package main

import (
	"fmt"
	"math"
)

func main() {
	engine := NewRPMEngine()
	engine.Push(2)
	engine.Push(3)
	engine.Compute("+")
	engine.Compute("^2")
	engine.Compute("SQRT")
	fmt.Println("Result", engine.Pop())
}

// RPMEngine is the engine for a RPN calculator
type RPMEngine struct {
	stack stack
}

// NewRPMEngine returns a RPMEngine
func NewRPMEngine() *RPMEngine {
	return &RPMEngine{
		stack: make(stack, 0),
	}
}

// Push a value to the internal stack
func (e *RPMEngine) Push(v int) {
	e.stack = e.stack.Push(v)
}

// Pop a value from the internal stack
func (e *RPMEngine) Pop() int {
	var v int
	e.stack, v = e.stack.Pop()
	return v
}

// Compute an operation
// If the operation results in a value, push it onto the stack
func (e *RPMEngine) Compute(operation string) error {
	switch operation {
	case "+":
		e.addTwoNumbers()
	case "-":
		e.subtractTwoNumbers()
	case "*":
		e.multiplyTwoNumbers()
	case "/":
		e.divideTwoNumbers()
	case "^2":
		e.pow2ANumber()
	case "SQRT":
		e.sqrtANumber()
	case "C":
		e.Pop()
	case "AC":
		e.stack = make(stack, 0)
	default:
		return fmt.Errorf("Operation %s not supported", operation)
	}
	return nil
}

func (e *RPMEngine) addTwoNumbers() {
	op2 := e.Pop()
	op1 := e.Pop()
	e.Push(op1 + op2)
}

func (e *RPMEngine) subtractTwoNumbers() {
	op2 := e.Pop()
	op1 := e.Pop()
	e.Push(op1 - op2)
}

func (e *RPMEngine) multiplyTwoNumbers() {
	op2 := e.Pop()
	op1 := e.Pop()
	e.Push(op1 * op2)
}

func (e *RPMEngine) divideTwoNumbers() {
	op2 := e.Pop()
	op1 := e.Pop()
	e.Push(op1 * op2)
}

func (e *RPMEngine) pow2ANumber() {
	op1 := e.Pop()
	e.Push(op1 * op1)
}

func (e *RPMEngine) sqrtANumber() {
	op1 := e.Pop()
	e.Push(int(math.Sqrt(float64(op1))))
}