Started porting audio code from 0.9

src/Discord.Net.Audio/Opus/OpusDecoder.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Discord.Audio.Opus
+{
+    internal unsafe class OpusDecoder : OpusConverter
+    {
+        [DllImport("opus", EntryPoint = "opus_decoder_create", CallingConvention = CallingConvention.Cdecl)]
+        private static extern IntPtr CreateDecoder(int Fs, int channels, out OpusError error);
+        [DllImport("opus", EntryPoint = "opus_decoder_destroy", CallingConvention = CallingConvention.Cdecl)]
+        private static extern void DestroyDecoder(IntPtr decoder);
+        [DllImport("opus", EntryPoint = "opus_decode", CallingConvention = CallingConvention.Cdecl)]
+        private static extern int Decode(IntPtr st, byte* data, int len, byte[] pcm, int frame_size, int decode_fec);
+
+        public OpusDecoder(int samplingRate)
+            : base(samplingRate)
+        {
+            OpusError error;
+            _ptr = CreateDecoder(samplingRate, 2, out error);
+            if (error != OpusError.OK)
+                throw new InvalidOperationException($"Error occured while creating decoder: {error}");
+        }
+        
+        /// <summary> Produces PCM samples from Opus-encoded audio. </summary>
+        /// <param name="input">PCM samples to decode.</param>
+        /// <param name="inputOffset">Offset of the frame in input.</param>
+        /// <param name="output">Buffer to store the decoded frame.</param>
+        public unsafe int DecodeFrame(byte[] input, int inputOffset, int inputCount, byte[] output)
+        {
+            int result = 0;
+            fixed (byte* inPtr = input)
+                result = Decode(_ptr, inPtr + inputOffset, inputCount, output, inputCount, 0);
+
+            if (result < 0)
+                throw new Exception(((OpusError)result).ToString());
+            return result;
+        }
+
+        protected override void Dispose(bool disposing)
+        {
+            if (_ptr != IntPtr.Zero)
+            {
+                DestroyDecoder(_ptr);
+                _ptr = IntPtr.Zero;
+            }
+        }
+    }
+}