Implement precise seeking

Codecs/SoundFlow.Codecs.FFMpeg/FFmpegDecoder.cs

@@ -111,8 +111,29 @@ public bool Seek(int sampleOffset)
         if (IsDisposed || !_stream.CanSeek) return false;
 
         var frameIndex = sampleOffset / Channels;
-        var result = FFmpeg.SeekToPcmFrame(_handle, frameIndex);
-        return result == FFmpegResult.Success;
+        var result = FFmpeg.SeekToPcmFrame(_handle, frameIndex, out long resultFrameIndex);
+
+        if (result == FFmpegResult.Success)
+        {
+            if(resultFrameIndex >= 0)
+            {
+                // Compute how many samples to skip
+                var toSkip = (int)(frameIndex - resultFrameIndex);
+
+                if(toSkip > 0)
+                {
+                    // TODO!!! This could probably be optimized, but this is the quickest way to add this
+                    Span<float> dummySamples = stackalloc float[toSkip * Channels];
+
+                    // Dummy decode samples to skip them so we are at exact location we want
+                    Decode(dummySamples);
+                }
+            }
+
+            return true;
+        }
+        else
+            return false;
     }
 
     private unsafe nuint OnRead(IntPtr pUserData, IntPtr pBuffer, nuint bytesToRead)

Codecs/SoundFlow.Codecs.FFMpeg/Native/FFmpeg.cs

@@ -194,7 +194,7 @@ public static partial FFmpegResult InitializeDecoder(SafeDecoderHandle decoder,
     public static partial FFmpegResult ReadPcmFrames(SafeDecoderHandle decoder, IntPtr pFramesOut, long frameCount, out long outFramesRead);
 
     [LibraryImport(LibraryName, EntryPoint = "sf_decoder_seek_to_pcm_frame")]
-    public static partial FFmpegResult SeekToPcmFrame(SafeDecoderHandle decoder, long frameIndex);
+    public static partial FFmpegResult SeekToPcmFrame(SafeDecoderHandle decoder, long frameIndex, out long resultFrameIndex);
 
     [LibraryImport(LibraryName, EntryPoint = "sf_decoder_free")]
     public static partial void FreeDecoder(IntPtr decoder);

Native/ffmpeg-codec/soundflow-ffmpeg.c

@@ -305,7 +305,7 @@ SF_FFMPEG_API SF_Result sf_decoder_read_pcm_frames(SF_Decoder* decoder, void* pF
     return SF_RESULT_SUCCESS;
 }
 
-SF_FFMPEG_API SF_Result sf_decoder_seek_to_pcm_frame(SF_Decoder* decoder, int64_t frameIndex) {
+SF_FFMPEG_API SF_Result sf_decoder_seek_to_pcm_frame(SF_Decoder* decoder, int64_t frameIndex, int64_t* resultFrameIndex) {
     if (!decoder || !decoder->format_ctx || decoder->stream_index < 0) return SF_RESULT_ERROR_INVALID_ARGS;
 
     AVStream* stream = decoder->format_ctx->streams[decoder->stream_index];
@@ -317,20 +317,29 @@ SF_FFMPEG_API SF_Result sf_decoder_seek_to_pcm_frame(SF_Decoder* decoder, int64_
 
     int ret = av_seek_frame(decoder->format_ctx, decoder->stream_index, timestamp, AVSEEK_FLAG_BACKWARD);
     if (ret < 0) {
+        *resultFrameIndex = -1;
         return SF_RESULT_DECODER_ERROR_SEEK_FAILED;
     }
 
     // Read and discard packets until we reach the desired position
+    int64_t packetTimestamp = -1;
+
     AVPacket* pkt = av_packet_alloc();
     while (av_read_frame(decoder->format_ctx, pkt) >= 0) {
         if (pkt->stream_index == decoder->stream_index) {
+            packetTimestamp = pkt->pts;
             av_packet_unref(pkt);
             break;
         }
         av_packet_unref(pkt);
     }
     av_packet_free(&pkt);
 
+    if (packetTimestamp >= 0)
+        resultFrameIndex = av_rescale_q(packetTimestamp, stream->time_base, (AVRational) { 1, stream->codecpar->sample_rate });
+    else
+        resultFrameIndex = -1;
+
     return SF_RESULT_SUCCESS;
 }