using System;
using Core.Interfaces;

namespace Core.Cpu
{
    public partial class Z80
    {
        // Main Register Set
        public RegisterPair AF;
        public RegisterPair BC;
        public RegisterPair DE;
        public RegisterPair HL;

        // Alternate Register Set
        public RegisterPair AF_Prime;
        public RegisterPair BC_Prime;
        public RegisterPair DE_Prime;
        public RegisterPair HL_Prime;

        // Index Registers
        public RegisterPair IX;
        public RegisterPair IY;

        // Special Purpose Registers
        public ushort PC; // Program Counter
        public ushort SP; // Stack Pointer
        public byte I;    // Interrupt Vector
        public byte R;    // Memory Refresh

        // The Memory Bus
        private readonly IMemory _memory;

        public Z80(IMemory memory)
        {
            _memory = memory;
            Reset();
        }

        public void Reset()
        {
            PC = 0x0000;
            // The Z80 initializes SP to 0xFFFF on boot
            SP = 0xFFFF;

            AF.Word = 0;
            BC.Word = 0;
            DE.Word = 0;
            HL.Word = 0;
        }

        public int Step()
        {
            // Fetch the next opcode and increment the Program Counter
            byte opcode = _memory.Read(PC++);

            // Decode and execute
            return ExecuteOpcode(opcode);
        }

        private int ExecuteOpcode(byte opcode)
        {
            switch (opcode)
            {
                case 0x00: // NOP (No Operation)
                    return 4; // Takes 4 T-states

                // We will expand this massive list soon!

                default:
                    throw new NotImplementedException($"Opcode 0x{opcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
            }
        }
    }
}