package cpu

import "fmt"

const ARCH = "MOS6502"

// bits for the status register
const (
	FlagCarry     = 1 << 0 // Bit 0
	FlagZero      = 1 << 1 // Bit 1
	FlagInterrupt = 1 << 2 // Bit 2
	FlagDecimal   = 1 << 3 // Bit 3
	FlagBreak     = 1 << 4 // Bit 4
	FlagUnused    = 1 << 5 // Bit 5
	FlagOverflow  = 1 << 6 // Bit 6
	FlagNegative  = 1 << 7 // Bit 7
)

type Bus interface {
	Read(address uint16) uint8
}

type CPU struct {
	A      uint8  // Accumulator
	X      uint8  // X Register
	Y      uint8  // Y Register
	PC     uint16 // Program Counter
	SP     uint8  // Stack Pointer
	Status uint8  // Status Flags
	Bus    Bus    // Connection to the Bus
}

func (c *CPU) Reset() {
	c.A = 0
	c.X = 0
	c.Y = 0
	c.SP = 0xFD
	c.Status = 0x24 
	c.PC = 0x0000
}

// Small helper func to flip the status register bits
func (c *CPU) setFlag(flag uint8, set bool) {
	if set {
		c.Status |= flag
	} else {
		c.Status &= ^flag
	}
}

// fetch -> decode -> execute cycle
func (c *CPU) Step() {
	// fetch with program counter and bus memory
	opcode := c.Bus.Read(c.PC)

	fmt.Printf("Executing at PC: %04X | Opcode: %02X\n", c.PC, opcode)

	// decode + exe need to split this out to handle all instructions etc
	// Look at : https://www.ahl27.com/posts/2023/01/6502-emu1/
	switch opcode {
	case 0xEA: //NOP
		// does nothing its a no op
	default:
		fmt.Printf("Unknown opcode: %02X\n", opcode)
	}

	// Move the Program counter
	c.PC++
}