CheckCal.kt

class CheckCal {
    private val MAX = 128
    private val valStack = Stack<Int>(MAX)
    private val opStack = Stack<Char>(MAX)
    private var buf = CharArray(MAX)

    fun startCheck(inStream: InputStream) {
        val reader = inStream.bufferedReader()
        var prev: Int? = null
        loop@ while(reader.read(buf) != -1) {
            t@ for (t in buf) {
                try {
                    when (t) {
                        ' ' -> continue@t
                        '(' -> {
                            opStack.push(t)
                            prev = null
                        }
                        ')' -> {
                            closeBracket()
                            prev = null
                        }
                        in nums -> {
                            if(prev!=null) valStack.pop()
                            val intOfT = Character.getNumericValue(t)
                            prev = prev?.times(10)?.plus(intOfT) ?: intOfT
                            valStack.push(prev)
                        }
                        in operators -> {
                            if(!opStack.isEmpty)
                                if(highPriority(t) && opStack.top != '(')
                                    operate(opStack.pop())
                            opStack.push(t)
                            prev = null
                        }
                        '\n' -> {
                            finalCheck()
                            break@loop
                        }
                        else -> close("Only numbers and \"+-*/()\" are accepted!")
                    }
                } catch (e: Exception) {
                    e.printStackTrace()
                    close("Exceed close brackets!")
                }
            }
        }

        println("Result is ${valStack.top}")
    }

    private fun highPriority(t: Char) = findPriority(opStack.top as Char) < findPriority(t)

    private fun finalCheck() {
        while (!opStack.isEmpty && !valStack.isEmpty) {
            if (valStack.size == 1) throw Exception("Statement Error!")
            operate(opStack.pop())
        }
    }

    private fun closeBracket() {
        for (op in opStack) {
            if (op == '(') break
            if (valStack.size < 2) throw Exception("Statement Error!")
            operate(op)
        }
    }

    private fun operate(tmp: Char) {
        val cal = calculate(valStack.pop(), valStack.pop())
        val ret = when (tmp) {
            '+' -> cal(Int::plus)
            '-' -> cal(Int::minus)
            '*' -> cal(Int::times)
            '/' -> cal(Int::div)
            else -> throw Exception("Fuck")
        }
        valStack.push(ret)
    }

    private fun calculate(arg1: Int, arg2: Int) : ((Int, Int) -> Int) -> Int {
        return fun (op: (Int, Int) -> Int) : Int {
            return op(arg2, arg1)
        }
    }

    private fun close(txt: String) {
        println(txt)
        System.exit(1)
    }

    private val nums = "1234567890".toCharArray()
    private val operators = "+-*/".toCharArray()

    val priority = arrayOf(
            arrayOf('(', ')'),
            arrayOf('*', '/'),
            arrayOf('+', '-')
    )
    
    fun findPriority(t: Char) = priority.indexOf(priority.first{it.contains(t)})
}