package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var stack []float64

func pop() float64 {
	var x float64
	if len(stack) > 0 {
		x, stack = stack[len(stack)-1], stack[:len(stack)-1]
	}
	return x
}

func push(v float64) {
	stack = append(stack, v)
}

func addOp(v1, v2 float64) float64 {
	fmt.Printf("%f + %f = %f\n", v1, v2, (v1 + v2))
	return v1 + v2
}
func subtractOp(v1, v2 float64) float64 {
	fmt.Printf("%f - %f = %f\n", v1, v2, (v1 - v2))
	return v1 - v2
}
func multiplyOp(v1, v2 float64) float64 {
	fmt.Printf("%f * %f = %f\n", v1, v2, (v1 * v2))
	return v1 * v2
}
func divideOp(v1, v2 float64) float64 {
	fmt.Printf("%f / %f = %f\n", v1, v2, (v1 / v2))
	return v1 / v2
}

func main() {
	fmt.Println("stack calculator")
	var in string
	reader := bufio.NewReader(os.Stdin)
	for {
		fmt.Print(stack)
		fmt.Print(" > ")
		in, _ = reader.ReadString('\n')
		in = strings.TrimSpace(in)
		switch in {
		case ".":
			break
		case "+":
			doStackOperation(addOp)
		case "-":
			doStackOperation(subtractOp)
		case "*":
			doStackOperation(multiplyOp)
		case "/":
			doStackOperation(divideOp)
		default:
			val, err := strconv.ParseFloat(in, 64)
			if err != nil {
				fmt.Println(err.Error())
				fmt.Println("Valid inputs: +, -, /, *, <number>")
				fmt.Println("'.' by itself to quit")
			} else {
				push(val)
			}
		}
		if in == "." {
			break
		}
	}
}

func doStackOperation(op func(float64, float64) float64) {
	if len(stack) < 2 {
		fmt.Println("Not enough on stack to add")
		return
	}
	v1, v2 := pop(), pop()
	push(op(v1, v2))
}