package gmachine

import (
	"fmt"
	"log"
)

// DefaultMemSize is the number of 64-bit words of memory which will be
// allocated to a new G-machine by default.
const DefaultMemSize = 1024

const (
	OpHALT uint64 = iota
	OpNOOP
	OpINCA
	OpDECA
	OpSETA
)

type gMachine struct {
	// Memory 64-bit words of memory
	Memory []uint64
	// P (Program counter)
	P uint64
	// A (Register A)
	A uint64
	// Debugging flag, if turned on will print OpCodes
	DebugOn bool
}

func New() *gMachine {
	return &gMachine{
		P:       0,
		Memory:  make([]uint64, DefaultMemSize),
		DebugOn: false,
	}
}

func (m *gMachine) NewProgram(program []uint64) {
	copy(m.Memory, program)
	m.Run()
}

func (m *gMachine) Run() {
	for {
		pointer := m.P
		m.P++
		switch m.Memory[pointer] {
		case OpHALT:
			m.printIfDebug("OpHALT")
			return
		case OpNOOP:
			m.printIfDebug("OpNOOP")
		case OpINCA:
			m.printIfDebug("OpINCA")
			m.A++
		case OpDECA:
			m.printIfDebug("OpDECA")
			m.A--
		case OpSETA:
			m.A = m.Memory[m.P]
			m.P++
			m.printIfDebug(fmt.Sprintf("OpSETA %d", m.A))
		default:
			m.printIfDebug(fmt.Sprintf("COULD NOT DETERMINE OP CODE, code:%d, pointer:%d", m.Memory[pointer], pointer))
			panic(pointer)
		}
	}
}

func (m *gMachine) printIfDebug(message string) {
	if m.DebugOn {
		log.Println(message)
	}
}