[audio] added sdl audio backend and in-tree atrac9 decoder

src/SharpEmu.LibAtrac9/Atrac9Rng.cs

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+#nullable disable
+namespace LibAtrac9
+{
+    /// <summary>
+    /// An Xorshift RNG used by the ATRAC9 codec
+    /// </summary>
+    internal class Atrac9Rng
+    {
+        private ushort _stateA;
+        private ushort _stateB;
+        private ushort _stateC;
+        private ushort _stateD;
+
+        public Atrac9Rng(ushort seed)
+        {
+            int startValue = 0x4D93 * (seed ^ (seed >> 14));
+
+            _stateA = (ushort)(3 - startValue);
+            _stateB = (ushort)(2 - startValue);
+            _stateC = (ushort)(1 - startValue);
+            _stateD = (ushort)(0 - startValue);
+        }
+
+        public ushort Next()
+        {
+            ushort t = (ushort)(_stateD ^ (_stateD << 5));
+            _stateD = _stateC;
+            _stateC = _stateB;
+            _stateB = _stateA;
+            _stateA = (ushort)(t ^ _stateA ^ ((t ^ (_stateA >> 5)) >> 4));
+            return _stateA;
+        }
+    }
+}