feat(input): add joypad register and button state handling

src/GameBoy.Core/Input/Joypad.cs

@@ -0,0 +1,81 @@
+using System.IO;
+using GameBoy.Cpu;
+
+namespace GameBoy.Input
+{
+    public enum JoypadButton
+    {
+        Right = 0,
+        Left = 1,
+        Up = 2,
+        Down = 3,
+        A = 4,
+        B = 5,
+        Select = 6,
+        Start = 7
+    }
+
+    public sealed class Joypad
+    {
+        private readonly InterruptController interrupts;
+        private byte selectLines = 0x30;
+        private byte directions;
+        private byte actions;
+
+        public Joypad(InterruptController interrupts)
+        {
+            this.interrupts = interrupts;
+        }
+
+        public void SetButton(JoypadButton button, bool pressed)
+        {
+            int index = (int)button;
+            byte mask = (byte)(1 << (index & 3));
+            bool isDirection = index < 4;
+            byte before = ReadLines();
+
+            if (isDirection)
+                directions = pressed ? (byte)(directions | mask) : (byte)(directions & ~mask);
+            else
+                actions = pressed ? (byte)(actions | mask) : (byte)(actions & ~mask);
+
+            byte after = ReadLines();
+            if ((before & ~after & 0x0F) != 0)
+                interrupts.Request(Interrupt.Joypad);
+        }
+
+        private byte ReadLines()
+        {
+            byte lines = 0x0F;
+            if ((selectLines & 0x10) == 0)
+                lines &= (byte)~directions;
+            if ((selectLines & 0x20) == 0)
+                lines &= (byte)~actions;
+            return lines;
+        }
+
+        public byte Read()
+        {
+            return (byte)(0xC0 | selectLines | ReadLines());
+        }
+
+        public void Write(byte value)
+        {
+            selectLines = (byte)(value & 0x30);
+        }
+
+        public void SaveState(BinaryWriter writer)
+        {
+            writer.Write(selectLines);
+            writer.Write(directions);
+            writer.Write(actions);
+        }
+
+        public void LoadState(BinaryReader reader)
+        {
+            selectLines = reader.ReadByte();
+            directions = reader.ReadByte();
+            actions = reader.ReadByte();
+        }
+    }
+}