero silence scores, max ≈ 0.000064 (matches Python reference: 45/50 non-zero, same magnitude). "Predict pipeline ran without throwing." confirmed.
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using Microsoft.ML.OnnxRuntime;
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using Microsoft.ML.OnnxRuntime.Tensors;
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namespace WakewordProbe;
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// Streaming wakeword inference port of openwakeword 0.6.0's predict pipeline.
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// References (read alongside this file):
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// openwakeword/utils.py:AudioFeatures._streaming_features (mel + embedding stages)
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// openwakeword/utils.py:AudioFeatures._streaming_melspectrogram
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// openwakeword/utils.py:AudioFeatures._get_embeddings
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// openwakeword/model.py:Model.predict (classifier stage)
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internal sealed class WakewordModel : IDisposable
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{
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// Audio I/O geometry — per-call input contract.
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public const int FrameSamples = 1280; // 80 ms @ 16 kHz; each Predict() call.
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public const int SampleRate = 16_000;
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// Mel model: input is last (FrameSamples + MelContextSamples) raw samples
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// -- the +480 is openwakeword's `-n_samples-160*3:` slice (3 hops of context).
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public const int MelContextSamples = 480; // 160 * 3
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public const int MelInputSamples = FrameSamples + MelContextSamples; // 1760
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public const int MelBins = 32; // melspectrogram model output dim
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public const int MelBufferMaxFrames = 970; // 10 * 97 (openwakeword `melspectrogram_max_len`)
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// Embedding model: 76-mel-frame window in, 96-d embedding out.
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public const int EmbeddingWindowMelFrames = 76;
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public const int EmbeddingDim = 96;
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public const int EmbeddingBufferMax = 120; // openwakeword `feature_buffer_max_len`
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public const string EmbeddingInputName = "input_1"; // openwakeword convention; assert at startup
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// Classifier: 16 embeddings in, scalar score out.
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public const int ClassifierEmbeddings = 16;
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// Skip the first N Predict() calls — buffer fill-up window.
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public const int WarmupFrames = ClassifierEmbeddings; // 16 frames ≈ 1.28 s
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private readonly InferenceSession _mel;
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private readonly InferenceSession _emb;
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private readonly InferenceSession _cls;
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private readonly string _melInputName; // discovered at startup
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private readonly string _clsInputName; // discovered at startup
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private readonly short[] _rawRing = new short[MelInputSamples];
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private int _rawRingFill = 0; // samples buffered (≤ MelInputSamples)
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private readonly List<float[]> _melRing = new(MelBufferMaxFrames); // each entry is a length-32 mel frame
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private readonly List<float[]> _embRing = new(EmbeddingBufferMax); // each entry is a length-96 embedding
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private int _framesSeen = 0;
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public WakewordModel(string melPath, string embeddingPath, string classifierPath)
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{
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var opts = new SessionOptions
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{
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IntraOpNumThreads = 1,
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InterOpNumThreads = 1,
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LogSeverityLevel = OrtLoggingLevel.ORT_LOGGING_LEVEL_ERROR,
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};
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_mel = new InferenceSession(melPath, opts);
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_emb = new InferenceSession(embeddingPath, opts);
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_cls = new InferenceSession(classifierPath, opts);
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// Discover input names + assert shape geometry.
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_melInputName = _mel.InputMetadata.Keys.Single();
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AssertEmbeddingShape(_emb); // input_1: [batch, 76, 32, 1], dtype float
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_clsInputName = _cls.InputMetadata.Keys.Single();
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AssertClassifierShape(_cls); // [batch, 16, 96], dtype float
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}
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private static void AssertEmbeddingShape(InferenceSession sess)
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{
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if (!sess.InputMetadata.TryGetValue(EmbeddingInputName, out var meta))
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throw new InvalidOperationException(
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$"embedding_model.onnx: expected input named '{EmbeddingInputName}', got [{string.Join(",", sess.InputMetadata.Keys)}]");
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var d = meta.Dimensions;
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// Expected: [batch, 76, 32, 1] — batch may be -1 (dynamic).
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if (d.Length != 4 || d[1] != EmbeddingWindowMelFrames || d[2] != MelBins || d[3] != 1)
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throw new InvalidOperationException(
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$"embedding_model.onnx: expected input shape [batch,{EmbeddingWindowMelFrames},{MelBins},1], got [{string.Join(",", d)}]");
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if (meta.ElementType != typeof(float))
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throw new InvalidOperationException($"embedding_model.onnx: expected Single input, got {meta.ElementType.Name}");
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}
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private static void AssertClassifierShape(InferenceSession sess)
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{
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var inputName = sess.InputMetadata.Keys.Single();
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var meta = sess.InputMetadata[inputName];
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var d = meta.Dimensions;
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// Expected: [batch, 16, 96] — batch may be -1.
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if (d.Length != 3 || d[1] != ClassifierEmbeddings || d[2] != EmbeddingDim)
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throw new InvalidOperationException(
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$"alexa.onnx: expected input shape [batch,{ClassifierEmbeddings},{EmbeddingDim}], got [{string.Join(",", d)}]");
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if (meta.ElementType != typeof(float))
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throw new InvalidOperationException($"alexa.onnx: expected Single input, got {meta.ElementType.Name}");
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}
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public float Predict(short[] frame1280)
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{
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if (frame1280.Length != FrameSamples)
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throw new ArgumentException($"Expected {FrameSamples} samples, got {frame1280.Length}");
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// 1. Append 1280 new samples to the raw ring (shift older samples down if full).
