Reset resampler on seek to fix off-by-one audio sample count (#1604)

Author: NicolasHug

src/torchcodec/_core/CpuDeviceInterface.cpp

@@ -476,6 +476,11 @@ CpuDeviceInterface::maybe_flush_audio_buffers() {
       /*length=*/actual_num_remaining_samples);
 }
 
+void CpuDeviceInterface::flush() {
+  DeviceInterface::flush();
+  swr_context_.reset();
+}
+
 std::string CpuDeviceInterface::get_details() {
   return std::string("CPU Device Interface.");
 }

src/torchcodec/_core/CpuDeviceInterface.h

@@ -43,6 +43,8 @@ class CpuDeviceInterface : public DeviceInterface {
   virtual std::optional<torch::stable::Tensor> maybe_flush_audio_buffers()
       override;
 
+  void flush() override;
+
   void convert_av_frame_to_frame_output(
       const AVFrame& av_frame,
       FrameOutput& frame_output,

test/test_decoders.py

@@ -3082,6 +3082,29 @@ def test_downsample_empty_frame(self):
             frames_44100_to_8000.data, frames_8000.data, atol=0.03, rtol=0
         )
 
+    def test_resample_seek_sample_count(self):
+        # Non-regression test for https://github.com/meta-pytorch/torchcodec/issues/1601
+        # When resampling, the swresample context buffers samples and tracks a
+        # fractional sample position across calls. If it isn't reset on a
+        # mid-stream seek, stale state leaks into the next range decode and the
+        # output sample count can be off by one.
+        # The exact condition in which this happens is unclear to me but claude
+        # managed to find this test that reproduces consistently - and the fix
+        # was to reset the swresample context on a mid-stream seek, which seems
+        # like a very normal thing to do.
+        asset = SINE_MONO_S32_44100
+        assert asset.sample_rate == 44_100
+        assert asset.duration_seconds == 4
+
+        out_sample_rate = 16_000
+        decoder = AudioDecoder(asset.path, sample_rate=out_sample_rate)
+
+        decoder.get_samples_played_in_range(start_seconds=2.0, stop_seconds=4.8)
+        tail = decoder.get_samples_played_in_range(start_seconds=3.6, stop_seconds=4.8)
+
+        # [3.6, 4.0) of audio at out_sample_rate.
+        assert tail.data.shape[1] == round(0.4 * out_sample_rate)
+
     def test_decode_s16_ffmpeg4(self):
         # Non-regression test for https://github.com/pytorch/torchcodec/issues/843
         # Ensures that decoding s16 on FFmpeg4 handles