feat(audio): add wave channel with programmable RAM

src/GameBoy.Core/Audio/WaveChannel.cs

@@ -0,0 +1,150 @@
+using System.IO;
+
+namespace GameBoy.Audio
+{
+    internal sealed class WaveChannel
+    {
+        private readonly byte[] waveRam;
+
+        private byte nr30, nr31, nr32, nr33, nr34;
+
+        private int frequencyTimer;
+        private int position;
+        private int sampleBuffer;
+
+        private bool enabled;
+        private bool dacEnabled;
+
+        private int lengthCounter;
+        private bool lengthEnabled;
+
+        public WaveChannel(byte[] waveRam)
+        {
+            this.waveRam = waveRam;
+        }
+
+        public bool Enabled => enabled;
+        public bool DacEnabled => dacEnabled;
+
+        private int Frequency => ((nr34 & 0x07) << 8) | nr33;
+
+        private int VolumeShift => ((nr32 >> 5) & 0x03) switch
+        {
+            0 => 4,
+            1 => 0,
+            2 => 1,
+            _ => 2
+        };
+
+        public int Output() => enabled ? (sampleBuffer >> VolumeShift) : 0;
+
+        public float Analog() => dacEnabled ? (Output() / 15f * 2f - 1f) : 0f;
+
+        public byte Read(int reg)
+        {
+            return reg switch
+            {
+                0 => nr30,
+                1 => nr31,
+                2 => nr32,
+                3 => nr33,
+                4 => nr34,
+                _ => 0xFF
+            };
+        }
+
+        public void Write(int reg, byte value)
+        {
+            switch (reg)
+            {
+                case 0:
+                    nr30 = value;
+                    dacEnabled = (value & 0x80) != 0;
+                    if (!dacEnabled)
+                        enabled = false;
+                    break;
+                case 1:
+                    nr31 = value;
+                    lengthCounter = 256 - value;
+                    break;
+                case 2:
+                    nr32 = value;
+                    break;
+                case 3:
+                    nr33 = value;
+                    break;
+                case 4:
+                    nr34 = value;
+                    lengthEnabled = (value & 0x40) != 0;
+                    if ((value & 0x80) != 0)
+                        Trigger();
+                    break;
+            }
+        }
+
+        public void Step(int cycles)
+        {
+            frequencyTimer -= cycles;
+            while (frequencyTimer <= 0)
+            {
+                frequencyTimer += (2048 - Frequency) * 2;
+                position = (position + 1) & 31;
+                sampleBuffer = ReadNibble(position);
+            }
+        }
+
+        private int ReadNibble(int index)
+        {
+            byte packed = waveRam[index >> 1];
+            return (index & 1) == 0 ? (packed >> 4) : (packed & 0x0F);
+        }
+
+        public void ClockLength()
+        {
+            if (lengthEnabled && lengthCounter > 0)
+            {
+                lengthCounter--;
+                if (lengthCounter == 0)
+                    enabled = false;
+            }
+        }
+
+        private void Trigger()
+        {
+            enabled = dacEnabled;
+            if (lengthCounter == 0)
+                lengthCounter = 256;
+            frequencyTimer = (2048 - Frequency) * 2;
+            position = 0;
+            sampleBuffer = ReadNibble(0);
+        }
+
+        public void PowerOff()
+        {
+            nr30 = nr31 = nr32 = nr33 = nr34 = 0;
+            frequencyTimer = 0;
+            position = 0;
+            sampleBuffer = 0;
+            enabled = false;
+            dacEnabled = false;
+            lengthCounter = 0;
+            lengthEnabled = false;
+        }
+
+        public void SaveState(BinaryWriter w)
+        {
+            w.Write(nr30); w.Write(nr31); w.Write(nr32); w.Write(nr33); w.Write(nr34);
+            w.Write(frequencyTimer); w.Write(position); w.Write(sampleBuffer);
+            w.Write(enabled); w.Write(dacEnabled);
+            w.Write(lengthCounter); w.Write(lengthEnabled);
+        }
+
+        public void LoadState(BinaryReader r)
+        {
+            nr30 = r.ReadByte(); nr31 = r.ReadByte(); nr32 = r.ReadByte(); nr33 = r.ReadByte(); nr34 = r.ReadByte();
+            frequencyTimer = r.ReadInt32(); position = r.ReadInt32(); sampleBuffer = r.ReadInt32();
+            enabled = r.ReadBoolean(); dacEnabled = r.ReadBoolean();
+            lengthCounter = r.ReadInt32(); lengthEnabled = r.ReadBoolean();
+        }
+    }
+}