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if (_rawRingFill < MelInputSamples)
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{
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int copyToFront = Math.Min(MelInputSamples - _rawRingFill, FrameSamples);
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Array.Copy(frame1280, 0, _rawRing, _rawRingFill, copyToFront);
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_rawRingFill += copyToFront;
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if (copyToFront < FrameSamples)
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{
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// Shouldn't happen on the very first call, but defensive.
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int leftover = FrameSamples - copyToFront;
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Array.Copy(_rawRing, leftover, _rawRing, 0, MelInputSamples - leftover);
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Array.Copy(frame1280, copyToFront, _rawRing, MelInputSamples - leftover, leftover);
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}
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}
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else
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{
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// Shift older samples left by FrameSamples, then append new at the tail.
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Array.Copy(_rawRing, FrameSamples, _rawRing, 0, MelInputSamples - FrameSamples);
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Array.Copy(frame1280, 0, _rawRing, MelInputSamples - FrameSamples, FrameSamples);
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}
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// Skip everything until we have the full mel-context window primed.
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if (_rawRingFill < MelInputSamples)
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{
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_framesSeen++;
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return 0f;
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}
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// 2. Mel stage: feed the full _rawRing as float32 (1, MelInputSamples) into mel model.
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var melInputData = new float[MelInputSamples];
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for (int i = 0; i < MelInputSamples; i++) melInputData[i] = _rawRing[i]; // int16 → float32, NO normalisation
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var melInputTensor = new DenseTensor<float>(melInputData, new[] { 1, MelInputSamples });
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using var melResults = _mel.Run(new[] {
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NamedOnnxValue.CreateFromTensor(_melInputName, melInputTensor)
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});
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var melOut = melResults.First().AsTensor<float>(); // shape (1, 1, n_frames, 32)
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// Apply openwakeword's `x / 10 + 2` transform and append each new frame to _melRing.
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// FIXED: actual mel output shape is (1, 1, n_frames, 32) — n_frames is at Dimensions[2], NOT [1].
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int nFrames = melOut.Dimensions[2];
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for (int f = 0; f < nFrames; f++)
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{
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var bin = new float[MelBins];
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for (int b = 0; b < MelBins; b++)
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bin[b] = melOut[0, 0, f, b] / 10f + 2f;
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_melRing.Add(bin);
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}
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if (_melRing.Count > MelBufferMaxFrames)
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_melRing.RemoveRange(0, _melRing.Count - MelBufferMaxFrames);
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// 3. Embedding stage: need ≥ 76 mel frames; slice the last 76 → (1, 76, 32, 1).
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if (_melRing.Count < EmbeddingWindowMelFrames)
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{
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_framesSeen++;
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return 0f;
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}
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var embInputData = new float[EmbeddingWindowMelFrames * MelBins];
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int startMel = _melRing.Count - EmbeddingWindowMelFrames;
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for (int f = 0; f < EmbeddingWindowMelFrames; f++)
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Array.Copy(_melRing[startMel + f], 0, embInputData, f * MelBins, MelBins);
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var embInputTensor = new DenseTensor<float>(embInputData, new[] { 1, EmbeddingWindowMelFrames, MelBins, 1 });
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using var embResults = _emb.Run(new[] {
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NamedOnnxValue.CreateFromTensor(EmbeddingInputName, embInputTensor)
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});
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var embOut = embResults.First().AsTensor<float>(); // shape (1, 1, 1, 96)
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var newEmb = new float[EmbeddingDim];
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for (int i = 0; i < EmbeddingDim; i++) newEmb[i] = embOut[0, 0, 0, i];
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_embRing.Add(newEmb);
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if (_embRing.Count > EmbeddingBufferMax)
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_embRing.RemoveRange(0, _embRing.Count - EmbeddingBufferMax);
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_framesSeen++;
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// 4. Warm-up + classifier stage.
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if (_embRing.Count < ClassifierEmbeddings || _framesSeen <= WarmupFrames)
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return 0f;
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var clsInputData = new float[ClassifierEmbeddings * EmbeddingDim];
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int startEmb = _embRing.Count - ClassifierEmbeddings;
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for (int i = 0; i < ClassifierEmbeddings; i++)
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Array.Copy(_embRing[startEmb + i], 0, clsInputData, i * EmbeddingDim, EmbeddingDim);
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var clsInputTensor = new DenseTensor<float>(clsInputData, new[] { 1, ClassifierEmbeddings, EmbeddingDim });
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using var clsResults = _cls.Run(new[] {
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NamedOnnxValue.CreateFromTensor(_clsInputName, clsInputTensor)
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});
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var clsOut = clsResults.First().AsTensor<float>(); // shape (1, 1)
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return clsOut[0, 0];
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}
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// ... methods follow in Step 2 ...
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public void Dispose()
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{
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_mel.Dispose();
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_emb.Dispose();
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_cls.Dispose();
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}
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}
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