From 5342fc360f1592c6e98698c330c5eb50710d7542 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Tue, 25 Aug 2026 13:14:16 +0200
Subject: [PATCH] feat(audio): add noise channel synthesis

---
 src/GameBoy.Core/Audio/NoiseChannel.cs | 166 +++++++++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 src/GameBoy.Core/Audio/NoiseChannel.cs

diff --git a/src/GameBoy.Core/Audio/NoiseChannel.cs b/src/GameBoy.Core/Audio/NoiseChannel.cs
new file mode 100644
index 0000000..a9d9229
--- /dev/null
+++ b/src/GameBoy.Core/Audio/NoiseChannel.cs
@@ -0,0 +1,166 @@
+using System.IO;
+
+namespace GameBoy.Audio
+{
+    internal sealed class NoiseChannel
+    {
+        private static readonly int[] Divisors = { 8, 16, 32, 48, 64, 80, 96, 112 };
+
+        private byte nr41, nr42, nr43, nr44;
+
+        private int frequencyTimer;
+        private int lfsr = 0x7FFF;
+
+        private bool enabled;
+        private bool dacEnabled;
+
+        private int lengthCounter;
+        private bool lengthEnabled;
+
+        private int volume;
+        private int envelopePeriod;
+        private bool envelopeAdd;
+        private int envelopeTimer;
+
+        public bool Enabled => enabled;
+        public bool DacEnabled => dacEnabled;
+
+        private int Period => Divisors[nr43 & 0x07] << (nr43 >> 4);
+        private bool WidthMode => (nr43 & 0x08) != 0;
+
+        public int Output() => enabled ? ((~lfsr & 1) * volume) : 0;
+
+        public float Analog() => dacEnabled ? (Output() / 15f * 2f - 1f) : 0f;
+
+        public byte Read(int reg)
+        {
+            return reg switch
+            {
+                1 => nr41,
+                2 => nr42,
+                3 => nr43,
+                4 => nr44,
+                _ => 0xFF
+            };
+        }
+
+        public void Write(int reg, byte value)
+        {
+            switch (reg)
+            {
+                case 1:
+                    nr41 = value;
+                    lengthCounter = 64 - (value & 0x3F);
+                    break;
+                case 2:
+                    nr42 = value;
+                    dacEnabled = (value & 0xF8) != 0;
+                    if (!dacEnabled)
+                        enabled = false;
+                    envelopePeriod = value & 0x07;
+                    envelopeAdd = (value & 0x08) != 0;
+                    break;
+                case 3:
+                    nr43 = value;
+                    break;
+                case 4:
+                    nr44 = value;
+                    lengthEnabled = (value & 0x40) != 0;
+                    if ((value & 0x80) != 0)
+                        Trigger();
+                    break;
+            }
+        }
+
+        public void Step(int cycles)
+        {
+            frequencyTimer -= cycles;
+            while (frequencyTimer <= 0)
+            {
+                frequencyTimer += Period;
+                ClockLfsr();
+            }
+        }
+
+        private void ClockLfsr()
+        {
+            int xor = (lfsr & 1) ^ ((lfsr >> 1) & 1);
+            lfsr >>= 1;
+            lfsr |= xor << 14;
+            if (WidthMode)
+            {
+                lfsr &= ~(1 << 6);
+                lfsr |= xor << 6;
+            }
+        }
+
+        public void ClockLength()
+        {
+            if (lengthEnabled && lengthCounter > 0)
+            {
+                lengthCounter--;
+                if (lengthCounter == 0)
+                    enabled = false;
+            }
+        }
+
+        public void ClockEnvelope()
+        {
+            if (envelopePeriod == 0)
+                return;
+            if (envelopeTimer > 0)
+                envelopeTimer--;
+            if (envelopeTimer == 0)
+            {
+                envelopeTimer = envelopePeriod;
+                int next = volume + (envelopeAdd ? 1 : -1);
+                if (next >= 0 && next <= 15)
+                    volume = next;
+            }
+        }
+
+        private void Trigger()
+        {
+            enabled = dacEnabled;
+            if (lengthCounter == 0)
+                lengthCounter = 64;
+            frequencyTimer = Period;
+            envelopeTimer = envelopePeriod;
+            volume = nr42 >> 4;
+            lfsr = 0x7FFF;
+        }
+
+        public void PowerOff()
+        {
+            nr41 = nr42 = nr43 = nr44 = 0;
+            frequencyTimer = 0;
+            lfsr = 0x7FFF;
+            enabled = false;
+            dacEnabled = false;
+            lengthCounter = 0;
+            lengthEnabled = false;
+            volume = 0;
+            envelopePeriod = 0;
+            envelopeAdd = false;
+            envelopeTimer = 0;
+        }
+
+        public void SaveState(BinaryWriter w)
+        {
+            w.Write(nr41); w.Write(nr42); w.Write(nr43); w.Write(nr44);
+            w.Write(frequencyTimer); w.Write(lfsr);
+            w.Write(enabled); w.Write(dacEnabled);
+            w.Write(lengthCounter); w.Write(lengthEnabled);
+            w.Write(volume); w.Write(envelopePeriod); w.Write(envelopeAdd); w.Write(envelopeTimer);
+        }
+
+        public void LoadState(BinaryReader r)
+        {
+            nr41 = r.ReadByte(); nr42 = r.ReadByte(); nr43 = r.ReadByte(); nr44 = r.ReadByte();
+            frequencyTimer = r.ReadInt32(); lfsr = r.ReadInt32();
+            enabled = r.ReadBoolean(); dacEnabled = r.ReadBoolean();
+            lengthCounter = r.ReadInt32(); lengthEnabled = r.ReadBoolean();
+            volume = r.ReadInt32(); envelopePeriod = r.ReadInt32(); envelopeAdd = r.ReadBoolean(); envelopeTimer = r.ReadInt32();
+        }
+    }
+}