package rpn

import (
    "stack"
    "strconv"
    "strings"
)

var operators = map[string]func(int, int) int{
    "+": func(left int, right int) int { return left + right },
    "-": func(left int, right int) int { return left - right },
    "/": func(left int, right int) int { return left / right },
    "*": func(left int, right int) int { return left * right },
}

/*
Evaluate a postfix expression. The following operators are supported: + - * /
*/
func Evaluate(input string) int {
    parts := strings.Split(input, " ")
    var s stack.Stack

    for _, part := range parts {
        if operator, ok := operators[part]; ok {
            right := s.Pop()
            left := s.Pop()
            s.Push(operator(left, right))
        } else if number, err := strconv.Atoi(part); err == nil {
            s.Push(number)
        } else {
            panic("Didn't recognize input.")
        }
    }

    if s.Size() != 1 {
        panic("Unbalanced expression!")
    }

    return s.Pop()
}