// Simple parser example
use std, types

operations = {
  "+" : def(a,b) = a+b,
  "-" : def(a,b) = a-b,
  "*" : def(a,b) = a*b,
  "/" : def(a,b) = a/b,
  "%" : def(a,b) = a%b,
  ">" : def(a,b) = a>b,
  "<" : def(a,b) = a<b
}

def calculate(expression) {
  pos = 0
  len = length(expression)

  def isDigit(c) = 48 <= c && c <= 57

  def parseNumber() {
    buffer = ""
    while (pos < len && isDigit(expression.charAt(pos))) {
      buffer += toChar(expression.charAt(pos))
      pos++
    }
    return number(buffer)
  }

  def parseOperation() {
    while (pos < len && !arrayKeyExists(toChar(expression.charAt(pos)), operations)) {
      pos++
    }
    return operations[toChar(expression.charAt(pos++))]
  }

  num1 = parseNumber()
  op = parseOperation()
  num2 = parseNumber()
  return op(num1, num2)
}

println calculate("2+2")
println calculate("400*16")
println calculate("400/160")
println calculate("3>4")