package interpreter

import "fmt"

type Interpreter struct {
	program            []int
	instructionPointer int
}

func Init(program []int) *Interpreter {
    return &Interpreter {
        program: program,
        instructionPointer: 0,
    }
}

func (i *Interpreter) Next() bool {
	opcode := i.getValueAtIndex(i.instructionPointer)
	ipIncrement := 1

	switch opcode {
	case 1:
        i.add()
        ipIncrement = 4
	case 2:
        i.mul()
        ipIncrement = 4
	case 99:
		return true
	default:
		panic(fmt.Sprintf("unhandled opcode %v at IP %v", opcode, i.instructionPointer))
	}

	i.instructionPointer += ipIncrement

	return false
}

func (i *Interpreter) getValueAtIndex(index int) int {
    return i.program[index]
}

func (i *Interpreter) setValueAtIndex(index, value int) {
    i.program[index] = value
}

func (i *Interpreter) add() {
    resIndex := i.getValueAtIndex(i.instructionPointer + 3)
    arg1Index := i.getValueAtIndex(i.instructionPointer + 1)
    arg2Index := i.getValueAtIndex(i.instructionPointer + 2)

    i.setValueAtIndex(resIndex, i.getValueAtIndex(arg1Index) + i.getValueAtIndex(arg2Index))
}

func (i *Interpreter) mul() {
    resIndex := i.getValueAtIndex(i.instructionPointer + 3)
    arg1Index := i.getValueAtIndex(i.instructionPointer + 1)
    arg2Index := i.getValueAtIndex(i.instructionPointer + 2)

    i.setValueAtIndex(resIndex, i.getValueAtIndex(arg1Index) * i.getValueAtIndex(arg2Index))
}